Loudspeaker Impedance Meter

Also suitable for Headphones Operates in conjunction with a DVM

A simple Impedance Meter can be useful to measure the actual impedance a loudspeaker or headphone is presenting @ 1kHz standard frequency. The circuit, designed on request, relies on an earlier design (Spot-frequency Sine wave Generator) to obtain a stable, low distortion 1kHz sine wave avoiding the use of thermistors, bulbs or any special amplitude-limiting device. The sine wave output, after some amplitude setting obtained by means of P1, is sent to the device under measurement through a resistor.

A regulated supply is necessary to obtain a stable output waveform. D1 and D2 force IC1 to deliver 6.2V output instead of the nominal 5V. The measurement is done in two stages: as a constant current supply of the device under test is necessary, this can be set at first by adjusting P1 and measured across the series resistor (R7 or R8, depending on the impedance value to be measured); then, the meter is switched across the device under test and the actual impedance will be read directly on the meter display.

Circuit diagram:

Loudspeaker Impedance Meter Circuit DaigramLoudspeaker Impedance Meter Circuit Diagram 

P1_______________4K7  Linear Potentiometer
R1______________12K 1/4W Resistor
R2_______________2K2 1/4W Resistor
R3_______________1K 1/2W Trimmer (Cermet)
R4_______________1K5 1/4W Resistor
R5_______________4K7 1/4W Resistor
R6_______________3K3 1/4W Resistor
R7_____________100R 1/4W Resistor (See Notes)
R8_______________1K 1/4W Resistor (See Notes)
R9_______________1K 1/4W Resistor (Optional)
C1______________22nF 63V Polyester Capacitor
C2_____________330nF 63V Polyester Capacitor
C3______________22µF 25V Electrolytic Capacitor
D1,D2_________1N4148 75V 150mA Diodes
D3_______________3mm Red LED (Optional)
Q1,Q2,Q3_______BC550C 45V 100mA Low noise High gain NPN Transistors
IC1____________78L05 5V 100mA Regulator IC
SW1,SW2_________SPDT Toggle or Slider Switches
SW3_____________SPST Toggle or Slider Switch
B1________________9V PP3 Battery

Clip for PP3 Battery

Circuit set-up using an oscilloscope:

Connect the oscilloscope in place of the DVM and rotate P1 fully clockwise.

Short the speaker output and adjust R3 to obtain a sine wave of about 2.2V peak-to-peak amplitude.

"By ear" circuit set-up:

Connect a small loudspeaker or one of the two earpieces forming a pair of headphones to the circuit output and rotate P1 to obtain a moderate output sound level.

Carefully adjust R3 until the output sound will stop; then turn back the trimmer very slowly and stop adjusting immediately when the sound will start again.


  • Connect a Digital Voltage Meter set to 200mV ac range to the DVM output terminals

  • Connect the device under test to the Speaker terminals

  • Switch SW1 in the position towards R7 if the impedance value to be measured is below 100 Ohm or towards R8 if above

  • With SW2 in the "Set" position power-on the circuit by means of SW3

  • Adjust P1 in order to read exactly 100.0mV on the DVM display

  • Switch SW2 in the "Measure" position and read directly the loudspeaker or headphones impedance value on the DVM display, e.g. 8.5mV = 8.5 Ohm

  • Please note that when measuring devices with impedance values above 100 Ohm (SW1 set towards R8), the decimal point in the DVM reading must be ignored. E.g. if the display shows 70.5mV, the impedance will be 705 Ohm


  • For very precise measurements use 1% or 2% tolerance resistors for R7 and R8.

  • D3 LED pilot light and its current limiting resistor R9 are optional.

Source : red circuits

Slave Flash With Red-Eye Delay

Digital cameras are becoming more and more affordable. At the economy end of the market cameras are usually equipped with a small built-in flash unit that is ideal for close-ups and simple portraiture. The power rating of the built-in flash unit is quite low so that any subject further away than about 2 to 3 metres (maybe 4 m if you are lucky) tends to disappear into the gloom. You soon become aware of the limitations if you need to photograph a larger group of people say at a function under artificial light in a large hall or outdoors.

Project image :

Slave Flash With Red-Eye Delay  Circuit Imag Slave Flash With Red-Eye Delay Image

The majority of these cameras are not fitted with an accessory socket so it is not possible to simply connect a second flash unit to increase the amount of light. Single lens reflex cameras also need additional lighting (e.g. fill-in flash) to reduce the harsh contrast produced by a single light source. For all these cases an additional slave flashgun is a useful addition to the equipment bag. Rather than shelling out lots of cash on a professional slave flashgun, the circuit here converts any add-on flashgun into a slave flash unit triggered by light from the camera flash.

Circuit diagram :

Slave Flash With Red-Eye Delay-Circuit Diagram

Slave Flash With Red-Eye Delay Circuit Diagram

Simple slave flash circuits can have problems because most modern cameras use a red eye reduction pre-flash sequence. This pre-flash is useful for portraiture. It is designed to allow time for the subjects pupils to contract so that the red inner surface of the eye is not visible when the picture is taken. Some cameras use information gathered at this preflash time to estimate the light power required for the main flash period and some use this time to fine-tune the auto focus. A simple slave flash circuit will be triggered by the pre-flash sequence and will therefore not provide any additional lighting when the main flash occurs and the picture is actually taken.

The circuit shown here is quite simple but neatly solves the pre-flash problem. With switch S1 set to ‘Normal’, the pulse produced by D1 when it detects the camera flash will trigger both monoflops IC1a and IC1b. The output of IC1.A does not perform any useful action in this mode because the logic level on the other side of resistor R4 is pulled high by D3. The output of IC1.B will go high for approximately 10 ms switching T1 on and causing the triac to conduct and trigger the slave flash. The use of a triac optocoupler here has the advantage that the circuit can be used on older types of flashgun triggered by switching a voltage of around 100 V as well as newer types that require only a few volts to be switched.

Parts :


  • R1, R3 = 100kΩ
  • R2 = 100Ω
  • R4, R5 = 220kΩ
  • R6 = 1kΩ


  • C1, C3 = 10µF 16 V radial
  • C2, C4 = 100nF
  • C5 = 47nF


  • D1 = TLRH180P
  • D2, D3 = BAT85
  • IC1 = 4538P
  • IC2 = MOC3020
  • T1 = BC547B


  • Bt1 = two 1.5V batteries (LR44) with PCB mount holder
  • S1 = 3-position slide switch
  • Cable or adaptor for external flasher

PCB Layout :

Slave Flash With Red-Eye Delay-PCB

With switch S1 in the delay position the first flash will trigger IC1.A and its output will enable IC1.B but the low pass characteristics of the filter formed by R4 and C5 slow the rising edge of this waveform so that IC1.B will only be enabled 10 ms after the first flash is detected. IC1.B is now enabled for a period of about 1s (governed by R1 and C3). When the main flash occurs in this time window it will immediately trigger IC1.B and the triac will be switched as described above. The circuit requires a supply of 3 V and draws very little current from the two 1.5 V button cells. It will run continuously for quite a few days, should it be accidentally left on. Switch S1 can be either a three-position toggle or slider type.

Circuit construction is greatly simplified and the finished unit looks much neater if it is built on the available PCB. Space is also provided to fit the PCB mounted battery holders. A suitable flash extension cable or adapter can be found in most photo shops.

