LM338 Power Supply +13.8V 5A

This ac to dc power supply can output 5A in continous operation and 12A peak current. This kind of dc power supplies uses a PCB so you can use two case types for IC1, TO-220 or TO-3. The regulation of this 12 volt power supply is made with TR1 ( multiturn ). IC1 must be placed on proper heatsink.

LM338 Power Supply Circuit Diagram :

 13.8V 5A power sp-Circuit Diagram

A Doorbell for the Deaf

This circuit provides a delayed visual indication when a door bell switch is pressed. In addition, a DPDT switch can be moved from within the house which will light a lamp in the door bell switch. The lamp can illuminate the words "Please Wait" for anyone with walking difficulties.

A Doorbell for the Deaf Circuit Diagram :

A Doorbell for the Deaf-Circuit Diagram

Notes :

The circuit uses standard 2 wire doorbell cable or loudspeaker wire. In parallel with the doorbell switch, S1, is a 1N4001 diode and a 12 volt 60mA bulb. The bulb is optional, it may be useful for anyone who is slow to answer the door, all you need to do is flick a switch inside the house, and the bulb will illuminate a label saying Please Wait inside the doorbell switch or close to it. The double pole double throw switch sends the doorbell supply to the lamp, the 22 ohm resistor is there to reduce current flow, should the doorbell switch, S1 be pressed while the lamp is on. The resistor needs to be rated 10 watts, the 0.5 Amp fuse protects against short circuits.

When S2 is in the up position (shown as brown contacts), this will illuminate the remote doorbell lamp. When down, (blue contacts) this is the normal position and will illuminate the lamp inside the house. Switch S1 will then charge the 47u capacitor and operate the transistor which lights the lamp. As a door bell switch is only pressed momentarily, then the charge on the capacitor decays slowly, resulting in the lamp being left on for several seconds. If a longer period is needed then the capacitor may be increased in value.

9V Automatic Battery NiCd Charger

This automatic NiCd charger for 9V NiCd batteries is using 555 timer properties and is very easy to build. Why is an automatic 9 volts NiCd battery charger? Because you can leave the battery for charging as much as you like: it will be always completely charged and ready for use when is needed. It wont be overcharged and it will not discharge.

9V Automatic Battery NiCd Charger Circuit Diagram :


With the values presented in the circuit diagram, the battery charger NiCd circuit is suitable for 6V and 9V batteries. 9 volt types with 6 and 7 cells are charging with 20mA; P1 must be adjusted so that the NiCd charger disconnects after 14 hours. Window inferior level is set at 1V below this value with P2.

5V battery type with 4 or 5 cells are charged at 55mA. Again, with P1 adjust the NiCd charger circuit so it disconnects after 14 hours. Window inferior level must be set at 0.8V below this value.

USB Standby Killer

When turning a computer on and off, various peripherals (such as printers, screen, scanner, etc.) often have to be turned on and off as well. By using the 5-V supply voltage from the USB interface on the PC, all these peripherals can easily be switched on and off at the same time as the PC. This principle can also be used with other appliances that have a USB interface (such as modern TVs and radios).

USB Standby Killer Circuit Diagram :

USB-Standby-Killer-Circuit Diagram

This so-called ‘USB-standby-killer’ can be realised with just 5 components.

The USB output voltage provides for the activation of the triac-opto driver (MOC3043) which has zero-crossing detection. This, in turn, drives the TRIAC, type BT126.

The circuit shown is used by the author for switching loads with a total power of about 150 W and is protected with a 1-A fuse. The circuit can easily handle much larger loads however. In that case and/or when using a very inductive load a so-called snub-ber network is required across the triac. The value of the fuse will also need to be changed as appropriate.

The circuit can easily be built into a mains multi-way power board. Make sure you have good isolation between the USB and mains sections (refer to the Electrical Safety page published regularly in this magazine).

Author : Wim Abuys - Copyright : Elektor

Ultra Low Drop Linear Regulator

The circuit is a MOSFET based linear voltage regulator with a voltage drop of as low as 60 mV at 1 ampere. Drop of a fewer millivolts is possible with better MOSFETs having lower  RDS(on)  resistance.

