Remote Washing Machine Alert

It is often the case these days that the washing machine and  tumble dryer are installed in an outbuilding  or corner of a garage. This not only makes the kitchen a much quieter place but also leaves room for a dish washer and gives additional cupboard space. The problem now is how to tell when the wash cycle is finished. In bad weather you don’t want to make too many fruitless trips down the garden path just to check if the wash cycle is finished. The author was faced with this problem when he remembered a spare wireless door chime he had. With a few additional components and a phototransistor to passively detect when the washing machine’s ‘end’ LED comes on, the problem was solved.

Circuit diagram :

Remote Washing Machine Alert-Circuit Diagram

Remote Washing Machine Alert Circuit Diagram

C1 smoothes out any fluctuations in the LED light output (they are often driven by a multiplex signal) producing a more stable DC voltage to inputs 2 and 6 of IC1. The circuit is battery powered so the CMOS version of the familiar 555 timer is used for IC1 and IC2. The output of IC1 (pin 3) keeps IC2 reset (pin 4) held Low while there is no light falling on T1. When the wash cycle is finished the LED lights, causing T1 to conduct and the voltage on C1 starts to fall. Changing  the value of R1 will increase sensitivity if the LED is not bright enough.

When the voltage on C1 falls  below 1/3 of the supply volt-age IC1 switches its output  (pin 3) High, removing the  reset from IC2. T2 conducts  and LED D1 is now lit, sup-plying current to charge C2.  When  the  voltage  across  C2 reaches 2/3 supply IC2  switches its output Low and  C2 is now discharged by pin  7 via R3. The discharge time  is roughly one minute before  the transistor is again switched on. The process repeats as long as light is falling on T1.

Transistor T2 is a general-purpose small signal NPN type. The open collector output is  wired directly in parallel with the bell push  (which still functions if the transistor is not  switched on). Ensure that transistor output is  wired to the correct bell push terminal (not the side connected to the negative battery  terminal).

Each timer consumes about 60 µA quiescent and the circuit can be powered from the transmitter battery. Alternatively a 9 V battery can be substituted; it has much greater capacity than the original mini 12 V battery fitted in the bell push. Before you start construction, check the range of the wireless doorbell to make sure  the signal reaches from the washing machine to wherever the bell will be fitted.

Author : Götz Ringmann - Copyright : Elektor

In-Car Food And Beverage Warmer

This is a very useful device for those who are frequently on the move. It will keep your tea, coffee or food warm while consuming little power.  The circuit is simple. The ubiquitous timer 555 is used as a free-running astable multivibrator. Diodes 1N4148 are connected in reverse direction to facilitate maximum variation of the duty cycles.

 In-Car Food

Power transistor T1 is Darlington type with 5A capacity and output of more than 60 watts. The chosen discrete components assure fixed frequency of 1 Hz (approximately) at pin 3 of timer IC1 (555). Resister R1 and potmeter VR1 (1-mega-ohm) allow adjustment of the duty cycle. The higher the duty cycle, the higher the output of the heater.

You can connect up to five 10W heating elements in parallel, totaling 50 watts. The consumption of current will be significantly less if fewer coil elements are connected in parallel through toggle switches S2 through S4. Each of these switches has a 6A rating.

Assemble the circuit on a general-purpose PCB. Mount power transistor TIP120 on a thick heat-sink. Isolate the circuit from the heating elements using only two wire connections. Use wires that can carry more than 6A current. Fix the coil elements below an aluminium or steel rectangular plate which is at least 1mm thick. Do not forget to insulate the heating plate from the elements. Use the car battery for the power supply with a proper current-carrying-capacity wire.

Author :Ashok K. Doctor - Copyright : EFY

Microphone Preamplifier for Radio Amateurs

The technical demands on microphones used with radio equipment are not stringent in terms of sound quality: a frequency response from around 50 Hz to 5 kHz is entirely adequate for speech. For fixed CB use or for radio amateurs sensitivity is a more important criterion, so that good intelligibility can be achieved with-out always having to hold the microphone directly under your nose. Good micro-phones with extra built-in amplifiers can be bought, but, with the addition of a small preamplifier, an existing micro-phone will do just as well.

Circuit diagram :

Simple Microphone Preamplifier for Radio Amateurs

Simple Microphone Preamplifier for Radio Amateurs Circuit Diagram

The project described here uses only a few discrete components and is very undemanding. With a supply voltage of between 1.5 V and 2 V it draws a current of only about 0.8 mA. If you prefer not to use batteries, the adaptor circuit shown, which uses a 10 kΩ resistor, three series-connected diodes and two 10 µF electrolytic smoothing capacitors, will readily generate the required voltage from the 13.8 V supply that is usually available. There is little that need be said about the amplifier itself. Either an ordinary dynamic microphone or a cheaper electret capsule type can be connected to the input. In the latter case a 1 kΩ  resistor needs to be connected between the 1.5 V supply volt-age and the positive input connection. The impedance of the microphone and of the

following stage in the radio apparatus are not of any great importance since the available gain of 32 dB (a factor of 40) is so great that only in rare cases does P1 have to be set to its maximum position. With a frequency range from 70 Hz to about 7 kHz, low distortion, and small physical size, the preamplifier is ideal for retrofitting into the enclosure of the radio equipment or into the base of a micro-phone stand.

In case you are concerned about our somewhat cavalier attitude towards distortion: for speech radio the ‘fi’ does not need to be ‘hi’. Quite the reverse, in fact: the harmonics involved in a few percent of distortion can actually improve intelligibility  it’s not a bug, it’s a feature!

Author : Ludwig Libertin - Copyright : Elektor

Room Noise Detector Schematic Circuit

This circuit is intended to signal, through a flashing LED, the exceeding of a fixed threshold in room noise, chosen from three fixed levels, namely 50, 70 & 85 dB. Two Op-amps provide the necessary circuit gain for sounds picked-up by a miniature electret microphone to drive a LED. With SW1 in the first position the circuit is off. Second, third and fourth positions power the circuit and set the input sensitivity threshold to 85, 70 & 50 dB respectively. Current drawing is 1mA with LED off and 12-15mA when the LED is steady on.

Circuit diagram :

Parts List :

R1____________10K 1/4W Resistor
R2,R3_________22K 1/4W Resistors
R4___________100K 1/4W Resistor
R5,R9,R10_____56K 1/4W Resistors
R6_____________5K6 1/4W Resistor
R7___________560R 1/4W Resistor
R8_____________2K2 1/4W Resistor
R11____________1K 1/4W Resistor
R12___________33K 1/4W Resistor
R13__________330R 1/4W Resistor

C1___________100nF 63V Polyester Capacitor
C2____________10µF 25V Electrolytic Capacitor
C3___________470µF 25V Electrolytic Capacitor
C4____________47µF 25V Electrolytic Capacitor

D1_____________5mm. Red LED

IC1__________LM358 Low Power Dual Op-amp

Q1___________BC327 45V 800mA PNP Transistor

MIC1_________Miniature electret microphone

SW1__________2 poles 4 ways rotary switch

B1___________9V PP3 Battery

Clip for PP3 Battery

Use :
  • Place the small box containing the circuit in the room where you intend to measure ambient noise.
  • The 50 dB setting is provided to monitor the noise in the bedroom at night. If the LED is steady on, or flashes bright often, then your bedroom is inadequate and too noisy for sleep.
  • The 70 dB setting is for living-rooms. If this level is often exceeded during the day, your apartment is rather uncomfortable.
  • If noise level is constantly over 85 dB, 8 hours a day, then you are living in a dangerous environment.

Source :

Short-Wave Superregenerative Receiver

Superregenerative receivers are characterised by their high sensitivity. The purpose of this experiment is to deter-mine whether they are also suitable for short-wave radio. Superregenerative receivers are relatively easy to build. You start by building a RF oscillator for the desired frequency. The only difference between a superregenerative receiver and an oscillator is in the base circuit. Instead of using a voltage divider, here we use a single, relatively high-resistance base resistor (100 kΩ to 1MΩ).