Author :Paul Goossens – Copyright : Elektor

LED driven tail/brake Light Cluster

Constant current circuitry 12V Battery operation

This circuit was designed on request to drive a Light-cluster formed by several LEDs that can be mounted in the vehicle as a tail and brake light. When SW1 is on, the cluster will illuminate at medium brightness. When brakes are operated, SW2 will be closed and the cluster will shine at maximum brightness. These two brightness levels of the cluster are obtained by a constant current source drive formed by Q1 and Q2. The two constant current levels are set by R2 and R3 values.

Circuit diagram :

BrakeLightCircuit Diagram

LED driven tail/brake Light Cluster Circuit Diagram


Parts :

R1___10K  1/4W Resistor
R2___33R  1/4W Resistor (See Notes)
R3___15R  1/4W Resistor (See Notes)

D1___1N5819   40V 1A Schottky-barrier Diode (See Notes)
D2--D13___LEDs   High brightness, high efficiency red types (See Notes)

Q1___BC547   45V 100mA NPN Transistor
Q2___BC337   45V 800mA NPN Transistor

SW1___SPST   Tail Light Switch
SW2___SPST   Brake Light Switch


  • The cluster can be formed by up to 12 LEDs as shown in the circuit diagram. Common cluster types usually range from 5 to 10 LEDs.
  • Using the values shown above, stand-by current was 1mA; SW1 on = 20mA, SW2 on = 60mA.
  • Constant output current value can be changed by varying R2 and/or R3.
  • The formula is: R = 0.6/I (in Amperes).
  • Please note that the brake current is obtained by paralleling R2 and R3 values.
  • Use high brightness, high efficiency red LED types of suitable size and change R2 and R3 values to suit LED's Absolute Maximum Ratings.
  • Any Schottky-barrier type diode can be used in place of the 1N5819: the BAT46 type will be a very good choice.

Source : www.redcircuits.com

Noise Suppression For R/C Receivers

Receiver interference is hardly an unknown problem among model builders. Preventive measures in the form of ferrite beads fitted to servo cables are often seen in relatively large models and/or electrically driven models, to prevent the cables from acting as antennas and radiating interference to the receiver. If miniature ferrite beads are used for this purpose, the connector must be first be taken apart, after which the lead must be threaded through the bead (perhaps making several turns around the core) and then soldered back onto the connector.

Project Image :

Noise Suppression For R-C Receivers Image

Noise Suppression For R/C Receivers Image

An interference source can also cause problems in the receiver via the power supply connection.The battery is normally connected directly to the receiver, with the servos in turn being powered from the receiver.  The servos can draw high currents when they operate, which means they can create a lot of noise on the supply line. This sort of interference can be kept under control by isolating the supply voltage for the receiver from the supply voltage for the servos. All of these measures can easily be implemented ‘loose’ in the model, but it’s a lot nicer to fit everything onto a single small circuit board. That makes everything look a lot tidier, and it takes up less space.

Circuit diagram :

Noise Suppression For R-C Receivers- Circuit Daigram

Noise Suppression For R/C Receivers Circuit diagram

The schematic diagram is shown in Figure 1. Connectors K1–K8 are located at the left. They are the inputs for the servo signals, which are connected to the receiver by the servo leads. The outputs (K9–K16) are located on the right. That is where the servos are connected. Finally, the battery is connected to K17. Interference on the supply voltage line due to the motors and servos is suppressed by a filter formed by L10, R1, C1 and C2. L10 is a ferrite-core coil with an impedance of 2000 ohms at 30 MHz. In combination with C1 and C2, it forms a substantial barrier to interference in the 35-MHz R/C band.

Parts :


R1 = 1Ω


C1 = 100nF

C2 = 22pF


L1-L8,L10 = ferrite inductor

L9 = common-mode coil

K1-K8 = servo cable

K9-K17 = 3-way SIL pinheader

PCB Layout :

PCB Laout

Noise Suppression For R/C Receivers PCB Layout

Signals with frequencies close to the 10.4-MHz intermediate frequency (which is used in many receivers) are also effectively blocked by this filter. L9 filters out common-mode noise on the supply line for the servos, which effectively means that it prevents the supply lines to the servos from acting as antennas. Finally, high-frequency currents on the servo signals are filtered out by ferrite beads in order to limit the antenna effects of these connection lines.

Author : Paul Goossens – Copyright : Elektor

70 A Solid-state Starter Relay

Overall, electro-mechanical scooter starter  solenoids are cheap enough but the down-side is that they’re not very reliable. The contact resistance increases over time, the coil  can be open-circuited due to the vibration,  and sometimes the power contacts weld up.  One solution is to replace them with a solid-state relay. In DC mode, we’ll need to use a  MOSFET transistor.

As is often the case in automotive systems, the supply negative is connected to the chassis ground, which means we’ll need to use a P-channel MOSFET. The current to be switched is relatively high, between 55 and 100 A (depending on engine capacity and compression), so we need a transistor with a  very low RDS(on) capable of carrying a large IDS. Since the starter is a DC motor with brushes, it generates considerable voltage spikes that are  quite destructive for the driving device, whence  the need to protect everything very well.


70 A Solid-state Starter Relay-Circuit Diagram 70 A Solid-state Starter Relay Circuit Diagram

A look at the wiring diagrams for various  scooters reveals that the safety switch on the  brake (which has to be applied first) supplies  +12 V, but the starter button (to be operated  next) connects to ground. One simple solution is to use an opto-isolator. While we’re on  the subject, let’s just note that this technique  means this circuit can be used for many other  applications too.

And finally, the circuit must be ‘Plug-n-Play’, i.e. usable with the original connector,  thereby limiting the circuit dimensions to  50 × 50 mm. Building a PCB capable of handling a current  of 70 A needs a few calculations. The resistance RT of a copper track with thickness E of  35 μm (0.035 mm) with length L and width W  is calculated from : RT = 1.7 × 10-5 × L / (E × W)   [Ω] , where E, L, and W are in mm, and T = 25 °C). 

The component positions mean our tracks  can be 15.25 × 44 mm, thus each track rep-resents 1.4 mΩ, or 0.7 mΩ if we use a double-sided board. At 75 A, the total voltage  drop will be around 100 mV and the power  dissipated 7.5 watts. The SUP75P03-07-E3  MOSFET from Vishay Siliconix (Farnell part no.  179 4 812) off er san RDS(on) of 7 mΩ at 75 A, i.e.  3.5 mΩ if we put two in parallel. In this case,  the voltage drop is 0.263 V and the power dissipated in each transistor is around 10 watts. The end result is that we get an overall volt-age drop of around 360 mV and a total dissipation of around 27.5 watts.

Let’s take a look now at the circuit diagram. On  the left, everything within the dashed rectangle  corresponds to the original wiring of the major-ity of Chinese scooters. R1 sets the current in  the 4N28 opto-isolator LED to around 25 mA  and R2 biases the base of the phototransistor. The phototransistor collector is connected  directly to the gates of the two MOSFETs T1  wired in parallel. At rest, the MOSFETs are held off by R3, but start to conduct when both contacts S1 and S2 are made, thanks to D3 and the  low impedance of the starter motor. Once the  starter turns, the charge on C2 ensures that the  circuit will continue to function.

Components C1, D1, C2, D2, and D3 protect  the circuit against the interference produced  by a load that is anything but purely resistive. Tests and measurements have been carried  out on a scooter using a GY6 engine type  CJ12M. The average consumption was 53 A:  49 A at bottom dead centre (minimum compression) as against 57 A at top dead centre  (maximum compression). The voltage drop  measured at the circuit terminals was strictly  identical to the theoretical value. After three  hours’ testing, at a rate of one start every five  minutes, no heating was detected.