The circuit in Fig.1 uses 15V-0-15V secondary output from a step-down transformer and employs an n-channel MOSFET IRF540 to get the regulated 12V output from DC input, which could be as low as 12.06V. The gate drive voltage required for the MOSFET is generated using a voltage doubler circuit consisting of diodes D1 and D2 and capacitors C1 and C4. To turn the MOSFET fully on, the gate terminal should be around 10V above the source terminal which is connected to the output here. The voltage doubler feeds this voltage to the gate through resistor R1. Adjustable shunt regulator TL431 (IC2) is used here as an error amplifier, and it dynamically adjusts the gate voltage to maintain the regulation at the output.

Ultra Low Drop Linear Regulator Circuit Fig-1 Circuit Fig.1

With adequate heatsink for the MOSFET, the circuit can provide up to 3A output at slightly elevated minimum voltage drop. Trimpot VR1 in the circuit is used for fine adjustment of the output voltage. Combination of capacitor C5 and resistor R2 provides error-amplifier compensation.

The circuit is provided with a short-circuit crow-bar protection to guard the components against over stress during accidental short at the output. This crow-bar protection will work as follows: Under normal working conditions, the voltage across capacitor C3 will be 6.3V and diode D5 will be in the off state since it will be reverse-biased with the output voltage of 12V. However, during output short-circuit condition, the output will momentarily drop, causing D5 to conduct and the opto-triac MOC3011 (IC1) will get triggered, pulling down the gate voltage to ground, and thus limiting the output current. The circuit will remain latched in this state, and input voltage has to be switched off to reset the circuit.

Ultra Low Drop Linear Regulator fig-2 Circuit Fig.2

The circuit shown in Fig.2 follows a similar scheme. It can be utilised when the regulator has to work from a DC rail in place of 15V-0-15V AC supply. The gate voltage here is generated using an LM555 charge pump circuit as follows:

When 555 output is low, capacitor C2 will get charged through diode D1 to the input voltage. In the next half cycle, when the 555 output goes high, capacitor C3 will get charged to almost double the input voltage. The rest of the circuit works in a similar fashion as the circuit of Fig. 1.

The above circuits will help reduce power-loss by allowing to keep input voltage range to the regulator low during initial design or even in existing circuits. This will keep the output regulated with relatively low input voltage compared to the conventional regulators.

The minimum voltage drop can be further reduced using low RDS(on) MOSFETs or by paralleling them.

Author :P.S .SINI - Copyright : EFY

Oil Temperature Gauge for 125 cc Scooter

Lots of Far-Eastern scooters are fitted with GY6 engines. These already elderly units are sturdy and economical, but if you want to  “push” the power a bit (so called ‘Racing’  kits, better handling of the advance, etc.), you soon find yourself faced with the problem  of the engine temperature, and it becomes essential to f it a heat sink (of ten wrongly  referred to as a ‘radiator’) on the oil circuit. Even so, in these circumstances, it’s more than reassuring for the user to have a constant clear indication of the oil temperature. Here are the specifications we set for the temperature gauge we wanted to build:

Oil Temperature Gauge Circuit Diagram :

Oil Temperature Gauge-Circuit Diagram

  • no moving parts (so not meter movement), as scooters vibrate a lot!;
  • as cheap as possible (around £12);
  • robust measuring transducer (avoid NTC thermistors and other ‘exotic’ sensors);
  • temperature range 50–140 °C. (122 – 291 °F);
  • audible and visual warning in case of dangerous temperature;
  • compact;
  • waterproof.

Let’s start by the sensor. This is a type-K thermocouple, as regularly used by multimeter manufacturers. Readily available and fairly cheap, these are robust and have excellent linearity over the measurement range we’re interested in here. The range extends from 2 mV to 5.7 mV for ten measurement points. The positive output from the thermocouple is applied to the non-inverting input of IC3.A,  wired as a non-inverting amplifier. Its gain  of 221 is determined by R1 and R2. IC3 is an LM358, chosen for its favourable characteristics when run from a single-rail supply. IC3.B is wired as a follower, just to avoid leaving it powered with its pins floating.