Superregenerative oscillation occurs when the amplitude of the oscillation is sufficient to cause a strong negative charge to be applied repeatedly to the base. If the regeneration frequency is audible, adjust the values of the resistors and capacitors until it lies somewhere above 20 kHz. The optimum setting is when you hear a strong hissing sound. The subsequent audio amplifier should have a low upper cutoff frequency to strongly attenuate the regeneration signal at its output while allowing signals in the audio band to pass through. This experimental circuit uses two transistors. A Walkman headphone with two 32-Ω earphones forms a suitable output device.

Circuit diagram :

Short-Wave  circuit

Short-Wave Superregenerative Receiver Circuit Diagram


The component values shown in the schematic diagram have proven to be suitable for the 10–20 MHz region. The coil consists of 27 turns wound on an AA battery serving as a winding form. The circuit produces a strong hissing sound, which diminishes when a station is received. The radio is so sensitive that it does not require any antenna to be connected. The tuned circuit by itself is enough to receive a large number of European stations. The circuit is usable with a supply voltage of 3 V or more, although the audio volume is greater at 9 V.

One of the major advantages of a superregenerative receiver is that weak and strong stations generate the same audio level, with the only difference being in the signal to noise ratio. That makes a volume control entirely unnecessary. However, there is also a specific drawback in the short-wave bands: interference occurs fairly often if there is an adjacent station separated from the desired station by some-thing close to the regeneration frequency. The sound quality is often worse than with a simple regenerative receiver. However, this is offset by the absence of the need for manual feedback adjustment, which can be difficult.

Author :Burkhard Kainka  - Copyright : Elektor

Tandem Doorbell

The author had a problem: the neighbours  had exactly the same type of doorbell as he did (actually a 50 Hz buzzer), so it wasn’t always clear who needed to answer the door. To avoid confusion, the author augmented the existing doorbell with a wireless model a reasonably inexpensive option at current  prices. All that was necessary for this was to arrange for the existing button and wiring to also actuate the wireless doorbell.

Circuit diagram :

Tandem Doorbell-Circuit Diagram

Tandem Doorbell Circuit Diagram

The author opened up the button enclosure of the wireless doorbell and used a multi-meter to find out which set of contacts were closed when the button was pressed. This is where the relay output should be connected (see the schematic diagram). The circuit is virtually self-explanatory: when the existing doorbell button is pressed to actuate the  buzzer, the voltage is rectified by the bridge rectifier and regulated at 5 V by the 7805. This voltage drives the relay directly, causing the switch in the wireless doorbell button to be shorted. As a result, along with the buzzer a  sizeable Big Ben chime indicates that some-one is at the door. Now the author just hopes that his neighbour doesn’t copy his idea.

Author : A. René Bosch - Copyright : Elektor

Long Range FM Transmitter

The use of transmitters which have a more powerful output than the ‘flea-power’ are sometimes required when there are many obstacles in the path of the surveillance transmitter and monitoring station receiver, or the distance between them is too far so as to make a low powered device feasible. Whereas a typical micro transmitter will produce an RF power in the order of just a few milliwatts, i.e. a few thousandths of a watt, the VHF-FM transmitter described has a power output of between around a half and 2 watts, depending on the power source, which may be anywhere between 6 volts and 30 volts d.c. .

Circuit diagram :

long-range-fm-transmitter Circuit Diagram

Long Range FM Transmitter Circuit diagram

The battery or batteries should be of the alkaline high power type, since the current drain will be found to be relatively higher when compared to microtransmitter current drain The power output of this device is somewhat proportional to the current drain and so therefore both may be decreased by altering the value of R6 to a higher resistance, or a variable resistor with a value of around 1k may be introduced in series with the existing R6, so as to give a variable power output. The variable resistormust not be a wirewound device because this would act as an inductor which will cause feedback problems.

The audio input to the power oscillator, which incidentally is formed by TR2 and associated components, is derived from a piezoelectric microphone which drives the simple audio frequency amplifier TR1. The input of the audio amplifier is controlled by the gain pot R1, which selects the correct amount of voltage that is generated by the piezoelectric microphone, then connects this signal to the base of audio amplifier TR1 via C7. It may be found that there is insufficient housing space for a bulky piezoelectric microphone, so with a slight modification to the circuit, it is possible to employ an electret microphone insert as shown.

Since the RF field that is generated by this transmitter is relatively large, the problem of RF feedback may very well be encountered. This may be overcome by placing the transmitter inside a metal enclosure, keeping all internal wiring as short as possible and the aerial wire.

Component listing for 1 watt transmitter

Resistors Semiconductors R1 = 27k TR1 = BC547 R2 = 330k TR2 = 2N2219 fitted with heat sink R3 = 5k6 MIC = piezoelectric microphone R4, 5 = 10k R6 = 100R

L = 6 turns 22 gauge enameled wire wound on 3⁄16″ former


C1, 2, 3 = 330 pF C4 = 2–10 pF trimmer C5 = 4p7 C6 = 1 nF

C7,C8 = 40uF/25V Electrolytic

Source : Circuittoday

LED Bicycle Lights

Before getting started an acknowledgement is due, the circuit presented here uses an ingenious method of controlling a flyback  converter by the voltage developed on a cur-rent sensing resistor, this was published by Andrew Armstrong in the July 1992 issue of ETI magazine.

Circuit diagram :

LED Bicycle Lights-Circuit Daigram

LED Bicycle Lights Circuit Diagram

The reworked circuit is quite simple. At the instant that power is applied only a small current flows to charge C4 so insufficient voltage is developed on R3 to switch T2 on. Also, D1 allows C2 to charge from the 6 V battery, so  R1 feeds enough voltage to switch on T1 this shunts the voltage across L1 and the current in it starts to rise. At a certain point the  current which returns via R3 will develop sufficient voltage to switch on T2 which shunts the gate voltage to T1 causing it to switch off,  initiating the flyback voltage from L1. The fly-back pulse forces a current around the circuit,  charging C4 and feeding the LEDs.

As the return current is via the current sensing resistor R3, this keeps T2 turned on and T1 turned  off, so the flyback phase is not clamped until it has given up all its energy. Capacitor C3 provides positive feedback to ensure reliable oscillation and sharpen up the switching edges. Components D1, D2 & C2 form a bootstrap boost circuit for the MOSFET gate, although it is logic level it only guarantees the stated RD-S(on) at a Vg level of about 8 V — by happy coincidence the combined Vf of four ultrabright red LEDs is about 8.8 V and  this is the value that the output is normally clamped to.

There are some notes on the components specified. For position T1 an n-channel MOS- FET with a very low RD-S(on) of 15 mΩ (at 10 V) Is suggested, although its high ID rating (35 A) is not strictly necessary. Purists may wish to use Schottky barrier diodes for D2 and D4, but a quick look at the data sheet for the popular BAT85 shows that with a Trr of 4 ns it is not actually any faster than the 1N4148. It is doubtful whether the lower Vf would make any noticeable difference.

Zener diode D5 has been included as a safety measure in case the output should ever find  itself open circuit. The flyback converter can  develop a quite impressive voltage when  run without load and would have no difficulty damaging the MOSFET. If a higher voltage MOSFET is used then C4 could easily fall  prey to excessive voltage if the lead to the LED breaks. In the final working prototype D5 was a 1.3-watt 22-volt zener, but any value  between 18 and 24 V is fine. Bear in mind that with four white LEDs on the output the voltage will be somewhere in the region of 13 V. L1 is a 9 mm diameter 0.56 A 220 µH inductor with a low DC resistance (Farnell # 8094837); don’t even think about using those small axial lead inductors disguised as resistors even the fat ones last only a few seconds before failing with shorted turns.

On R3, this resistor is selected depending on  the configuration of LEDs. A value of 20 mA  is fairly typical for 5 mm LEDs, on this basis four red LEDs will need about 12 Ω; five red  LEDs about 10 Ω, and four white LEDs about 6.8 Ω. Resistor R4 (1 Ω 1%) is provided to use as a temporary connection for the LEDs’ negative lead so the volt drop can be measured to indicate the current flowing during setting the correct LED current by adjusting R3. The efficiency of the circuit depends on the LED current, which also determines to some extent the switching frequency. At 10 mA (4 white LEDs) 170 kHz was measured on the prototype and that’s about the maximum  normal electrolytic capacitors are able to withstand. If more current is drawn (e.g. three white LEDs at 30 mA) then the switching frequency drops to about 130 kHz and the efficiency rises to around 75%.  The circuit is simple enough to construct on stripboard, which can be built as a single or  double unit to suit whatever lamp housings are ready to hand. The double unit should fit comfortably in a 2x D cell compartment and  the single board is only a whisker bigger than a single C cell.