Author :By Georges Treels - Copyright : Elektor

Wireless Audio Transmitter

Sitting peacefully under a tree at the bottom of your garden, or stretched out beside your swimming pool, you may feel like listening to your favourite music from your hifi. Rather than turning the volume up beyond reasonable limits and risking upsetting all your neighbours or attracting the wrong audience, we suggest building this little wireless audio transmitter/receiver combination. Using the UHF ISM (industrial, scientific, medical) band and quality FM (frequency modulation), it won’t impair the sound quality and will let you listen nice and discreetly.

Aurel FM transmitter

The transmitter uses a well-known module manufactured for some years now by Aurel as their ‘FM audio transmitter’. It works in the licence free 433.92 MHz band and so allows our project to operate completely legally as the transmitter is type-approved to quite strict technical specifications. Note however the frequency you’re using is not exclusive as it is shared by many other wire-less devices such as headphones and key fobs for garage doors and so on. The equipment is low-power however and should have a short range.

The Aurel module is a complete FM audio transmitter designed for powering from 12V. The only external components required, R5, R6, and C5, form the pre-emphasis (high-boost) network specific to frequency modulation.Used alone, this module offers a typical audio input sensitivity of 100 mV rms. So we are driving it from an opamp with gain adjustable between 0.5 and 5, extending the voltage range from 50 to 500 mV, to make it compatible with any audio device line output. Note in passing that, if you reduce resistor R1 to 2.2 kΩ, you can increase the sensitivity to 2.5 mV so that the transmitter could then be used as a UHF radio mic for use in shows and events, for example.

Aurel FM receiver module

The power supply can be obtained from a 12 V battery or a ‘plug-top’ power supply; diode D1 protects the circuit from reversed polarity by. The receiver is just as simple, since it uses the complementary module to the previous one, again from Aurel, and naturally called their ‘FM audio receiver’. This receiver has a squelch (FM noise silencing) adjustment, set by the voltage applied to pin 15. Potentiometer P1 connected to this makes it possible to adjust the squelch threshold so as to have a receiver that won’t output noise in the absence of a sig-nal, using the information provided on pin 18. This is High when a signal is present and Low when absent. Here it drives an 8-into-1 CMOS analogue multiplexer, of which only input 8 is used. This solution employs a very cheap, good-quality analogue switch that is easy to use.

Its output passing via the volume control P2 and is applied to the well-known small integrated power amplifier LM386. The transmitter’s RF output power of a few hundred milli-watts is more than adequate for such an application, and its quality likewise, especially if you combine it with a loud-speaker worthy of the name, with a pair of headphones as the next best alternative. The Aurel receiver module and CMOS multiplexer both require a 5 V supply; this is stabilized by a standard 3-terminal regulator. The circuit as a whole is powered from 9V, and is also protected against possible reverse polarity by diode D1. Given the relatively high current consumption of the amplifier, especially if you use it for longer periods, rechargeable NiMH batteries will obviously be preferable to primary cells, which wouldn’t last very long and will turn out rather expensive in the long term, as well as bad for the environment.

As far as the antennas are concerned, for both transmission and reception, simple quarter-wave whips ensure a range of a hundred metres or so even more if line-of-sight. You can of course buy such antennas ready-made, but a simple piece of stiff wire around 17 cm long (i.e.a quarter wavelength at 433.92 MHz) will do the job just as well, and cost a lot less. Equipped with these two modules you can make the most of your music wherever you like. Don’t forget, though, that outdoors, the best music of all is that of birds, that is, the feathered variety.

Author : C. Tavernier – Copyright : Elektor

LED Tester

This simple LED tester consists of a current source with a potentiometer that can be used to adjust the current. The current source is implemented using a type TL081 opamp.

Circuit diagram :

LED Tester-Circuit Diagram

LED Tester Circuit Diagram

The output current of the opamp  flows  through the diode and R2. The voltage drop across R2 is fed back to the inverting  input and compared with the reference voltage, which is set with R1 and applied to the  non-inverting  input. The adjustment range is approximately 0–30 mA, which is suitable for testing all normal LEDs. If you wish, you can connect a multi-meter across the LED to measure the voltage on the LED.

For the power source, a good option is to  use a small laboratory power supply with the output voltage set to 5 V. It is convenient to fit the potentiometer with a scale so you can see directly how much current is flowing through the LED. In order to calibrate the scale, you can temporarily connect an ammeter in place of the LED.

Author : Henry Schouwstra – Copyright : Elektor

Fog Lamp Sensor

For several years now, a rear fog lamp has been mandatory for trailers and caravans in order to improve visibility under foggy conditions.

Circuit diagram :

Fog Lamp Sensor-Circuit Diagram

Fog Lamp Sensor Circuit Diagram

When this fog lamp is switched on, the fog lamp of the pulling vehicle must be switched off to avoid irritating reflections. For this purpose, a mechanical switch is now built into the 13-way female connector in order to switch off the fog lamp of the pulling vehicle and switch on the fog lamp of the trailer or caravan. For anyone who uses a 7-way connector, this switching can also be implemented electronically with the aid of the circuit illustrated here.

Here a type P521 optocoupler detects whether the fog lamp of the caravan or trailer is connected. If the fog lamp is switched on in the car, a current flows through the caravan fog lamp via diodes D1 and D2. This causes the LED in the optocoupler to light up, with the result that the phototransistor conducts and energises the relay via transistor T1. The relay switches off the fog lamp of the car.

For anyone who’s not all thumbs, this small circuit can easily be built on a small piece of perforated circuit board and then fitted somewhere close to the rear lamp fitting of the pulling vehicle.

Author :Harrie Dogge - Copyright : Elektor

High Voltage Generator

This high voltage generator was designed  with the aim of testing the electrical break-down protection used on the railways. These  protection measures are used to ensure that  any external metal parts will never be at a  high voltage. If that were about to happen,  a very large current would flow (in the order  of kilo-amps), which causes the protection  to operate, creating a short circuit to ground effectively earthing the metal parts. This hap-pens when, for example, a lightning strike hits  the overhead line (or their supports) on the  railways.

This generator generates a high voltage of  1,000 V, but with an output current that is limited to few milliamps. This permits the electrical breakdown protection to be tested with-out it going into a short circuit state. The circuit uses common parts throughout: a  TL494 pulse-width modulator, several FETs or  bipolar switching transistors, a simple 1.4 VA  mains transformer and a discrete voltage multiplier. P1 is used to set the maximum current  and P2 sets the output voltage.

Circuit diagram :

High Voltage Generator-Circuit Diagram

High Voltage Generator Circuit Diagram

The use of a voltage multiplier has the advantage that the working voltage of the smoothing capacitors can be lower, which makes them easier to obtain. The TL494 was chosen  because it can still operate at a voltage of  about 7 V, which means it can keep on working even when the batteries are nearly empty.  The power is provided by six C-type batteries, which keeps the total weight at a reason-able level.

The 2x4 V secondary of AC power transformer  (Tr1) is used back to front. It does mean that  the 4 V winding has double the rated voltage  across it, but that is acceptable because the  frequency is a lot higher (several kilo-Hertz)  than the 50 Hz (60 Hz) the transformer is  designed for. The final version also includes a display of the  output voltage so that the breakdown volt-age can be read.