IC3.B output is connected to pin 5 of IC1, an LM3914. This very common IC is an LED display driver. We can choose ‘point’ or ‘bar’ mode operation, according to how pin 9 is connected. Connected as here to the + rail, the display will be in ‘bar’ mode. Pin 8, connected to ground, sets the full scale to 1.25 V. R3 sets the average LED current. Pin 4, via the potential divider R7/R8+R9, sets the offset  to 0.35 V. Using R8 and R9 in series like this avoids the need for precision resistors.

As per the LM3914 application sheet , R4-R5-R6 and C5 will make the whole display flash as soon as D10 lights (130 °C = 226 °F). Simultaneously, via R10 and T1, the (active) sounder will warn the user of overheating. Capacitor C6 avoids undesirable variations in the reference voltage in ‘flashing’ mode. IC2 is a conventional 7808 regulator and C1– C4 filter the supply rails. Do not leave these out! D1 protects the circuit against reverse polarity.

The author has designed two PCBs to be fit-ted as a ‘sandwich’ (CAD file downloadable  from [1]). In the download you’ll also find  a document with a few photos of the project. You’ll note the ultimate weapon in on-board electronics: hot-melt glue. Better than epoxy (undoable!) and quite effective against vibration.

Author : Georges Treels - Copyright : Elektor

Digital Switching System

This circuit can control any one out of 16 devices with the help of two push-to-on switches. An up/down counter acts as a master-controller for the system. A visual indication in the form of LEDs is also available.  IC1 (74LS193) is a presettable up/ down counter. IC2 and IC3 (74LS154) (1of 16 decoder/demultiplexer) perform different functions, i.e. IC2 is used to indicate the channel number while IC3 switches on the selected channel.

Digital Switching System Circuit Diagram :

Digital Switching System-Circuit Diagram

Before using the circuit, press switch S1 to reset the circuit. Now the circuit is ready to receive the input clock. By pressing pressing switch S2 once, the counter advances by one count. Thus, each pressing of switch S2 enables the counter to advance by one count. Likewise, by pressing switch S3 the counter counts downwards.

The counter provides BCD output. This BCD output is used as address input for IC2 and IC3 to switch one (desired channel) out of sixteen channels by turning on the appropriate triac and the corresponding LED to indicate the selected channel.  The outputs of IC3 are passed through inverter gates (IC4 through IC6) because IC3 provides negative going pulses while for driving the triacs we need positive-going pulses. The high output of inverter gates turn on the npn transistors to drive the triacs. Diodes connected in series with triac gates serve to provide unidirectional current for the gate-drive.

Author : Rajesh K.P - Copyright : EFY

A Simple Detector with Amplification

A simple shortwave radio detector is neither very sensitive nor very selective. However, with a little extra amplification we can improve the reception performance significantly.

The additional circuit is designed to compensate for the losses in the resonant circuit. A transistor is used to amplify the RF signal and feed it back into the resonant circuit. When the gain is set correctly we can make the amount of this feedback exactly equal to the losses. The resonant circuit is then critically damped and has a very high Qfactor. Now we can separate transmissions that are just 10 kHz apart, and we can tune in to very weak stations.

Detector with Amplification Circuit Diagram :

Detector with Amplification-Circuit Diagram

The tuning capacitor used has two gangs of vanes with capacitances of 240 pF and 80 pF. These two gangs are connected in parallel to make a 320 pF variable capacitance. The air-cored inductor has 25 turns on a diameter of 10 mm, with taps at 5-turn intervals. The resonant circuit so formed is capable of covering the full shortwave  band from 5 MHz to 25 MHz.

The short wave detector can be connected to a power amplifier, or, for exam-ple, amplified PC loudspeakers. The antenna does not have to  be very long: in experiments we used a one metre length of wire. Tuning the radio involves adjusting the variable capacitor to bring in the station and then adjusting the gain of the feed-back circuit for optimal output volume. If the potentiometer is turned up too far, the receiver will go into self-oscillation and become a mini-transmitter. At  the optimal setting the sound  quality is very pleasant and certainly no worse than many ordinary shortwave radios.

If you find shortwave detectors that use a battery and an amplifier a little new-fangled, you can get your fix of nostalgia by dispensing with the battery and connecting a crystal earpiece to the detector’s output. The radio will of course also work without the feedback circuit, but with rather poorer performance.