Suggested lamp housings are the Ever Ready and the Ultralight but there should be many others that can be modified to house the stripboard. In many cases the hole for the bulb will need 4 notches cut with a round file so that the LEDs can be pushed far enough through. These can be secured in place with a spot of hot melt glue. The battery and switch box can be surprisingly challenging, the unit built for a family member went on a bicycle with a wire basket so it was easy to bolt a Maplin ABS project box to that. With only the tubular frame to fix things onto, it’s not so easy. The  authors’ battery box for the present project  is an old Halfords lamp the one that drops  into a U shaped plastic clip that does nothing to deter thieves, but it’s far more secure when cut down to make a battery box and clamped to the handlebar with a jubilee clip. It easily holds a 6 V 1.3 Ah SLA battery from  Maplin but any nominal 6 V type can be used as per individual preference. Deep discharging should be prevented.

Please Note. Bicycle lighting is subject to legal restrictions, traffic laws and, addition-ally in some countries, type approval.

Author : Ian Field  – Copyright : Elektor

Mobile Phone Shield with Charger

This is the cell phone shield circuit which can be used as mobile charger. Give protection to your cell phone from unexpected use or theft working with this easy circuit. It is able to produce a loud chirping sound when someone tries to take away the mobile handset. The added function is that the circuit also operates as being a mobile charger.

Circuit diagram :


Mobile Phone Shield with Charger Circuit Diagram

The circuit is powered by a step-down transformer X1 with rectifier diodes D1 and D2 and filter capacitor C1. Regulator IC 7812 (IC1) together with noise filter capacitors C2 and C3 gives regulated power source. The cell phone shield circuit uses two NE555 timer ICs: One as being a very simple astable multivibrator (IC2) and then the 2nd as being a monostable multivibrator (IC3). The astable multivibrator has timing resistors R1 and R2 but no timing capacitor since it operates with stray capacitance. Its pins 6 and 2 are directly joined to a safeguarding shield built up of 10cm×10cm copper-clad board.

The inherent stray capacitance of the circuit is enough to supplied an output frequency of about 25 kHz with R1 and R2. This arrangement gives better sensitivity and allows the circuit with hand capacitance effect. Output pulses from the oscillator are immediately assigned to trigger pin 2 of the monostable multivibrator. The monostable utilizes a low-value capacitor C6, resistors R3 and preset VR1 for timing.

The output frequency of the monostable multivibrator is altered utilizing preset/trimmer VR1 such that it is slightly less than that of the astable multivibrator. This makes the circuit standby, as soon as there is no hand capacitance present. So in the standby mode, the astable’s output is going to be low. This tends to make the trigger input of monostable become low and output become high.

The warning indicator buzzer and LED1 are joined such that they come to be active only when the output of the monostable multivibrator sinks current. During the standby state, the LED1 continues to be “off” and also the buzzer is silent. As someone attempts to take the cell phone from the defending shield, his hand comes close to the shield or makes contact with the shield, which introduces hand capacitance within the circuit. Because of this, the astable’s frequency changes, which makes the trigger pin of the monostable become low and its output oscillates. This generates chirping sound from the buzzer and also makes the LED1 blink.

The circuit can even be utilized as being a mobile charger. It delivers output of 6V at 180 mA through regulator IC 7806 (IC4) and resistor R5 for charging the cell phone. Diode D3 defends the output from polarity reversal.

The circuit could be wired on a general PCB. Enclose it inside a appropriate case with provision for charger output leads. Produce the protective shield making use of 10cm×10cm copper-clad board or aluminium sheet. Hook it up towards the circuit working with a 15cm plastic wire. Leads of all capacitors ought to be short.

Fine-tune VR1 little by little working with a plastic screwdriver until eventually the buzzer stops sounding. Get the hand nearby to the shield and fine-tune VR1 right up until the buzzer sounds. With trial-and-error method, set it up for the highest level of sensitivity such that as shortly the hand comes close to the shield, the buzzer begins chirpring and also the LED blinks. As an alternative to applying the copper cladding for shield, a metallic cell phone holder can be utilized as being the shield.

Tachometer Pulse Divider

The author is a motorbike racer in the Classics class of a Dutch Motorcyclists Association. He recently replaced the contact points  on the engine of his motorbike (a 500-cc BSA  Goldstar with a single-cylinder four-stroke  motor) by an electronic ignition. The new  ignition system produces a spark for every  rotation of the motor, compared with a  spark for every two rotations with the con-tact points, so there are twice as many spark  pulses. As a result, the tachometer indication was no longer correct.

Circuit Diagram :

Tachometer Pulse Divider-Circuit Diagram

Tachometer Pulse Divider Circuit Diagram

A new tachometer suitable for use with an  electronic ignition (such as a Krober unit) is rather pricey at around €  175. Accord- ingly, the author first looked through past Elektor July & August issues for a suitable divider circuit — after all, it should be possible to solve this problem with a bit of electronics. It didn’t take long to find something suitable in the form of a mon-ostable multivibrator. The circuit shown here required only a couple of changes to the original design, and now the original tachometer again shows the right motor speed. Final tally: problem solved for € 5; € 170 saved, and the priceless pleasure of setting the bike right yourself.

Author : Sjabbo van Timmeren  – Copyright :Elektor

Call Bell with Welcome Indication

Here is a simple call bell circuit that displays a welcome message when somebody presses the call bell switch momentarily. the alphanumeric display can be fitted near the call bell switch. the circuit is built around two 555 ICs (IC1 and IC2), seven KLA511 common-anode alphanumeric displays (DIS1 through DIS7) and a few discrete components. For easy understanding,  the entire circuit can be divided into  two sections: controller and display. the controller section is built around  IC1 and IC2, while the display section is built around alphanumeric displays (DIS1 through DIS7).

As shown in the circuit, both IC1 and IC2 are wired as monostable  multivibrators having time periods of around 5 seconds and 2 minutes, respectively. You can change the time period of IC1 by changing the values of resistor R12 and capacitor  C3. Similarly, the time period of IC2  can be changed by changing the values of resistor R2 and capacitor C1. Alphanumeric displays DIS1 through DIS7 are wired such that they show ‘WELCOME’ when the output of IC2  goes high. the circuit is powered by a 6V battery. Else, you can use the 6V, 300mA power adaptor that is readily available in the market. the 6V battery or power adaptor provides regulated 6V required to operate the circuit.

Circuit diagram :

Call Bell with Welcome Indication-Circuit Diagram

Call Bell with Welcome Indication Circuit Diagram

A 6V DC socket is used in the circuit to connect the output of the adaptor if you don’t use the battery. Working of the circuit is simple. First, power-on the circuit using switch S2. LED1 glows to indicate presence of power supply in the circuit. Now if you press call bell switch S1  momentarily, it triggers  both the timers (IC1 and  IC2) simultaneously. IC1 produces a high output at its pin 3 for about five seconds. transistor t2 conducts and piezobuzzer PZ1 sounds for about five seconds indicating that there is  somebody at the door. At the same time, IC2  too produces a high out-put at its pin 3 for about two minutes. transistor  t1 conducts to enable the alphanumeric displays. the word ‘WEL-COME’ is displayed  for about two minutes  as DIS1 through DIS7  ground via transistor T1.

If switch S1 is pressed again within these two minutes, piezobuzzer PZ1 again  sounds for five seconds and the display continues to show ‘WEL-COME’. Assemble the complete circuit on a general purpose PCB and house in a small cabinet with call bell switch S1 and LED1 mounted on the front panel. At the rear side of the cabinet, connect a DC socket for the adaptor. Install the complete unit (along with the display) at the entrance of your house. Connect the 6V battery or 6V adaptor for powering the circuit. Configure switch 2 (used to enable/disable the call bell) in a switch board at a suitable location inside your house. If you don’t use a battery, connect the power adaptor to the DC socket on the rear of the cabinet. Close switch S2 only when you want to activate the circuit with  battery. Otherwise, keep it open when the 6V adaptor is in use.