From a historical perspective there follows a  bit of background information. In the past a different system was worked  out. Every high-voltage support post has a  protection system, and it isn’t clear when  the protection had operated and went into  a short-circuit state due to a large current  discharge.

Since very large currents were involved, a certain Mr. Van Ark figured out a solution for this.  He used a glass tube filled with a liquid containing a red pigment and a metal ball. When  a large current discharge occurred the metal  ball shot up due to the strong magnetic field,  which caused the pigment to mix with the liquid. This could be seen for a good 24 hours after the event. After a thunder storm it was  easy to see where a discharge current took  place: one only had to walk past the tubes  and have a good look at them.

Unfortunately, things didn’t work out as  expected. Since it often took a very long  time before a discharge occurred, the pigment settled down too much. When a dis-charge finally did occur the pigment no  longer mixed with the liquid and nothing was  visible. This system was therefore sidelined,  but it found its place in the (railway) history  books as the ‘balls of Van Ark’.

Author : By Jac Hettema – Copyright : Elektor

Low Cost Electronic Clock

This circuit can be used for the safety of precious and valuable items. The low-cost electronic lock is suitable for use in homes and banks for lockers etc.The circuit consists of three thumbwheel switches, three DIP switch sets (each having eight switches), three inverter ICs (7404), three quad-AND gate ICs (7408), one quad-NAND IC (7400), one timer IC (555), few diodes/transistors and other passive components.

Circuit diagram :

Low Cost Electronic Clock Circuit-Diagram

Low Cost Electronic Clock Circuit Diagram

The Dip switches are supposed to be hidden and the actual code to be set on the thumbwheel switches must match the Dip switch settings, which will result in enabling of all AND gates (G1 to G11). The high output of the last AND gate is used for energising relay RL1 via DPDT switch S1, which in turn results in energisation of solenoid coil from mains supply via its N/O contacts. This causes the steel rod of the solenoid to be pulled against the spring tension and the lock gets opened. If the code selected via thumbwheel switches does not match the DIP switch settings and DPDT switch S1 is put on, the low output of AND gate G11 will result in setting of latch formed by NAND gates N13 and N14 as well as sounding of buzzer through transistor T2 which gets forward biased. Resetting of buzzer as well as the latch is possible by momentary depression of switch S2. It is therefore desirable to install switch S2 also in a hidden place.

For proper operation of the circuit, switch S1 is initially kept in off position and desired code is selected with the help of the dip switches. This is followed by resetting of latch by momentary depression of switch S2. Now the system is ready for operation. Switch S1 is to be put on only after correct code is selected with the help of thumbwheel switches for opening of lock (for energisation of solenoid), else it will result in sounding of alarm (buzzer), as explained earlier. Diode D2 prevents the positive voltage present at pin 1 of NAND gate N13 (when switch S1 is off) from reaching transistor T1 base and energising relay RL1 and the solenoid. A lock using solenoid coil can be easily fabricated even if it is not readily available. If required, the solenoid used in electrical gong type of bell may be used for the purpose.

Copyright : EFY

Isolated Fuse Fail Indicator

This circuit uses standard components and shows a method of indicating the fuse status of mains powered equipment while providing electrical isolation from the mains supply.

Circuit diagram:

Isolated Fuse Fail Indicator-Circuit Diagram

Isolated Fuse Fail Indicator Circuit Diagram

A standard miniature low power mains transformer  (e.g. with an output of around 6 V at 1.5 VA) is used as a ‘sense’ trans-former with its primary winding (230 V) connected across the equipment’s input fuse so that when the fuse blows, mains voltage is applied to the transformer and a 6 V ac output volt-age appears at the secondary winding. The 1N4148 diode rectifies this voltage and the LED lights to indicate that the fuse has failed. The rectified voltage is now connected to an RC low-pass filter formed by the 10 kΩ resistor and 100 nF capacitor. The resulting positive signal can now be used as an input to an A/D converter or as a digital input to a microcontroller (make sure that the signal level is within the microcontroller input voltage level specification). The 1 MΩ resistor is used to discharge the capacitor if the input impedance of the connected equipment is very high.

As long as the fuse remains intact it will short out the primary winding of the ‘sense’ transformer so that its secondary out-put is zero.

Author : G. Kleine - Copyright : Elektor

Secret Lock

This secret lock, unlike a conventional code lock, gives away no hints to the unwanted visitor as to its existence: there are no buttons, switches or keypads. No code sequence need be learnt: you simply need an inconspicuous key. The idea is based on two magnetically-operated switches which, when operated simultaneously, cause two relays to close. These in turn could actuate an electric door latch or start a garage door motor.

This would not be particularly noteworthy (and rather easy to defeat) if simple reed switches were used, since they do not depend on the polarity of the magnetic field: they react equally to the north or the south pole of a magnet. Instead we use Hall effect ICs, which only react to south poles. In this way the would-be intruder, carrying just a powerful permanent magnet in his pocket, is frustrated in his nefarious deeds: horseshoe and bar magnets do not have two south poles. And if that is not secure enough, you can always add further Hall effect ICs and relays: just like a lock with more levers.

Secret Lock-Circuit Diagram Secret Lock Circuit Diagram

The sensor used in the circuit shown in Figure 1 is smaller than a transistor, and yet contains rather more: a unipolar sensing surface for the magnetic field, Hall generator and threshold generator, amplifier, Schmitt trigger and output transistor. With a field stronger than 20 millitesla the open-collector output transistor is turned on. The series-connected contacts of the 12 V miniature relays then complete the circuit via connection L. Relays with a coil current of 50 mA or less should be used in order not to overload the ICs.

The Hall effect ICs are fitted or glued at least 5 cm apart behind a sheet of glass, plastic or aluminium (perhaps the letterbox or doorbell), at most 4 mm thick, with the component marking towards the key. In no circumstances should iron or steel be used as these screen the sensors from the magnetic field. The sensors can either be wired to directly or fitted on a piece of perforated board. The position of the sensors should be suitably marked on the outside.

Secret Lock-Circuit Diagramw

The simplest way to make a key is from a piece of square section wood in which two small holes are bored for two cylindrical magnets (as used with reed switches). The two magnets should be glued in the same way round, which can easily be tested by checking that the poles repel. Alternatively, of course, the magnets can be fixed in a flat plastic box using hot-melt glue. Remember that only one side of the key will open the lock.

The secret lock can be safely used outside as long as it is fitted in a suitable watertight enclosure. It can save money compared to the services of a locksmith, and it will resist even the professional burglar. The lock is vandal-proof, operates independent of temperature, requires no battery in the key, can be cheaply extended and provided with any number of keys. The Hall effect ICs (Conrad Electronics order code 147508) are inexpensive. The operating voltage depends on the relays chosen, and should lie between 6 V and 24 V. The standby current for two ICs is about 7 mA at 12 V.

Author : W. Zeiller - Copyright : Elektor

LED Flasher

Project image :LED Flasher Image

LED Flasher Project Image

A flashing LED based on a 555 is not likely to win the first prize for originality, but such a circuit continues to be useful for all kinds of applications. The thing that makes this version special is the very low current consumption, because the LED flashes only very briefly each time (duty-cycle of only 10%) and because it was specifically designed to serve as a (fake) alarm indicator in a car. The circuit has been configured such that the flashing only starts when the ignition is switched off.