Author :Burkhard Kainka - Copyright : Elektor

10W Stereo Audio Amplifier Using TDA2009A

This is a schematic of a 10W stereo audio amplifier using TDA2009A amplifier IC. TDA2009A is a good IC provides quality sound. It has built in features like output current protection and thermal protection etc. The circuit can be operate between 8 to 24V DC with 1 to 2 amphere.

10W Stereo Audio Amplifier Circuit Diagram :


If you want to operate this 10 watt amplifier circuit with watt amplifier circuit with mains supply then use a filtered and stable power supply to reduce mains hum. 10 watt out put power can be obtained by providing 20V 1.5A to the circuit. Use good and thick heatsink with the IC.

A Headphone Monitoring Switch

In any recording situation, monitoring is critical to make sure you're getting what you want on tape. This is just as true in field recording, but in most cases, one's monitoring options are severely limited--stereo headphone is the only choice.

Headphone Monitoring Switch  :

A headphone monitoring Switch-Circuit Daigram

Since I often use dual-mono mics, hearing a stereo feed of the two is not always convenient. I wanted the option to hear JUST the left mic in BOTH ears, or just the right mic in both ears, as well as a normal stereo signal. This is simple enough to do with a big rotary switch. When completed, you can create a little box that your headphones plug into, which in turn is plugged into the stereo phone output of your deck. Then, by turning the knob on the switch box, you can hear normal stereo, left-only mono, right-only mono, left+right mono and even left-right reversed stereo (or normal stereo again).

Note the use of summing resistors in the left+right mono section. This was an attempt to prevent the two outputs from "fighting" each other if there were very different voltages in left and right outputs. I used 8 ohm resistors here, but a higher value might be better. Maybe ~20 ohms? Also, I initially decided to put normal stereo on both ends of the switch's travel so I'd always be able to find it without looking. However, I sometimes wish to have left-right reversed. If you'd like to try this, simply swap the leads on one of the "normal stereo" connections.

One final caveat: The left only/right-only mono positions are -6dB down, since only one half of the deck's headphone amp is driving your phones when the switch is in those positions.

Simple USB AVR-ISP Compatible Programmer

Modern PCs rarely have a serial or parallel  port any more, to the great regret of any-one who experiments with microcontrollers every now and then. In the old days  it was very simple to use the parallel port  of a standard PC and program just about  any type of AVR microcontroller with it.  When you want to do that now, you’re first  obliged to buy a programmer that communicates with the PC via USB, which immediately raises the threshold of getting started  with these microcontrollers. The circuit presented here offers a solution to this.

Simple USB AVR-ISP Compatible Programmer Circuit Diagram :

Simple USB AVR-ISP Compatible-Programmer-Circuit Diagram

As you can see from the schematic, this is  a very simple circuit, built around a cheap,  standard AVR microcontroller plus a handful of passive components. You may have  already observed that this microcontroller does not have a USB interface and the  circuit does not use a USB to serial converter either. The strength of this circuit is  found in the firmware. The USB interface  has been implemented in software, as we  have shown in an earlier article ‘AVR drives  USB’ in the March 2007 issue. The firmware  ensures that the circuit is recognised by the  PC as a serial port and communicates with  AVR Studio, the standard Atmel development environment, as if it were a ‘real’ AVR-ISP programmer.

The circuit is easily built on a small piece  of prototyping board or even on a bread board, since the controller is available in  a DIP-28 package. If you are going to pro-gram the controller yourself (via connector K2) then make sure that you set the  configuration fuses so that the internal  oscillator uses the external crystal as the  clock source.

Jumper K3 is provided in the event you would like to power the circuit to be programmed from the USB port. We do not  recommend that you do this, however,  but sometimes there is no other option.  K4 is a 10-way box header which has the  same standard pinout that Atmel uses  everywhere.