EFY note. 1. To avoid any shorting  during rain, waterproof the entire circuit assembly including alphanumeric displays (installed at the entrance) by covering it properly.

2.  the complete kit for this circuit is available with EFY associates  kits’n’spares.

Author : S.C. Dwivedi - Copyright : EFY

Simple Triple Power Supply

Inexpensive miniature transformers normally provide one or two secondary voltages, which is sufficient for generating a set of positive and negative supply volt-ages, such as are needed for operational amplifier circuits. But what can you do if you need an additional voltage that is higher than either of the supply voltages (such as a tuning voltage for a receiver?). This circuit shows a simple solution to this problem, and it certainly can be extended to suit other applications. Using a 2×15-V transformer, it generates positive 24-V and 12-V supply voltages and a negative 12-V supply voltage.

Circuit diagram :

Simple Triple Power Supply-Circuit diagram

Simple Triple Power Supply Circuit diagram

The little trick for generating the +24-V output consists of using IC1 to create a virtual ground. This is based on a well-known circuit with a voltage divider formed by two equal-valued resistors, which divide the voltage Ub across the rectifier from approximately 40 V down to 20 V. This Ub/2 potential is buffered by an opamp, which allows this virtual ground to drive a load. The present circuit uses the same principle, but instead of being divided by a factor of 2, the volt-age across the rectifier (approximately 40 V) is divided unequally by R1 and R2. The resulting potential, which is buffered by the opamp and the subsequent transistor output stage, lies approximately 15 V above the lower potential, and thus around 25 V below the upper potential. The three voltages are stabilised using standard 100-mA voltage regulators, as shown in the schematic.

The supply voltages for the opamp are also asymmetric. Thanks to the low cur-rent consumption, this can be managed using two Zener diodes.

You must bear in mind that the secondary voltage generated by an unloaded miniature transformer is significantly higher than its rated secondary voltage. The following results were obtained in a test circuit using a 1.6-VA transformer with two 15-V secondary windings: the positive and negative 12-V outputs could be loaded at around 10 mA each, and the 24-V output could be loaded with approximately 20 mA, all without any drop in any of the output voltages. For small circuits such as a 0(4)–20-mA instrumentation loop, this is fully adequate. For more complex circuits or switched loads, additional compensation may be necessary.

Author : Bernd Schädler - Copyright : Elektor

Flashing Lamp Lights

This flashing lights circuit can be used as beacon. The assembly consists basically of two blinking steps that commands two light bulbs. With the help of P1 you can adjust the flashing frequency between some limits.

Circuit diagram :


Flashing Lamp Lights Circuit Diagram

There are 2 parts for the circuit, the second one works the same way as the other but with the help of a wire bridge or a switch you can choose different operating modes. A bridge between M and 3 means: 2 independent blinks. If there is a bridge between M and 2, then the lamps lights alternatively with a frequency that can be adjusted with P1. And finally there is one more possibility for M and 1, where the lamps blinks at the same time.

The flashing lights circuit works with voltages between 3V and 15V.  The lamps voltage must be 2/3 of working voltage. R5 and R10 are chosen so that the lamps are about to light.

Automatic Curtain Opener

This circuit can be used with a timer clock to open and close curtains or (vertical) Venetian blinds. The curtain or blind is driven by  an electric motor with a reduction gearbox fitted to the control mechanism of the curtain or blind. This circuit is ideal for giving your home an occupied appearance while you are away on holiday or for some other reason. In the author’s house, this arrangement has provided several years of trouble-free service on a number of windows fitted  with Venetian blinds.

The original design was a simple relay circuit with pushbuttons for opening and closing and reed switches acting as limit switches. The mechanical drive is provided by a small DC motor with a reduction gearbox and pulley (all from Conrad Electronics).  It was later modified to work automatically with a timer clock. The timer operates a small  230-VAC (or 120-VAC) relay with a changeover contact. Thanks to the two timers, the motor stops after a few seconds if one of the reed switches is missed due to a mechanical defect.

Circuit diagram :

Automatic-Curtain Opener-Circuit Diagram

Automatic Curtain Opener Circuit Diagram

The circuit works as follows (see Figure 1). In the quiescent state, relays RE1–RE3 are de-energised and the motor is stopped. Open the blind:

When the timer clock applies power to the 230-V (120-V) relay RE3, the voltage at the junction of C1 and R1 goes high. IC1 (a 555)  then receives a trigger pulse on pin 2, which causes its output (pin 3) to go High and energise RE1, which in turn causes the motor to start running. When the magnet reaches reed  switch S1 (‘Open’), the 555 is reset. If the reed  switch does not operate for some reason, the relay is de-energised anyhow when the  monostable times out (time delay = 1.1 RC;  approximately 5 seconds). Close the blind:

The timer clock removes power from RE3, which causes a trigger pulse to be applied to the other 555 timer (IC2) via R5 and C4. Now the motor starts running in the other direction. The rest of the operation is the same as described above for opening the blind. Diodes D2 and D5 prevent the outputs of the 555 ICs from being pulled negative when the relay is de-energised, which could otherwise cause the timer ICs to malfunction.

All  components  of  the  mechanical  drive  come from Conrad Electronics [2]: a motor with a reduction gearbox (type RB32, order number 221936) and a pulley (V-belt pulley, order number 238341) on the output shaft. An O-ring is fitted to the pulley to provide  sufficient friction with the drive chain of the Venetian blind. The magnet for actuating the  reed switches is a rod magnet with a hole in the middle (order number 503659), and the chain of the Venetian blind is fed through this hole.

Author : Ton Smits  - Copyright : Elektor

Simple Automatic Loudness Control

( Simple add-on module Switchable "Control-flat" option )

In order to obtain a good audio reproduction at different listening levels, a different tone-controls setting should be necessary to suit the well known behaviour of the human ear. In fact, the human ear sensitivity varies in a non-linear manner through the entire audible frequency band, as shown by Fletcher-Munson curves. A simple approach to this problem can be done inserting a circuit in the preamplifier stage, capable of varying automatically the frequency response of the entire audio chain in respect to the position of the control knob, in order to keep ideal listening conditions under different listening levels. Fortunately, the human ear is not too critical, so a rather simple circuit can provide a satisfactory performance through a 40dB range.

Circuit diagram :

Loudness Circuit

Simple Automatic Loudness Control Circuit Diagram

The circuit is shown with SW1 in the "Control-flat" position, i.e. without the Automatic Loudness Control. In this position the circuit acts as a linear preamplifier stage, with the voltage gain set by means of Trimmer R7. Switching SW1 in the opposite position the circuit becomes an Automatic Loudness Control and its frequency response varies in respect to the position of the control knob by the amount shown in the table below. C1 boosts the low frequencies and C4 boosts the higher ones. Maximum boost at low frequencies is limited by R2; R5 do the same at high frequencies.


P1_________________10K   Linear Potentiometer (Dual-gang for stereo)

R1,R6,R8__________100K   1/4W Resistors
R2_________________27K   1/4W Resistor
R3,R5_______________1K   1/4W Resistors
R4__________________1M   1/4W Resistor
R7_________________20K   1/2W Trimmer Cermet

C1________________100nF   63V Polyester Capacitor
C2_________________47nF   63V Polyester Capacitor
C3________________470nF   63V Polyester Capacitor
C4_________________15nF   63V Polyester Capacitor
C5,C9_______________1µF   63V Electrolytic or Polyester Capacitors
C6,C8______________47µF   63V Electrolytic Capacitors
C7________________100pF   63V Ceramic Capacitor

IC1_______________TL072 Dual BIFET Op-Amp

SW1_____DPDT Switch (four poles for stereo)


  • SW1 is shown in "Control flat" position.
  • Schematic shows left channel only, therefore for stereo operation all parts must be doubled except IC1, C6 and C8.
  • Numbers in parentheses show IC1 right channel pin connections.
  • R7 should be set to obtain maximum undistorted output power from the amplifier with a standard music programme source and P1 rotated fully clockwise.