Circuit diagram :

LED Flasher-Circuit-Diagram LED Flasher Circuit Diagram

The latter could have been achieved in the usual way with the aid of two resistors and a transistor connected to the reset line. But by using a clever trick instead, this has become even simpler. The +12 V connection from ‘behind’ the ignition key is connected to the threshold input via diode D3. The astable multi-vibrator is held in the inactive state whenever this point is at +12 V. Only when the ignition is switched off, is the clamp removed and can the circuit begin to flash. You may then ask what the purpose of D3 is. Well, this has been added in order to prevent the various loads in the car from being connected in parallel with C2 — because that was not the intention, of course.

Part List :


R1 = 470k?
R2 = 1M?


C1 = 100nF
C2 = 220nF


D1 = LED
D2,D3 = 1N4149
IC1 = 555




LED Flasher PCB-Layout

A minuscule printed circuit board has been designed for the circuit, that, when fully built-up will fit exactly in an old fluorescent tube starter. The latter also immediately solves the problem of finding a suitable enclosure.

Copyright : Elektor Electronics

Wireless Baby Monitor

Walkie-talkies (also known as handheld or PMR, Personal Mobile Radio) can be bought at low prices even from department stores, and they can be operated without a licence in many countries. Considering the low cost, such a set would be very suitable for use as a wireless baby monitor, with the addition of several external components. These are connected to the jack sockets for an external loudspeaker/microphone and an external PTT (Push-To-Talk) switch, which are often found on these devices.

Project Image :

Wireless Baby Monitor Image

Wireless Baby Monitor Project Image

The walkie-talkie with the extra electronics and microphone is placed in the baby’s room. When the PTT switch on the other walkie-talkie is actuated for about a second the ‘baby’ walkie-talkie produces a series of tones, which the external electronics can detect. This then activates its own PTT switch for about 5 seconds, so it switches over to transmit. During this time the other device can hear what the external microphone picks up.

Circuit Diagram :

Wireless Baby Monitor-Circuit-Diagram

Figure 1-Wireless Baby Monitor Circuit Diagram

Figure 1 shows the circuit that the author designed for this. It has been designed specifically for a Tevion 3000 PMR sold some time ago by Aldi. This type of PMR has a combined jack socket that includes all the required connections.

The voltage present on the PTT connector is used to generate the supply voltage for the circuit via R3, D1 and C1/C2. When the loud-speaker output presents a series of tones (when the PTT switch on the other walkie-talkie is held down), it causes T1 to conduct. This also turns on T2 and T3, so that the external microphone is connected to ground. The resulting current that f lows through the microphone should be sufficient to activate the PTT circuit in the walkie-talkie, causing it to transmit. If the external microphone doesn’t draw sufficient current, a resistor (R8) should be connected in parallel. Some experimentation with the value of this resistor may be required. If you want to make use of the internal microphone then R8 should be replaced with a wire link.

Circuit diagram :

Wireless Baby Monitor-Circuit-Diagramwq

Figure 2-Wireless Baby Monitor Circuit Diagram

When the walkie-talkie switches to transmit the built-in amplifier stops producing a signal and T1 turns off. However, since electrolytic capacitor C3 has been charged up in the mean time, transistors T2 and T3 will keep conducting for several seconds until C3 has been almost discharged via R4. In the Elektor labs a simpler version with the same functionality (Figure 2) has been designed for use with a cheaper PMR set that can be obtained from Conrad Electronics (PMR Pocket Comm Active Pair, order number 930444). These walkie-talkies have separate jack sockets for the LS/Mic and PTT connections.

When there is a call a series of tones is produced that is used to turn on T1 via R3. T1 then activates the PTT function and the microphone amplifier is turned on. How-ever, it ’s not just the audio signal that is used, but also the DC offset produced when the internal output stage is turned on. Both the internal as well as external loudspeaker are driven via an output capacitor of 100 µF. When there is a call it charges up via R3 and the base-emitter junction of T1. If the walkie-talkie is called often there would be a danger that the output capacitor would remain charged and the DC offset of the audio signal would no longer be sufficient to turn on T1. To prevent this, D1 is connected in reverse across the base-emitter junction of T1, pro-viding a discharge path for the output capacitor.

To keep the circuit active for a minimum amount of time the microphone voltage is used to provide an extra base current. This is done by charging C1 via R1. When the transmitter is turned off the microphone and R2/ D1 provide a discharge path for the capacitor. C2 ensures that the circuit won’t react to spikes caused by interference. As can be seen from the second circuit diagram, use is made of two connectors, a 2.5 mm jack plug for an external headset and a 3.5 mm plug for the PTT function. These connectors are particular to the walkie-talkies we used here. With other types of walkie-talkie you should first check the connection details of the connectors before you connect the circuit up.

When the circuit is used as a baby monitor you should check that the microphone you’re using can pick up all the sounds. In our case the microphone didn’t appear to be very sensitive. The microphone amplifier has probably been designed for a voice that is near the PMR unit. When used as a baby monitor the microphone should therefore be positioned as close to the baby as possible.

Author : Wolfgang Papke - Ton Giesberts - Copyright : Elektor

Low Loss Step Down Converter

This circuit arose from the need of the author to provide a 5 V output from the 24 V battery of a solar powered genera-tor. Although solar power is essentially free it is important not to be wasteful especially for small installations; if the battery runs flat at midnight you’ve got a long wait before the sun comes up again. The basic requirement was to make an efficient step-down converter to power low voltage equipment; the final design shown here accepts a wide input voltage from 9 to 60 V with an output current of 500 mA. The efficiency is very good even with a load of 1 mA the design is still better than a standard linear regulator. The low quiescent current (200 µA) also plays a part in reducing losses.

Some of the components specified (particularly the power MOSFET) are not the most economical on the market but they have been deliberately selected with efficiency in mind.

Circuit diagram :

Low Loss  Step Down Converter-Circuit Diagram

Low Loss Step Down Converter Circuit Diagram

When power is applied to the circuit a reference voltage is produced on one side of R2. D1 connects this to the sup-ply (pin 7) of IC1 to provide power at start-up. Once the circuit begins switching and the output voltage rises to 5 V, D2 becomes forward biased and powers the IC from the output. Diode D1 becomes reverse biased reducing current through R1. When the circuit is first powered up the voltage on pin 2 of IC1 is below the reference voltage on pin 3, this produces a high level on output pin 6. The low power MOSFET T1 is switched on which in turn switches the power MOSFET T3 via R5 and the speed-up capacitor C4, the output volt-age starts to rise.

When the output approaches 5 V the voltage fed back to the inverting input of IC1 becomes positive with respect to the non inverting input (reference) and switches the output of IC1 low. T1 and T3 now switch off and C3 transfers this negative going edge to the base of T2 which conducts and effectively shorts out the gate capacitance of T3 thereby improving its switch off time.

The switching frequency is not governed by a fixed clock signal but instead by the load current; with no load attached the circuit oscillates at about 40 Hz while at 500 mA it runs at approximately 5 kHz. The variable clock rate dictates that the output inductor L1 needs to have the relatively high value of 100 mH. The coil can be wound on ferrite core material with a high AL value to allow the smallest number of turns and produce the lowest possible resistance. Ready-made coils of this value often have a resistance greater than 1 ? and these would only be suitable for an output load current of less than 100 mA.

The voltage divider ratio formed by R4 and R3 sets the output voltage and these values can be changed if a different out-put voltage is required. The output volt-age must be a minimum of 1 V below the input voltage and the output has a minimum value of 4 V because of the supply to IC1.