Author : Nand Eeckhout - Copyright : Elektor

Automotive Ignition Coil Buzz Box


Automotive Ignition Coil Buzz Box Circuit Diagram :

Ignition-Coil-Buzz-Box-Circuit diagram


This picture is a circuit for a buzz coil using a standard car battery to create. Dual timer IC 556 is used to set the frequency and the duty cycle of the coil current to be determined. One of the timer is used as an oscillator for generating the rectangular waveform 200 Hz to control (IRF740 MOSFET), while the second timer is stopped and the oscillator switching points are opened and closed (closed = a). The result is a steady stream of sparks of the ignition coil a distance of about 5 milliseconds, while the switching points are closed. Operation: Pin 8 and 12 the trigger inputs, and a timer which are driven by the points and an inverted signal of the clock output (pin 9) to produce.

When the pin 9 is grounded points high, and vice versa. The signal on pin 9 controls the reset line (pin 4) of the second timer and keeps the output at pin 5 is low, while pin 4 and pin 8 is low and 12 high (still open). The 15K and 47K resistors and capacitors are 0.33uF synchronization components that the frequency and duty cycle of the second clock, which is about 4 milliseconds to 2 milliseconds apart to secure positive and negative. During the time interval is positive, the doors are always high MOSFET causing the coil to the current height of about 4 amps.

This equates to approximately 80 milli joules of energy in the coil is released in the spark plug when the clock output (pin 5) moves on the ground, turn off the MOSFET. A zener diode 12 volts is placed on the node 10 and 27 ohms for the MOSFET gate input is above or below 12 volts -0.7 volts. A Zener diode 200 volts / 5 W used for the drain voltage of the MOSFET 200 and limit the useful life of the spark to expand. The circuit must operate reliably with a jumper, but the circuit operation with no load applied (the son of candle down, etc.) may cause a malfunction, because most of the energy absorbed by the Zener. You can also use a transient voltage suppressor (TVS) as 1.5KE300A 1.5KE200A or instead of the zener. This is probably a good hand, but difficult to obtain.

Auto Focus for Slide Projector

This circuit is intended as a replacement for the electronics in a partly or wholly defective autofocus driver in a slide projector. The mechanical parts in the autofocus system are assumed to be still functional.

Auto Focus for Slide Projector Fig. 1 :

 Auto Focus for Slide Projector-Fig-1 

Most automatic focusing systems in slide projectors  are based on the use of an optical module, which comprises a small lamp, a few lenses and mirrors, and a light sensor made from two series connected light dependent resistors (LDRs), which function as a potential divider. As shown in Fig. 1, lamp La projects a narrow beam onto the centre of the slide, A, whose surface reflects it onto the LDRs. When the slide surface bulges inside or outside, the projected image on the screen is blurred, and the beam from L is received on the surface of one of the LDRs (point 2 or 3). This is detected by a motor driver circuit, which ensures that the focal distance between the objective, 0, and the slide surface is corrected to maintain a sharp image, i.e., the objective is moved until the circuit detects that the reflected beam from L falls exactly in between the LDRs (point 1).

Auto Focus for Slide Projector Fig. 2 :

Auto Focus for Slide Projector-Fig-2

The circuit is based on the use of an existing set of  LDRs as part of the optical module in the slide projector. The symmetrical supply shown to the left,  and the motor plus decoupling capacitor, are also part of the projector. The inverting input of opamp IC1  is at ground potential when the above mentioned test beam falls in between the LDRs. The output of the opamp keeps the non-inverting input at 0 V as well, so that no motor voltage is available  at the emitters of power drivers Ti T2. Should the  reflected beam illuminate either one of the LDRs, the circuit arranges for the motor to move the objective glass towards the correct focal position, until no  voltage difference between the LDRs is detected.  The feedback gain of the circuit has been kept relatively low to keep the motor from continuously moving the objective glass past the target position, causing the system to oscillate slowly. Resistors R3 and R4 may have to be dimensioned differently than shown to achieve optimum response as regards speed and stability.

200w Audio Power Amplifier

This audio amplifier circuit delivers up to 200 W of top-class quality for loudspeaker from 4 to 16 ohm. Operating voltage is between 24 and 36 V, max 5 A. The frequency response is from 20 to 20000 Hz.  Please take special care that the transistors and the IC’s have been fixed firmly and solely one or two separated cooling elements with sufficient dimensions for this purpose (thermal resistance < 1K/W).