Technical data:

Frequency response referred to 1KHz and different control knob positions:


Total harmonic distortion at all frequencies and 1V RMS output: <0.01%

Source :redcircuits

Lighting Up Model Aircraft

This circuit provides aircraft modellers with extremely realistic beacon and marker lights at minimum  outlay. The project ’s Strobe out-put (A) provides four brief pulses repeated periodically for the wing  (white strobe) lights. In addition the Beacon output (B) gives a double pulse to drive a red LED for indicating the aircraft’s active operational status. On the proto-type this is usually a red rotating  beacon known as an Anti-Collision Light (ACL). The circuit is equally useful for road vehicle modellers, who can use it to flash headlights and blue emergency lights.

Circuit diagram :

Lighting Up Model Aircraft-Circuit Diagram

Lighting Up Model Aircraft Circuit Diagram

All signals are generated by a 4060 14-stage binary counter and some minimal output selection logic. Cycle time is determined by the way the internal oscillator is con-figured (resistor and capacitor on pins 9/10) and can be varied within quite broad limits. High-efficiency LEDs are your first choice for the indicators connected to the Bea-con and Strobe outputs (remember to fit series resistors appropriate to the operating voltage Ub and the current specified for the LED used).

The sample circuit is for operating voltages between 5 and 12 V. Cur- rent flow through the two BS170 FET devices must not exceed 500 mA.

Author : Werner Ludwig - Copyright : Elektor

Simple 8 Random Flashing LEDs

This project flashes eight LEDs in an apparently random manner. It uses a 4060 combined counter and display driver IC which is designed for driving 7-segment LED displays. 

Circuit diagram :


Simple 8 Random Flashing LEDs

The sequence is not really random because seven of the LEDs would normally be the display segments, the eighth LED is driven by an output that is normally used for driving further counters. The table below shows the sequence for the LEDs. You can use less than eight LEDs if you wish and the table may help you decide which ones to use for your purpose.

Balancing LiPo Cells

Things change fast in the electronics world, and that’s also true for recharge- able batteries. The rate of development of new types of rechargeable batteries has been accelerated by the steadily increasing miniaturisation of electronic equipment. LiPo cells have conquered the market in a relatively short time. Their price and availability have now reached a level that makes them attractive for use in DIY circuits.

Circuit diagram

BalancingBalancing LiPo Cells Circuit diagram

Unlike its competitors Elektor Electronics has already published several articles about the advantages and disadvantages of LiPo batteries. One of the somewhat less well-known properties of this type of rechargeable battery is that the cells must be regularly ‘balanced’ if they are connected in series. This is because no two cells are exactly the same, and they may not all have the same temperature. For instance, consider a battery consisting of a block of three cells. In this case the outer cells will cool faster than the cell in the middle. Over the long term, the net result is that the cells will have different charge states. It is thus certainly possible for an individual cell to be excessively discharged even when the total voltage gives the impression that the battery is not fully discharged. That requires action – if only to prolong the useful life of the battery, since LiPo batteries are still not all that inexpensive.

One way to ensure that all of the cells have approximately the same charge state is limit the voltage of each cell to 4.1 V during charging. Most chargers switch over to a constant voltage when the voltage across the batter terminals is 4.2 V per cell. If we instead ensure that the maximum voltage of each cell is 4.1 V, the charger can always operate in constant-current mode.

When the voltage of a particular cell reaches 4.1 V, that cell can be discharged until its voltage is a bit less than 4.1 V. After a short while, all of the cells will have a voltage of 4.1 V, with each cell thus having approximately the same amount of charge. That means that the battery pack has been rebalanced.

The circuit (Figure 1) uses an IC that is actually designed for monitoring the supply voltage of a microcontroller circuit. The IC (IC1) normally ensures that the microcontroller receives an active-high reset signal whenever the supply voltage drops below 4.1 V. By contrast, the out-put goes low when the voltage is 4.1 V or higher. In this circuit the output is used to discharge a LiPo cell as soon as the voltage rises above 4.1 V.

When that happens, the push-pull output of IC1 goes low, which in turn causes transistor T1 to con-duct. A current of approximately 1 A then flows via resistor R1. LED D2 will also shine as a sign that the cell has reached a voltage of 4.1 V. The function of IC2 requires a bit of explanation. The circuit built around the four NAND gates extends the ‘low’ interval of the signal generated by IC1. That acts as a sort of hysteresis, in order to prevent IC1 from immediately switching off again when the voltage drops due the internal resistance of the cell and the resistance of the wiring between the cell and the circuit. The circuitry around IC2 extends the duration of the discharge pulse to at least 1 s.

Balancing w3

Figure 2 shows how several circuits of this type can be connected to a LiPo battery. Such batteries usually have a connector for a balancing device. If a suit-able connector is not available, you will have to open the battery pack and make your own connections for it. The figure also clearly shows that a separate circuit is necessary for each cell.

Author :Paul Goossens - Copyright : Elektor

Shutter Guard

This sensitive vibration sensor is exclusively made for shops to protect against burglary. It will detect any mechanical or acoustic vibration in its vicinity when somebody tries to break the shutter and immediately switch on a lamp and sound a warning alarm. A 15-minute time delay after switch-on allows sufficient time for the shop owner to close the shutter.

Circuit diagram :

Shutter Guard Cri

Shutter Guard Circuit Diagram

The front end of the circuit has a timer built around the popular binary counter IC CD4060 (IC1) to provide 15-minute time delay for the remaining circuitry to turn on. Resistors R3 and R4 and capacitor C2 will make Q9 output high after 15 minutes. Di-ode D1 inhibits the clock input (pin 11) to keep the output high till the power is switched off. Blinking LED1 indicates the oscillation of IC1.

The high output from IC1 is used to enable reset pin 4 of IC2 so that it can function freely. Transistor T1 amplifies the piezo-sensor signal and triggers monostable IC2. The base of transistor T1 is biased using a standard piezo element that acts as a small capacitor and flexes freely in response to mechanical vibrations so that the output of IC2 is high till the prefixed time period.  In the standby mode, the alarm circuit built around IC3 remains dormant as it does not get current. Timing components R8 and C6 make the output of IC2 high for a period of three minutes.

When any mechanical vibration (caused by even a slight movement) disturbs the piezo element, trigger pin 2 of IC2 momentarily changes its state and the output of IC2 goes high. This triggers triac 1 and the alarm circuit activates. Triac BT136 completes the lamp circuit by activating its gate through resistor R9. IC UM3561 (IC4) generates a tone simulating the police siren with R11 as its oscillation-controlling resistor. Zener diode ZD1 provides stable 3.1V DC for the tone-generating IC.

Assemble the circuit on a general-purpose PCB and enclose in a suit-able, shockproof case. Connect the piezo element to the circuit by using a single-core shielded wire. Glue a circular rubber washer on the fine side of the piezo element and fix it on the shutter frame with the washer facing the frame so that the piezo element is flexible to sense the vibrations. Fix the lamp and the speaker on the outer side and the remaining parts inside the case. Since triac is used in the circuit, most points in the PCB will be at mains lethal potential. So it is advised not to touch any part of the circuit while testing.

Author :D. Mohan Kumar  - Copyright : EFY

Mains Failure Alarm

This circuit was designed to produce an audible alarm when the mains power is interrupted. Such an alarm is essential for anyone whose livelihood depends on keeping perishable foodstuffs in cold storage.

The circuit is powered by a 12-V mains adapter. LED D5 will light when the mains voltage is present. When the mains voltage disappears, so does the +12 V supply voltage, leaving the volt-age regulator IC1 and relay driver T1-T2 without power. The relay driver, by the way, is an energy-saving type, reducing the coil current to about 50% after a few seconds. Its operation and circuit dimensioning are discussed in the article ‘Relay Coil Energy Saver’.

Circuit diagram : 

Mains Failure Alarm-Circuit Diagram

Mains Failure Alarm Circuit Diagram

The value of the capacitor at the output of voltage regulator IC1 clearly points to a different use than the usual noise suppression. When the mains power disappears, Re1 is deenergised and the 0.22 F Gold-cap used in position C4 provides supply current to IC2. When the mains voltage is present, C4 is charged up to about 5.5 volts with IC1 acting as a 100-mA current limit and D10 preventing current flowing back into the regulator output when the mains voltage is gone. According to the Goldcap manufacturer, current limiting is not necessary during charging but it is included here for the security’s sake.