A maximum efficiency of around 90 % was achieved with this circuit using an input voltage between 9 and 15 V and supplying a current greater than 5 mA, even with an input voltage of 30 V the circuit efficiency was around 80 %. If the circuit is used with a relatively low input voltage efficiency gains can be made by replacing D4 with a similar device with a lower reverse breakdown voltage rating, these devices tend to have a smaller for-ward voltage drop which reduces losses in the diode at high currents. At higher input voltage levels the value of resistor R1 can be increased proportionally to reduce the quiescent current even further.

Author : Michel Franke - Copyright : Elektor

Measuring Milliohms with a Multimeter

Low values of resistance can be troublesome especially when large current s f low through them. A current of, say, 10 A passing through a terminal with a contact resistance of 50 m? will produce a voltage difference of 0.5 V. This resulting power loss of five watts is dissipated in the termination and can give rise to a dangerously high temperature which may degrade insulation around the wires.

Circuit diagram :

Measuring Milliohms with a Multimeter-Circuit Diagram

Measuring Milliohms with a Multimeter Circuit Diagram

Measuring low values of resistance is not easy. Low cost multimeters do not include a milliohm measurement range and specialist equipment is expensive. The simple circuit described here allows milliohm measurements to be made safely on a standard ist equipment is expensive. The simple circuit described here allows milliohm measurements to be made safely on a standard multimeter. The circuit consists of little more than a 6 V voltage regulator and a mains adapter capable of supplying around 300 mA at 9 to 12 V.

The circuit supplies a fixed cur-rent output of 100 mA or 10 mA selected by switch S1. This connects either the 60 ? or 600 ? resistor into the constant current generator circuit. The resistor values are produced by paralleling two identical resistors; 120 ? and 1.2 k? from the E12 standard resistor range. Two test leads with probes are used to deliver current to the test resistance. The resultant voltage drop is measured by the multimeter (M1). With the test current set to100 mA a measurement of 1 mV indicates a resistance of 10 m?. At 10 mA (with S1 in the position shown in the diagram) a measurement of 1 mV indicates a resistance of 100 m? while 0.1 mV is equal to 1 m?. Diode D1 protects the meter from too high an input voltage.

With the voltmeter connected as shown in the diagram it measures not only the voltage drop across RX but also that produced by the resistance of the test leads, and probes. To make a true measurement, first touch the probes close together on the same lead of the test resistance and note the reading, now place the probes across the test resistance and note the reading again. The first reading measures just the test leads and probes while the second includes the resistance RX. Subtract the first measurement from the second to get the value of RX.

The accuracy of the measurements are influenced by the contact resistance of switch S1, the precision of resistors R1 to R4, the 6 V supply level and of course the accuracy of the measuring voltmeter. For optimum decoupling C1 should be fitted as close as possible to pin1 of IC1. An additional electrolytic capacitor of around 500 µF can be used at the input to the circuit if the input voltage from the AC power adapter exhibits excessive ripple.

Author : Klaus Bertholdt - Copyright : Elektor

Acoustic Distress Beacon

An ELT (Emergency Locator Transmitter, also known as a distress beacon) is an emergency radio transmitter that is activated either manually or automatically by a crash sensor to aid the detection and location of aircraft in distress. This acoustic ELT project is intended for radio control (RC) model aircraft, which every now and then decide to go their own way and disappear into the undergrowth.

Circuit diagram :

Acoustic Distress Beacon-Circuit Diagram

Acoustic Distress Beacon Circuit Diagram

The audio locating device described here enables model aircraft that have landed ‘off limits’ to be found again and employs its own independent power supply. The small cam-era battery shown in the circuit activates an acoustic sounder when radio contact is lost and produces a short signal tone (bleep) every ten seconds for more than 25 hours. Current consumption in standby and passive (with jumper J1 set) modes is negligible. The timing generator for the alarm tone is the Schmitt trigger AND-gate IC1.B; its asymmetric duty cycle drives a 5 V DC sounder via MOSFET transistor T1. All the time that the RC receiver output is delivering positive pulses, the oscillator is blocked by IC1.A and diode D1. Setting jumper J1 parallel to C2 also disables the oscillator and serves to ‘disarm’ the distress beacon.

Author : Werner Ludwig - Copyright : Elektor

Fuel Reserve Indicator For Vehicles

Here is a simple circuit for monitoring the fuel level in vehicles. It gives an audiovisual indication when the fuel level drops alarmingly below the reserve level, helping you to avoid running out of petrol on the way. Nowadays vehicles come with a dash-mounted fuel gauge meter that indicates the fuel levels on an analogue display. The ‘reserve’ level is indicated by a red marking in some vehicles, but the needle movement through the red marking may be confusing and not precise. This circuit monitors the fuel tank below the reserve level and warns through LED indicators and audible beeps when the danger level is approaching.

Circuit diagram :

Fuel Reserve Indicator For Vehicles Circuit diagram

Fuel Reserve Indicator For Vehicles Circuit Diagram

The fuel sensor system consists of a tank-mounted float sensor and a current meter (fuel meter), which are connected in series. The float-driven sensor attached to an internal rheostat offers high resistance when the tank is empty. When the tank is full, the resistance decreases, allowing more current to pass through the meter to give a higher reading. The fuel monitoring circuit works by sensing the voltage variation developed across the meter and activates the beeper when the fuel tank is almost empty. Its point A is connected to the input terminal of the fuel meter and point B is connected to the body of the vehicle. The circuit consists of an op-amp IC CA3140 (IC1), two 555 timer ICs (IC2 and IC3) and decade counter CD4017 (IC4).

Op-amp IC CA3140 is wired as a voltage comparator. Its inverting input (pin 2) receives a reference voltage controlled through VR1. The non-inverting input (pin 3) receives a variable voltage tapped from the input terminal of the fuel meter through resistor R1. When the voltage at pin 3 is higher han at pin 2, the output of IC1 goes high and the green LED (LED1) glows. This condition is maintained until the voltage at pin 3 drops below that at pin 2. When this happens, the output of IC1 swings from high to low, sending a low pulse to the trigger pin of the monostable (usually held high by R3) via C1. The monostable triggers and its output goes high for a predetermined time based on the values of R5 and C2. With the given values, the ‘on’ time will be around four minutes.

The output of IC2 is used to power the astable circuit consisting of timer 555 (IC3) via diode D2. Oscillations of IC3 are controlled by R6, R7, VR2 and C4. With the given values, the ‘on’ and ‘off’ time periods are 27 and 18 seconds, respectively. The pulses from IC3 are given to the clock input (pin 14) of decade counter CD4017 (IC4) and its outputs go high one by one. When the circuit is switched on, LED1 and LED2 glow if your vehicle has sufficient petrol in the tank.

When the fuel goes below the reserve level, the output of IC1 goes low, LED1 turns off and a negative triggering pulse is received at pin 2 of IC2. The output of IC2 goes high for around four minutes and during this time period, clock pin 14 of IC4 receives the clock pulse (low to high) from the output of IC3. For the first clock pulse, Q0 output of IC4 goes high and the green LED (LED2) glows for around 50 seconds. On receiving the second clock pulse, Q1 goes high to light up the yellow LED (LED3) and sound the buzzer for around 45 seconds. This audio-visual signal warns you that the vehicle is running out of fuel. On receiving the third clock pulse, LED3 and the buzzer go off. There is a gap of around two-and-a-half minutes before Q5 output goes high.