200W Audio Power Amplifier Circuit Diagram :

200w-amplifier-schematic-Circuit diagram

Doing so it is necessary to mount the transistors and the IC’s insulated (with mica washes and plastic nipple). Please make sure before first operation that the transistors and the IC’s really do not have any electrical connection towards the cooling plate! The transistors have to be placed plane and firmly onto the cooling element! It is of extraordinary importance with this high-power amplifier that there is a considerable heat dissipation. The already mounted cooling element should be situated in a well ventilated case.

The PSU should be sufficiently powerful, power consumption of the amplifier may increase up to 5A. In case of using an unstabilised power supply. It is advisable to place a transformer of max 28V.

The amplifier will the show approx. 120W at a 4-Ohm loudspeaker, for it no-load voltage of the power supply will not be to high. If it is desired to use complete power, it is necessary to place a stabilised power supply with approx. 36V 5A. No-load voltage should not pass over 44V!

The cables leading the current supply and to the loudspeakers should have at least a cross section of min. 1.5 mm^2. The connected loudspeaker have to be equiped according to the high output power and should not have a lower impedance as 4 Ohm! With lower connection impedance and short circuit within the loudspeaker wiring, the transistors will be destructed.

The amplifier has an input sensitivity of approx. 500 … 800 mV. Therefore, it is possibile to connect directly at the amplifier tape decks, tuners, etc. In case there are connected signal sources with lower output voltage, it is necessary to pre-connect a preamplifier. Then it will alse be posible to connect microphones, etc.

200W audio amplifier PCB :


Parts List :

IC1, IC2 = 2 IC’s TDA2030
T1, T3 = 2 transistors KT818 or BD708
T2, T4 = 2 transistors KT819 or BD705
C1, C2, C3, C4, C7 = 5 capacitors 150 nF
C5 = 1 elca 10uF 63V
C8 – 1 capacitor 1.8 nF
R1, R7, R9 = 3 resistances 100K
R2, R3, R10, R11 = 4 resistances 2.2 Ohm
R4, R5 = 2 res. 2K
R6, R8 = 2 res. 1 Ohm
R12, R13 = 2 res. 2 res. 3.3K
D1…D4 = 1N4001, 1N4002, 1N4003
1 PCB board approx 56×51 mm

Car's Brake Lights Monitor Circuit

The circuit described below monitors your car's brake lights, and indicates by a light emiting diode 12V whether they both function correctly. In that sense, it can save you money by preventing your being fined for driving with defective brake lights, and it also leads to increasing road safety.

Car's Brake Lights Monitor Circuit Diagram :

Car's Brake Lights Monitor-Circuit Diagram

The monitor depends inevitably on the voltage drop across the supply lines to the two lamps. For the circuit to work correctly, that drop needs to be greater  than 0.6 V. If this is not so, the drop must be in- creased by adding a 5 V diode in series with each  lamp. Transistor Ti and T2 in figure 1 form a  Schmitt trigger, which reacts to the voltage drop  across the supply lines to the two brake lights. This  reaction manifests itself in Di lighting via T3. If  one of the brake lights is faulty, the switch-on cur- rent drawn by the other lamp will cause Di to light  briefly when the brake pedal is pressed. If both  brake lights are defective, Di will not light at all. All three possible states of the brake lights are thus indicated. sitivity of the circuit, can be adjusted within narrow limits with Pi. The preset is best adjusted with one lamp out of action in a manner which makes Di light briefly as described above.

If you find it disturbing that Di lights every time you brake, the operation can be reversed by replacing the BC557B in the T3 position by a BC547B  (n-p-n). The collector of T3 is then connected to the  positive supply line, and the emitter to R6. On the  printed circuit board this means that the flat edge  of T3 must be turned the other way. A second base  connection has also been provided on the PCB.

Note, however, that this configuration no longer makes it possible to ascertain whether one or both brake lights are faulty, i.e., when the LED lights, one or both lamps need replacing.

Alternating On-Off Switch with IC 4069

Get the circuit instead of a standard on-off switch. Switching is very gentle. If  we don’t use the PCB, connect unused input pins to an appropriate logic level (‘+’ or ‘-’).  Unused output pins *NEED* be left open!  On the Print Circuit Board this has completed already . One step ’push’ activates the relay, another ‘push’ de-activates the relay. IC1 (the 4069) is a regular Hex-inverter type and is constructed with MOS P-channel and N-channel enhancement mode devices in a single monolithic structure.