The CMOS 555 is configured in astable multivibrator mode here to save power, and so enable the audible alarm to sound as long as possible. Resistors R5 and R6 define a short ‘on’ time of just 10 ms. That is, however, sufficient to get a loud warning from the active buzzer. In case the pulses are too short, increase the value of R5 (at the expense of a higher average current drawn from the Goldcap).

Author : Myo Min – Copyright : Elektor

Simple Steam Whistle

This circuit consists of six square wave oscillators. Square waves are made up of a large number of harmonics. If six square waves with different frequencies are added together, the result will be a signal with a very large number of frequencies. When you listen to the result you’ll find that it is very similar to a steam whistle. The circuit should be useful in modelling or even in a sound studio.

Circuit diagram :

Simple Steam Whistle-Circuit Diagram

Simple Steam Whistle Circuit Diagram

This circuit uses only two ICs. The first IC, a 40106, contains six Schmitt triggers, which are all configured as oscillators. Different frequencies are generated by the use of different feedback resistors. The output signals from the Schmitt triggers are mixed via resistors. The resulting signal is amplified by IC2, an LM386. This IC can deliver about 1 W of audio power, which should be sufficient for most applications. If you leave out R13 and all components after P1, the output can then be connected to a more powerful amplifier. In this way a truly deafening steam whistle can be created. The ‘frequency’ of the signal can be adjusted with P2, and P1 controls the volume.

Simple Smoggy Circuit Schematic

Even if your good old (Sony) Walkman  sees little use nowadays it would be a  shame to get rid of it altogether. The more  so when just removing the tape head  would allow the built-in audio amplifier  to become an outstanding electrosmog  detector for a variety of purposes. Looking at the schematic, readers with RF  experience will have no difficulty in recognising the diodes and coils of the two  detector-receivers, which serve to capture and demodulate RF signals. With its  coil of four turns (L2) one receiver covers the higher frequency range of the  electromagnetic waves, whilst the sec-ond detector takes care of the lower frequency range.

Circuit diagram :

Simple Smoggy-Circuit Schematic

Simple Smoggy Circuit Diagram

For this reason a coil with a  greater number of turns is required: L1 is  an RF choke of about 250 µH. The precise  value is not critical and it could equally be  220 µH or 330 µH. The outputs of both detector-receivers  are connected to the cables disconnected  previously from the tape heads, feeding the  right and left channel inputs to the Walk-man’s audio amplifier. Please note here that  the screening of the tape head cable does not  have to be absolutely identical to the ground connection of the amplifier circuitry. As  we are dealing with a stereo amplifier,  we are listening into both channels and  thus both RF ranges at the same time.

One channel of the amplifier can also be  used to demodulate low-frequency magnetic alternating fields  via a capacitor  (C3) bypassing diode D1 and connecting either a third coil (L3, for instance;  a telephone recording adapter) as the  pickup device or else a long piece of wire  for acquiring low frequency AC electrical fields. Sources like this are discernible mainly by a distinct 50 Hz (or 60 Hz)  humming in the earphones. Predicting what you may hear down to  the very last detail is difficult, since every  locality has its own, individual interference sources. Nevertheless, with practice  users will succeed in identifying these  interference sources by their particular  audio characteristics.

To sum up, four different ‘sensors’ can be  connected to the inputs of this circuit:  ANT1 (approx. 50 cm long whip antenna),  ANT2 (3.5 cm short stub antenna), ANT3  (approx. 1 m long wire antenna for low frequency electrical fields) and a coil for magnetic fields. Finally, two more tips:

  1. Use only ‘good old’ germanium diodes for  D1 and D2. Sensitivity will be much reduced if  silicon diodes are used, as these have a higher  threshold voltage.
  2. Smoggy does not provide an absolute indi-cation of field strength and even more so can-not provide any guidance whether anything  it detects might be harmful. Its function is to detect electromagnetic signals and compare  their relative magnitude.

Author : Tony Ruepp  - Copyright : Elektor

Triple Power Supply

This low-cost, multipurpose power supply fulfils the requirements of almost all laboratory experiments. Nonetheless, it can be easily fabricated by hobbyists.

A single transformer is used to build this triple power supply. Regulator IC LM317 generates variable power supply of 1.25 to 20V, 1A. The dual ±12V, 1A power supply is generated  by regulators 7812 and 7912. Similarly, dual ±5V, 1A power supply is generated by regulators 7805 and 7905. ‘On’/‘off’ switches (S2 through S4) select the required power supply. Variable power supply is used to study the characteristics of devices. Fixed +5V power supply is used  for all digital, microprocessor and microcontroller experiments. Dual ±12V power supply is used for op-amp-based analogue circuit experiments.

Circuit diagram :

Triple Power Supply Fig. 1

Fig. 1: Triple power supply Circuit Daigram


Pin configurations Fig. 2: Pin configurations of regulators

Fig. 1 shows the circuit of the triple power supply, while Fig. 2 shows the pin configuration of the regulators used in the circuit. Transformer X1 steps down the mains power to deliver the secondary output of 18V-0-18V. The transformer output is rectified by full-wave bridge rectifier BR1, filtered by capacitors C1, C2, C3, C7 and C8, and regulated by IC1 through IC5. Regulator IC1 (LM317) provides variable voltages (1.25 to 20V), while IC2 and IC4 provide regulated +12V and –12V, respectively.The output of IC2 is fed to regulator IC3 (7805), which pro-vides fixed +5V. Similarly, the output of IC4 is fed to regulator IC5 (7905), which provides fixed –5V. Capacitors C4 through C6, and C9 through C11, are used for further filtering of ripples in positive and negative regulated power supplies. LED1 glows to indicate that +5V is available, while LED2 indicates that –5V is available.

Switch S1 is used for mains ‘on’/ ‘off’. Using  switches S2 through S4, any of the three supplies can be  independently turned off when not required in a particular experiment. This reduces unnecessary power dissipation and increases the life and reliability of the power supply. Since the circuit uses three terminal regulators, only capacitors are required at the input and output. The use of few components makes the circuit very simple. The three terminal regulators have heat-sink provision to directly deliver 1A output current. To ensure  the maximum output, do not forget to use heat-sinks for the regulators.

Proposed cabinet

Fig. 3: Proposed cabinet for power supply

The three-terminal regulators are almost non-destructible. These have inbuilt protection circuits including  the thermal shutdown protection. Even if there is overload or shorting of the output, the inbuilt overload protection  circuit will limit the current and slowly reduce the output voltage to zero. Similarly, if the temperature increases beyond a certain value due to excessive load and heat dissipation, the in-built thermal shutdown circuit will reduce  the output current and the output voltage (gradually) to zero. Thus complete  protection is provided to the circuitry. Assemble the circuit on a general-purpose PCB and enclose in a box as shown in Fig. 3.

The step-by-step procedure to build the triple power supply for the laboratory follows:

  1. Collect all the components shown in the circuit diagram.
  2. Connect switch S1, fuse, transformer and mains cord to the assembled PCB as well as the box.
  3. Keep the multimeter in DC voltage range (more than 25V DC) and measure the DC voltage across capacitors C1 and C7 (1000 µF, 35V). This voltage should be around 18V×1.41=25  to  26V  DC. Check  both positive and negative voltages with respect to ground.
  4. It is advisable to use three-wire mains cable and plug. If you are using any metallic box, earthing wire/pin of the mains plug should be soldered to the body of the metallic box using an earthing tag.
  5. If the 18V-0-18V transformer is replaced with 15V-0-15V transformer , the output voltage of the variable supply using LM317 will be correspondingly lower.
  6. Connect variable regulator LM317 to the circuit and check 1.25V to 20V output by varying the 2.2-kilo-ohm linear potentiometers.
  7. Now connect ICs 7812, 7912, 7805 and 7905 to the circuit and check their output voltage.
  8. Connect terminals, potmeter, switches and indicator LED on the front panel of the box and complete  the  connections. Close  the  box  by  us-ing screws.

Precaution. At the primary side of the transformer,  230V AC could give lethal shocks. So be careful not to touch this  part. EFY will not be responsible for any  resulting loss or harm to the user.