By the time Q5 goes high and the red LED (LED4) glows, four minutes elapse and the power supply to IC3 is cut off. The output state at Q5 will not change unless a low-to-high clock input is received at its pin 14. Thus LED4 will glow continuously along with the beep. The continuous glowing of the red LED (LED4) and the beep from the buzzer indicate that the vehicle will run out of fuel very shortly. Q6 output of IC4 is connected to its reset pin 15 via diode D3. This means that after ‘on’ state of Q5, the count will always start from Q0. Capacitor C5 provides power-on reset to IC4 when switch S1 is closed. The output of IC1 is also connected to reset pin of IC4 via diode D1 (1N4148). So when your vehicle is refueled above the reserve level, LED2 glows to indicate that the tank has sufficient fuel.

IC5 provides regulated 12V DC for proper functioning of the circuit even when the battery is charged to more than 12V. The circuit can be assembled on a perforated board. Adjust VR1 until the voltage at pin 2 of IC1 drops to 1.5V. When point A is connected to the fuel meter (fuel gauge) terminal that goes to the fuel sensor, green LEDs (LED1 and LED2) glow to indicate the normal fuel level. VR2 can be varied to set the ‘on’ time period of IC3 at around 20 seconds. Enclose the circuit in a small case and mount on the dashboard using adhesive tape. The circuit works only in vehicles with negative grounding of the body.

Author : D. Mohan Kumar - Copyright : EFY

Mini RS232 Data Switch

Only simple materials and a little bit of skill are needed to build an RS232 switch. All that you need are two 9-way sub-D plugs with solder pins, a small piece of sheet aluminium, two sets of screw retainer posts, a 4-pole double-throw switch, a strain relief sleeve and a suitable plastic connector shell for a 25-way sub-D connector, with both in-line and right-angle cable entries (such as Conrad Electronics #711322). What is important is that the side cable entry together with its associated strain relief leaves enough room for the switch. If necessary, you may have to cut away a few square millimetres of the sidewall or a few ribs of the plastic shell.


Project image :

 Mini RS232 Data Switch Image

Mini RS232 Data Switch Image


The switch is operated via the in-line cable opening, as can be seen from the photo. A suitable switch with an overall length of 29 mm can be found in the Conrad catalogue under order number 708232. The only modification that must be made to the connector shell is to drill two holes for the retaining screws for the switch (M2.6 screws) at a spacing of 24 mm.


Circuit diagram :

Mini RS232 Data Switch Circuit-Diagram

Mini RS232 Data Switch Circuit Diagram


Connect the two sub-D connectors together using the piece of aluminium and the screw retainer posts. Then solder the cable to the connectors and the switch as indicated. The two connectors are wired somewhat differently. While the upper sub-D plug is connected 1:1 with the input cable (with the switch in the appropriate position), the DCD, DTR, DSR and RI pins of the lower connector are left open. This is because RTS and CTS are fully sufficient for handshaking, as long as DTR and DSR are connected to each other. The only leads that are switched are RXD, RTS, TSD and CTS. The ground potential is fed from the cable to both connectors. After everything has been properly soldered together, you can fit everything into the cable shell as shown.

Author : H. J. Bohling - Copyright : Elektor

Computerised Universal Timer

This simple and flexible timer is more accurate than the real-time clock of the computer used for the purpose. It can be used in laboratories, dark rooms, kitchens, and for competitions in educational institutes. The program written in Q-Basic is self-explanatory. Generally, a universal timer provides the facility for switching on an electrical/electronic device after elapse of a certain time period, say, 5 minutes. The software does the same job here.


Circuit diagram :

Computerised Universal Timer-Circuit Diagram

Computerised Universal Timer Circuit Diagram


When the program is executed, it displays 0:0:0 on the monitor, indicating 0 hour, 0 minute, and 0 second. The display time 0:0:0 is increased by 10 seconds each time function key F1 of the computer keyboard is depressed. So by depressing function key F1 the required time is set for which the electrical or electronic device is to be switched on. However, in debate competitions the time allowed for a candidate to speak is filled the way it is discussed above. The program may be changed as indicated by REM statements and the single quote (‘) in the beginning of a program line may be accordingly removed in the program.


Now, after setting the time in the manner as discussed above, function key F2 of the computer keyboard is depressed to switch on any device. Simultaneously, the countdown of the time in the display box starts. The device will remain on until the display box shows 0:0:0 and then it will get switched off. The figure shows the relay interface circuit connected between D0 line (pin 2) and ground line (pin 25) of 378H output port of LPT1 printer port of the computer.


Author : D.K. Kaushik - Copyright : EFY

Going for Gold

The title refers to a popular TV game show where the contestants each have a big button.  The  game show  host  asks  a  question and the first contestant to press their but-ton makes an illuminated indicator light up on their desk. The other contestants’ buttons  are automatically inhibited, so that everyone can see who was the first contestant to press their button, and so is allowed to answer the question. The project described here shows how to build a similar sortof  refereeing device yourself, using simple resources and without needing a microcontroller, which is  pretty rare these days! The basic circuit is for  just two contestants, but the modular design  means it can easily be expanded.


Circuit diagram :

 Going for Gold-Circuit-Diagram

Going for Gold Circuit Diagram


The diagram shows three buttons: S2 and S3  are the buttons for the two contestants, S1 is  the button for the host, which allows them to  reset the circuit before each fresh question.  The ‘brains’ of the circuit is IC1, a 4013 dual D-type flip-flop, of which only the Set and Reset  inputs are used here. This circuit can handle  quite a wide supply voltage range, from 3 to  15 V, and so the project can easily be run off a 4.5 V battery pack (the power consumption is minimal).


IC1 is armed by pressing S1 (reset). In this  state, the non-inverting outputs (pins 1 and  13) are at 0 and the inverting outputs (pins 12  and 12) are at 1. Hence line A is pulled high  by R1, since diodes D2 and D4 are not biased  on. If contestant 1 presses button S2, the  non-inverting output of flip-flop IC1a goes  to logic 1, and LED D1 lights via T1 to indicate that contestant 1 has pressed the but-ton. At the same time, the flip-flop’s invert-ing output goes to logic 0, making diode  D2 conduct. Line A is now pulled down to 0,  and consequently contestant 2’s button S3  can no longer trigger the second flip-flop.  The reverse happens if it is contestant 2 who  presses their button S3 first.


The circuit can be extended to 4 or 6 contest-ants (or even more) by adding a second or  third (or more) 4013 IC. All you have to do is  repeat the circuit (minus R1, R2, and S1) and connect to the A, B, Vdd, and 0 V lines on the right-hand side.


Author : Joseph Kopff - Copyright : Elektor

Remote Control for Network Devices

Many devices connected to a local area net-work (LAN) are left on continuously, even  when they are not needed, including DSL  and cable modems, routers, wireless access  points, networked hard drives, printer servers and printers. The power consumption of  all these devices can add up to a considerable  fraction of one’s electricity bill. With the simple circuit described here we can ensure that  all these devices are only powered up when  at least one selected host device (such as a PC  or a streaming media client) is turned on. We insert a relay in the mains supply to the  devices whose power is to be switched, along  with a driver circuit controlled from the host  device over a two-wire bus. Optocouplers  provide galvanic isolation. One way to implement the bus is to use the spare pair of conductors that is often available in the existing  LAN cable.