Alternating On-Off Switch Circuit Diagram :

Alternating-ON-OFF-Switch Circuit Diagram

Parts List :

R1 = 10K
R2 = 100K
R3 = 10K
R4 = 220 Ohm (optional)
C1 = 0.1µF, Ceramic (100nF)
C2 = 1µF/16V, Electrolytic
D1 = 1N4001
Led1 = Led, 3mm, red (optional)
Q1 = 2N4401 (see text) IC1 = 4069, CMOS, Hex Inverter (MC14069UB), or equivalent
S1 = Momentary on-switch
Ry1 = Relay )

It is going to operate on voltages from 3 to 18 volts, but most applications are in the 5-15 volts.  Although the IC1 4069 contains protection circuitry against damage from ESD , use common sense when handling this device.  Depending on your application you may want to use an IC-socket with IC1.  It makes replacement easy if the IC ever fails.  The IC is CMOS so watch for static discharge!  You can use any type of 1/4 watt resistors including the metal-film type.

The type for D1 in not critical, even a 1N4148 will work.  But, depending on your application I would suggest a 1N4001 as a minimum if your relay type is 0.5A or more.  Any one in the 1N400x series diodes will work.  Any proper replacement for Q1 will work, including the european TUN’s.  Since Q1 is just a driver to switch the relay coil, almost any type for the transistor will do.  PN100, NTE123AP, BC547, 2N3904, 2N2222, 2N4013, etc. will all work for the relays mentioned here.  For heavier relays you may need to change Q1 for the appropriate type.

For C2, if you find the relay acts not fast enough, you can change it to a lower value. It is there as a spark-arrestor together with diode D1. For the relay I used an 8 volt type with the above circuit and a 9 volt battery.  Depending on your application, if the current-draw is little, you can use a cheap 5V reed-relay type.  Use a 8V or 9V relay type if your supply voltage is 12V.  Or re-calculate resistor R3 for a higher value.

The circuit and 9V will work fine and will pull the relay between 7 and 9 volt, the only thing to watch for is the working voltage of C2; increase that to 50V if you use a 12V supply.  The pcb was designed for an Aromat/Omron  relay, 12V/5A, #HB1-DC12V.  You can easily re-design the relay pads on the PCB for the relay of your choice.  If you wish to use something you already have, and you don’t want to re-design the PCB, you can glue the relay up-side-down on the pcb and wire the relay contacts manually to the pcb-holes or directly to your application. Use a 2N2222 transistor for Q1 if your supply voltage is higher than 9V and/or your relay is heavy duty, or doesn’t want to pull-in for any other reason.

Again, the pcb drawing is not to scale.  Use ‘page-setup’ to put the scale to 103% for a single pcb, vertically, and your scale should be correct.  I use a laser printer and so I don’t know if this scale of 103% is for all printers.  To check, print a copy onto regular paper and see if the IC pins fit the print.  If so, your copy is correct.  If not, change the scale up of down until a hardcopy fits the IC perfectly.

The Led is nice for a visual circuit indication of being ‘on’.  For use with 12V supply try making make R4 about 330 ohms.  The LED and R4 are of course optional and can be omitted.  Your application may already have some sort of indicator and so the LED and R4 are not needed.

L200 Charger Circuit

This circuit came about as the result of an  urgent need for a NiMH battery charger. No  suitable dedicated IC being immediately to  hand, the author pressed an L200 regulator and a 4.7 kΩ NTC thermistor into service.  Those components were enough to form the  basis of a charger with a cut-of f condition  based on cell temperature rise rather than  relying on the more common negative delta-V detection.

L200 Charger Circuit Diagram :

L200-Charger-Circuit Diagram

The circuit uses the L200 with the thermistor in the feedback loop. When ‘cold’ the  output volt age of the regulator is about 1.55 V per cell; when ‘warm’, at a cell temperature of about 35 °C to 40 °C, the out-put voltage is about 1.45 V per cell and the  thermistor has a resistance of about 3.3 kΩ.  This temperature sensing is enough to pre-vent the cells from being overcharged. P1  adjusts the charging voltage, and R2 limits  the charge current to 320 mA. The IC is fitted with a small 20 K/W heatsink as it dissipates around 1.2 watts in use.