Author : Sandip Trivedi And P.D. Lele – Copyright : EFY

Inductorless 3-to-5 Volts Converter

By configuring a comparator and a transistor to control the oscillator in a charge pump circuit, you enable the pump to generate a regulated output of in principle any desired value. Charge pump ICs can either invert or double an input voltage (for example, 3 V to –3 V or 3 V to 6 V). The charge pump itself does not regulate the output voltage and one running off 3 V is not normally capable of generating intermediate output voltage levels like 5 V. However, by adding a comparator and a reference device, you can create arbitrary output levels like 5 V and regulate them as well.

Circuit diagram  :

Inductorless 3-to-5 Volts Converter

Inductorless 3-to-5 Volts Converter Circuit Diagram

Charge pump IC1 (a MAX660) has an internal oscillator whose 45 kHz operation transfers charge from C1 to C2, causing the regulated output to rise.

When the feedback voltage (pin 3 of IC2) exceeds 1.18 V, the output of comparator IC2 (a MAX921) goes high, turning off the oscillator via T1. The comparator hysteresis (easily added on IC2) is zero here simply because no hysteresis is required in the control loop. The oscillator when enabled generates two cycles, which is sufficient to drive VOUT slightly above the desired level. Next, the feedback turns the oscillator off again.

The resulting output ripple will depend mainly on the input voltage and the output load current. Output ripple may be reduced at the expense of circuit efficiency by adding a small resistor (say, 1 ?) in series with C1. You’ll find that ripple also depends on the value and ESR associated with C1 - smaller values of C1 transfer less charge to C2, producing smaller jumps in V OUT.

Author: D. Prabakaran - Copyright: Elektor

Minimalist Dip Meter

In days gone by a radio amateur always had a dip meter close to hand in his ‘shack’. Now that people can afford oscilloscopes, the poor old dip meter has lost its importance and is  frequently no longer to be seen. Actually this is a shame because many tasks are much easier to carry out with a dip meter. Anyone who’s interested (perhaps the second time around) can easily build one rapidly with this very simple but adequate circuit. The interesting question is namely what do you actually need from a dip meter?

Minimalist Dip Meter-Circuit Diagram Minimalist Dip Meter Circuit Diagram

  • A visual display of the dip? Nope, the ‘scope can handle that task.
  • A large frequency scale? Not necessary, as you can connect a frequency counter for this.
  • A selection of coils? We don’t need these because we can use a jumper to change range (no coils to lose any more!).

The sensor coil L1 has ten turns and is wound  using an AA-size battery as a former. This coil will allow us to over the range from 6 MHz to 30 MHz. With jumper JP1 open an additional fixed inductance of 10 μH comes into circuit. The frequency measurement range is then from 2.5 MHz to 10 MHz. The switch may be replaced by a jumper.

To take measurements you hold a resonant circuit close to the sensor coil. Tune the rotary capacitor C1 slowly to and fro in order to find the resonant frequency, at which the oscillator amplitude decreases somewhat. The frequency can then be read directly off the oscilloscope.

To obtain a very accurate measurement you can additionally connect your frequency counter to the second output.

Author : Burkhard Kainka - Copyright : Elektor

Whistling Kettle

Most electric kettles do not produce a whistle and just switch off when they have boiled. Fitting a box of electronics directly onto an electric kettle (or even inside!) to detect when the kettle has boiled is obviously out of the question. The circuit shown here detects when the kettle switches off, which virtually all kettles do when the water has boiled. In this way, the electronics can be housed in a separate box so that no modification is required to the kettle. The box is prefer-ably a type incorporating a mains plug and socket.


In this application, the current flowing in coil L1 provides a magnetic field that actuates reed switch S1. Since the current drawn by the kettle element is relatively large (typically 6 to 8 amps), the coil may consist of a few turns of wire around the reed switch. The reed switch is so fast it will actually follow the AC current flow through L1 and produce a 100-Hz buzz. The switching circuit driven by the reed switch must, therefore, disregard these short periods when the contacts open, and respond only when they remain open for a relatively long period when the kettle has switched off.

Circuit diagram :

Whistling Kettle Circuit d

Whistling Kettle Circuit Diagram

The circuit is based on a simple voltage controlled oscillator formed around T2 and T3. Its operation is best understood by considering the circuit with junction R4/R5 at 0 V and C4 discharged. T2 will receive base current through R5 and turn on, causing T3 to turn on as well. The falling collector voltage of T3 is transmitted to the base of T2 by C4 causing this transistor to conduct harder. Since the action is regenerative, both transistors will turn on quickly and con-duct heavily. C4 will therefore charge quickly through T2’s base-emitter junction and T3. Once the voltage across C4 exceeds about 8.5 V (leaving less than 0.5 V across T2’s b-e junction), T2 will begin to turn off. This action is also regenerative so that soon both transistors are switched off and the collector volt-age of T3 rises rapidly to +9 V. With C4 still charged to 8.5 V, the base of T2 will rise to about 17.5 V holding T2 (and thus T3) off. C4 will now discharge relatively slowly via R5 until T2 again begins to conduct whereupon the cycle will repeat. The voltage at the collector of T3 will therefore be a series of short negative going pulses whose basic frequency will depend on the value of C4 and R5. The pulses will be reproduced in the piezo sounder as a tone.

The oscillation frequency of the regenerative circuit is heavily dependent on the voltage at junction R4/R5. As this voltage increases, the frequency will fall until a point is reached when the oscillation stops altogether. With this in mind, the operation of the circuit around T1 can be considered. In the standby condition, when the kettle is off, S1 will be open so that C1 and C2 will be discharged and T1 will remain off so that the circuit will draw no current. When the kettle is switched on, S1 is closed, causing C1 and C2 to be discharged and T1 will remain off. C3 will remain discharged so that T2 and T3 will be off and only a small current will be drawn by R1. Although S1 will open periodically (at 100 Hz), the time constant of R1/C1 is such that C1 will have essentially no voltage on it as the S1 contacts continue to close.

When the kettle switches off, S1 will be permanently open and C1/C2 will begin to charge via R1, causing T1 to switch on. C3 will then begin to charge via R4 and the falling voltage at junction R4/R5 will cause T2/T3 to start oscillating with a rising frequency. However, once T1 has switched off, C3 will no longer be charged via R4 and will begin to discharge via R3 and R5 causing the voltage at R4/R5 to rise again. The result is a falling frequency until the oscillator switches off, returning the circuit to its original condition. As well as reducing the current drawn by the circuit to zero, this mimics the action of a conventional whistling kettle, where the frequency rises as more steam is produced and then falls when it is taken off the boil.

The circuit is powered directly by the mains using a ‘lossless’ capacitive mains dropper, C6, and zener a diode, D2, to provide a nominal 8 V dc supply for the circuit.  A 1-inch reed switch used in the prototype required about 9 turns of wire to operate with a 2-kW kettle element. Larger switches or lower current may require more turns. In general, the more turns you can fit on the reed switch, the better, but do remember that the wire has to be thick enough to carry the current. It is strongly recommended to test the circuit using a 9-volt battery instead of the mains-derived supply voltage shown in the circuit diagram. A magnet may be used to operate S1 and so simulate the switching of the kettle.

Warning. This circuit is connected directly to the 230-V mains and none of the components must be touched when the circuit is in use. The circuit must be housed in an approved ABS case and carry the earth connection to the load as indicated. Connections and solder joints to components with a voltage greater than 200 volts across them (ac or dc) must have an insulating clearance of least 6 mm. An X2 class capacitor must be used in position C6.

Author :Bart Trepak  –Copyright : Elektor

Network RS232

With an ever increasing number of off the shelf electronic modules and boards available at low prices, designers are inclined to use these instead of making all their electronics from scratch. In many cases this makes sense as developing  say, a PID motor controller or a GPS receiver from scratch requires considerable skill, time and effort. A surprising number of modules still have an interface based on RS232. No wonder, as RS232 is easy implemented on a microcontroller with two I/O pins and a line driver such as the MAX232. In the case where the master is a PC, the serial port is relatively easy to access on both Windows and Linux. Usually modules implement a text terminal interface that decodes single line commands with arguments and generate a reply like this:

Tx: cmd arg0 arg1 ... argX/n
Rx: cmd arg0 arg1 ... argX/n
replyline1/n … replylineY/n

A complication occurs when there are a number of RS232 modules in a project, as each requires a serial interface at the master. A hardware solution in the form of an RS232  multiplexer would be a solution but wouldn’t  it be nice to get this functionality for free! By deviating from the original aim of RS232 as a point-to-point link, we can have an RS232 network in which all the modules share both transmit and receive lines to one master inter-face. All modules operate at the same speed, start and stop bits with no flow control.