The circuit diagram shows an example con-figuration where there are two controlling  host devices (a streaming media client and a  PC) and three network devices (a DSL router,  a networked hard drive and a networked  printer). We will assume that all the media  files are held on the networked hard drive.  The DSL router (to provide an internet connection) and the hard drive are to be powered up when either the PC or the media client is powered up; the printer only when the  PC is powered up.


Circuit diagram :

Remote Control for Network Devices-Circuit Diagram

Remote Control for Network Devices Circuit Diagram


We can think of the devices as being in two  groups, the first group consisting of the DSL  router and the hard drive, the second just the  printer. An optocoupler is powered from each  of the controlling host devices: these ensure  that the devices are isolated from one another  and from the rest of the circuit. The relay circuit, located close to the networked devices,  is controlled from the outputs of the optocouplers. The relay circuits are powered from  (efficient) mains adaptors: modified mobile  phone chargers do an admirable job.


In the circuit shown a 5 V supply from the  controlling devices is used to drive each optocoupler. Host 1 (the streaming client) drives  optocoupler IC1, host 2 (the PC) drives opto-couplers IC2 and IC3. Optocouplers IC1 and IC2 both control the  networked devices in group 1: networked  device 1 is the DSL router, switched by relay  RE1, and networked device 2 is the hard drive,  switched by relay RE2. Optocoupler  IC3  controls  the  networked  device in group 2, namely the printer. This is  switched by relay RE3.  The connections between the optocouplers  and the relay stages can be thought of as a  kind of bus for each group of devices. The  devices in a given group can be switched on  by simply shorting its bus, and this gives an  easy way to test the set-up. Resistors R2, R6  and R10 at the collectors of the transistors in  the optocouplers protect them in case power  should accidentally be applied to the bus.


The supply voltages V1 and V2 shown in the  example circuit diagram are derived from the  mains adaptors as mentioned above and are  used to power the relays. We have assumed  that the networked hard drive and the printer  are located near to one another, and so it is  possible to use a single mains adaptor to provide both voltages. Another possibility  would be to add a third wire to the bus to  carry power: this would allow all relays, wherever they were located, to be powered from  a single supply.  It is worth noting that network attached storage (NAS) devices such as networked hard  drives normally require an orderly shutdown  process before power is removed. Devices  that use Ximeta’s NDAS technology do not  suffer from this problem.


Author : Werner Rabl - Copyright : Elektor

Automatic TV Lighting Switch

The author is the happy owner of a television set with built-in Ambilight lighting in the living room. Unfortunately, the television set in  the bedroom lacks this feature. To make up for this, the author attached a small lamp to the wall to provide background lighting, This makes  watching television a good deal more enjoyable, but it ’s  not the ideal solution. Although the TV set can be  switched off with the remote  control, you still have to get out of bed to switch off the lamp.


Circuit diagram :

Automatic TV Lighting Switch-Circuit-Diagram


Automatic TV Lighting Switch Circuit Diagram


Consequently, the author devised this automatic lighting switch that switches the background light on and off along with the T V set. The entire circuit is fitted in series with the mains cable of the TV set, so there’s no need to tinker with the set. It works as follows: R1 senses  the current drawn by the TV  set. It has a maximum value  of 50 mA in standby mode,  rising  to around   500 m A  when  the  set  is  operating. The voltage across R1 is limited by D5 during negative  half- cycles  and  by  D1– D4  during positive half-cycles.  T he  voltage  across  these  four diodes charges capacitor C1 via D6 during positive  half-cycles. This voltage drives the internal LED of solid-state switch TRI1 via R2, which causes the internal triac to conduct and pass the mains voltage to the lamp.   Diode D7 is not absolutely necessary, but  it is recommended because the LED in the  solid-state switch is not especially robust  and cannot handle reverse polarisation. Fuse  F1 protects the solid-state switch against  overloads. T he  value  of  use d  here  (10 Ω)  for  resistor R1 works nicely with an 82-cm (32 inch)  LCD screen.


With smaller sets having lower  power consumption, the value of R1 can be  increased to 22 or 33 Ω, in which case you  should use a 3-watt type. Avoid using an  excessively high resistance, as otherwise TRI1 will switch on when the TV set is in standby mode.  Some TV sets have a half-wave rectifier in the  power supply, which places an unbalanced  load on the AC power outlet. If the set only  draws current on negative half-cycles, the cir-cuit won’t work properly. In countries with  reversible AC power plugs you can correct  the problem by simply reversing the plug. Compared with normal triacs, optically cou-pled solid-state relays have poor resistance  to high switch-on currents (inrush currents).


For this reason, you should be careful with  older-model TV sets with picture tubes (due  to demagnetisation circuits). If the relay fails,  it usually fails shorted, with the result that the TV background light remains on all the time. If you build this circuit on a piece of perf-board, you must remove all the copper next  to conductors and components carrying  mains voltage. Use PCB terminal blocks with a spacing of 7.5 mm. This way the separation between the connections on the solder  side will also be 3 mm. If you fit the entire  arrangement as a Class II device, all parts of  the circuit at mains potential must have a  separation of at least 6 mm from any metal  enclosure or electrically conductive exterior  parts that can be touched.


Author :Piet Germing - Copyright : Elektor

1-Watt LED Driver (PR4401)

The PR4401 chip from Prema can be used to drive an LED directly,  but not a high-power LED like one of the popular 1-watt types currently available on the market. The circuit shows that the drive signal at the Vout terminal of the PR4401 chip (pin 2) turns a medium-power PNP switching transistor (T1) on and off. When T1 is switched  into conduction, inductor L1 is charged. When T1 is switched off, the  inductor discharges its stored energy through the LED during flyback  with enough current to allow a one-watt LED to light up at nominal  brightness.


Circuit diagram :

PR4401 1-Watt LED Driver-Circuit-Diagram


PR4401 1-Watt LED Driver Circuit Diagram


During the ‘on’ time of transistor T1, the current through inductor L2 ramps up linearly to a peak value as expressed by. IL2(pk) = [(Vbatt – VCEsat(T1)) ×Ton ] / L2


Where VCEsat(T1) is the collector-to-emitter saturation voltage of T1 (here, a type BD140 is suggested).


During T1’s ‘off’ time, the inductor voltage reverses, forward-biasing  the LED and discharging through it at a constant voltage roughly  equal to the forward voltage of the LED, while its current ramps down  to zero. Because this cycle repeats at a high rate, the LED appears  to be always on, its brightness depending on the device’s average  current, which is proportional to the peak value. The LED current is  roughly a triangular pulse with a peak current approximately equal  to the inductor’s current because of the finite turn-off time of T1. The  estimated average current may be calculated from ILED(avg) = 1/2 × IL2peak × [Tdis  / (Ton + Toff)]


Where Tdis is the discharge time of inductor L2 through the LED. The  LED’s brightness can be increased or decreased by varying the inductance of L2. In practice, any value between 10 and 56 µH will work just  fine. The inductor current increases on each cycle until T1 goes out  of saturation, hence a small resistance (R1) is required at the base of  T1. Without the ‘stopper resistor’, the final current goes out of control  due to the DC gain of T1. A transistor with a high DC current gain and  low collector-to-emitter saturation voltage is the best choice if you  want to tweak the circuit for efficiency. Regarding L2, make sure the  peak current through it is below the saturation level.


Author :  T.A. Babu - Copyright : Elektor