Circuit diagram :

Network RS232-Circuit Diagram

Network RS232 Circuit Diagram

When idle, all the modules are listening for commands from the master and have their transmitters disabled. Each module is configured with an identifier consisting of a number that the master sends as a single line (e.g. ‘2/n’ selects module 2). If a module receives an identifier that matches its own, it is selected and can decode commands and drive its transmitter for the duration of the reply. Conversely, if the identifier does not match it must not decode commands and ensure its transmitter remains disabled.

In addition to some firmware sup-port, the RS232 driver electronics must be able to tri-state the transmitter while keeping the receiver operational. Sadly, the classic MAX232 driver is unsuitable but the ICL3321 and MAX242 are possible candidates for our purpose. These have low-power shutdown modes that power-down the charge pump and transmitters but keep the receivers enabled for monitoring RS232 activity.

The number of modules in your RS232 network is limited by the (nominal) 5 kΩ pull-down at the receiver input of the line driver device. Multiple modules increase the loading on this signal, reducing the maximum operating speed and cable length. Using the circuit shown here, running an application with five modules at 9,600 bps located within 1 meter  of each other did not present any problem.

Modules need a means of enabling the network mode and setting the unique identifier. This can be done via switches, jumpers or, if I/ O pins are scarce, by storing the configuration in the user EEPROM/Flash provided by many  microcontrollers. If the latter is done, it is reasonable to assume the module will only be configured with normal RS232. Special configuration commands can then be provided that are always decoded irrespective of the identifier match.

It is unlikely that commercially available modules can be tweaked to support ‘network’  RS232 unless the vendor has used a suitable RS232 line driver and is prepared to provide the firmware code. However, it is possible to implement on DYI modules and perhaps module designers can take note and enhance the functionality of their future designs.

Smart Laptop Docking Station

You can easily convert your ordinary docking station into a smart electronic laptop docking station with antitheft alarm.

Circuit diagram :

Smart Laptop Docking Station-Circuit diagram

Fig. 1: Circuit for laptop docking station with antitheft alarm

The add-on sensor circuit required for this is built around IC CNY70 (IC1) and IC CD4060 (IC2) as shown in Fig. 1. IC CNY70 is an integrated reflective-type opto-sensor that contains a photo-transistor and an infrared LED. The LED emits infrared light and the transistor works as a receiver. The current flowing through the phototransistor depends on the intensity of the light detected.  IC CD4060 is a 14-stage ripple-carry binary counter. The counter is reset to zero by a gating positive voltage at the reset input independent of clock.

Power supply to the circuit is derived from AC mains by using step-down transformer X1. The transformer output is rectified by a full-wave bridge rectifier comprising diodes D1 through D4 and smoothed by capacitor C1. When power switch S1 is in ‘on’ position, the circuit gets power supply and power-on indicator LED1 lights  up. At the same time, the mains socket also gets the AC mains supply. This mains socket can be used to connect the laptop charger and/or a desktop lamp, etc. Working of the circuit is simple.

When the laptop is in the docking station, the phototransistor inside IC1 receives the IR light from the LED, reflected by the laptop surface. The phototransistor conducts to make reset pin 12 of IC2 high, so IC2 does not oscillate. When someone lifts up the laptop from the docking station, the photo-transistor cuts off and pin 12 of IC2 goes low. As a result, IC2 starts oscillating. After a few seconds, delay pin 3 of IC2 goes high to drive transistor  T1. The piezobuzzer starts beeping to raise an alert and the LED2 glows to indicate that someone has stolen the laptop from the dockyard.

Proposed assembly :

Proposed assembly for docking station

Fig. 2: Proposed assembly for docking station

The simplicity of the circuit makes it ideal for construction on a small  PCB. After completion of wiring, check the circuit for proper functioning of all the sections and enclose the unit in a suitable ABS case. Mount the finished unit beneath the docking station using small screws/double-sided glue pads so that the opto-sensor is exactly at the centre of the docking-station base  plate. Refer Fig. 2 for the arrangement.

If your laptop computer is black in colour, it will reflect far less IR light. You can overcome this drawback by attaching a white sticker suitably at the bottom of the laptop. Calibrate the circuit before first use. Set preset VR1 at the centre and place the laptop in the docking station. Now turn VR1 slowly until IC2 goes to standby (no-oscillation) mode. Then remove the laptop from the docking station, ensure that IC2 is enabled  (pin 12 is low) and wait for the alarm sound. Repeat the process and adjust VR1 until you get the correct result. Note that the LED in the opto sensor is  permanently powered via resistor R2. Similarly, you are free to experiment with the values of IC2 timing components C5, R3 and R4 for increasing or  decreasing the delay time.

EFY note. During testing at EFY  Lab, we used CX sensor from OMRON in place of CNY70. 

Author :T.K Hareendran - Copyright : EFY

Single-cell Power Supply

Many modern electronic devices and micro-controller based circuits need a 5 V or 3.3 V power  supply. It is important  that  these voltages are constant and so a regulator of some kind is essential, including in battery powered devices. The simplest approach is to select a (perhaps rechargeable) battery whose voltage is rather higher than that required by the circuit and use an ordinary  linear voltage regulator. Unfortunately this solution is rather wasteful of precious energy and space: for a 5 V circuit at least six NiCd or NiMH cells would be required.

Both these disadvantages can be tackled using a little modern electronics. A good way to minimise energy losses is to use a switching regulator, and if we use a regulator with a step-up topology then we can simultaneously reduce the number of cells needed to power the circuit. Fortunately it is not too difficult to design a step-up converter suitable for use in portable equipment as the semi-conductor manufacturers make a wide range of devices aimed at exactly this kind of application. The Maxim MAX1708 is one example. It is capable of accepting an input voltage anywhere in the range from 0.7 V to 5 V, and with the help of just five external capacitors, one resistor, a diode and a coil, can generate a fixed output voltage of 3.3 V or 5 V. With two extra resistors the output voltage can be set to any desired value between 2.5 V  and 5.5 V.

Circuit diagram :

Single-cell Power Supply-Circuit Diagram

Single-cell Power Supply Circuit Diagram


  • Input voltage from 0.7 V to 5 V
  • Output voltage from 2.5 V to 5.5
  • Maximum output current 2 A
  • Can run from a single cell

The technical details of this integrated circuit can be  found on the manufacturer’s website [1], and the full datasheet is available for download. An important feature of  the device is that it includes an internal reference and integrated power switching MOSFET, capable of handling currents of up to 5 A. It is, for example, possible to convert 2 V at  5 A at the input to the circuit into 5 V at 2 A at the output, making it feasible to build a 5 V regulated supply powered from just two NiCd  or NiMH cells. With a single cell the maximum possible current at 5 V would  be reduced to around 1 A.

The example circuit shown here is configured for an output voltage of 5 V. The capacitor connected to pin 7 of the IC  enables the ‘soft start’ feature. R2 provides current limiting  at slightly more than 1 A. For maximum output current R2  can be dispensed with. Pins 1 and 2 are control inputs that allow the device to be shut down. To configure the device  for 3.3 V output, simply connect pin 15 to ground.

The coil and diode need to be selected carefully, and depend on the required current output. To minimise  losses D1 must be a Schottky type: for a 1A output current the SB140 is a suitable choice.

For L1 a fixed power inductor, for example from the Fastron PISR series, is needed. A fundamental limitation of the step-up converter is that the input voltage must be lower than the output voltage. For example, it is not possible to use a  3.7 V  lithium-polymer cell (with a terminal voltage of 4.1 V fully charged) at the input and expect to be able to generate a 3.3 V output, as diode D1 would  be  permanently conducting. On the other hand, there is no difficulty in generating a 5 V  output from a lithium-polymer cell.


Author : Harald Broghammer - Copyright : Elektor