The charger circuit can be connected permanently to the battery pa ck . Charging  starts when a ‘ wall wart ’ adaptor is connected to the input of the charger. The unregulated 12 V supply used by the author  delivered an open- circuit voltage of 18 V,  dropping to 14 V under load. Even though  the charge voltage is reduced when charging is complete, the cells should not be left  permanently on charge.

The author uses the circuit to charge the battery in a torch. After three years and some 150  charge cycles the cells are showing no signs of losing any capacity.

Author : Wolfgang Driehaus - Copyright : Elektor

Simple Car-Reversing Horn with Flasher

Here is a simple circuit that starts playing the car horn whenever your car is in reverse gear. The circuit (refer Fig. 1) employs dual timer NE556 to generate the sound. One of the timers is wired as an astable multivibrator to generate the tone and the other is wired as a monostable multivibrator.

Circuit diagram :

 Simple Car-Reversing Horn with Flasher- Circuit Daigram

Fig. 1: Car reverse horn Circuit Diagram

Working of the circuit is simple. When the car is in reverse gear, reverse-gear switch S1 of the car gets shorted and the monostable timer triggers to give a high output. As a result, the junction of diodes D1 and D2 goes high for a few seconds depending on the time period developed through resistor R4 and capacitor C4. At this point, the astable multivibrator is enabled to start oscillating. The output of the astable multivibrator is fed to the speaker through capacitor C6. The speaker, in turn, produces sound until the output of the monostable is high.

When the junction of diodes D1 and D2 is low, the astable multivibrator is disabled to stop oscillating. The output of the astable multivibrator is fed to the speaker through capacitor C6. The speaker, in turn, does not produce sound.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Connect the circuit to the car reverse switch through two wires such that S1 shorts when the car gear is reversed and is open otherwise. To power the circuit, use the car battery.

The flasher circuit (shown in Fig. 2) is built around timer NE555, which is wired as an astable multivibrator that outputs square wave at its pin 3. A 10W auto bulb is used for flasher. The flashing rate of the bulb is decided by preset VR1.

Flasher-circuit diagram

Fig. 2: Flasher Circuit Diagram

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. The flasher bulb can be mounted at the car's rear side in a reflector or a narrow painted suitable enclosure.

EFY note. A higher-wattage bulb may reduce the intensity of the headlight. You can enclose both the car-reversing horn and flasher circuits together or separately in a cabinet in your car.

Author :

Ashok K. Doctor - Copyright : EFY

Audio Processor Circuit using IC SSM2045

This audio processor circuit features the SSM2045 IC which was developed specially for electronic music applications and the 741 opamp IC. The circuit is configured as a low pass filter with a DC voltage control for gain. The input signal is set to a working level of 150mVpp through the resistor R1.

Audio Processor Circuit Diagram :

Audio-Processor Circuit Diagram

The filter has 2 buffered outputs: the 2-pole output at pin 1 and 4-pole output at pin 8. Internally, the outputs are connected to 2 voltage-controlled-amplifiers (VCA). The R15 and R16 are connected to these outputs to achieve optimum offset and control voltage suppression. P4 is the volume control. The current that flows to the pins 15 and 16 should not go beyond the maximum of 250 µA. The balance of the two VCAs and the entire filter is being controlled be a voltage range of -250 mV to + 250 mV at pin 14. This voltage can be set by P2.

The input can be driven with source impedances up to a maximum of 200 Ω. With an input level of 0dBm, the VCA weakens by 6 dB. The bias current needed at pin 17 is between 120 µA and 185 µA. The cutoff frequency can be shifted between 20 Hz and 20 kHz with a variable voltage at pin 5. This can be varied through P1. The capacitor values were selected to give the filter its Butterworth characteristics.

The output current of the SSM2045 IC is converted to a voltage output by the 741 opamp. Any sybsequent circuit must be DC decoupled from IC2. The noise-voltage ration is about 80 dB.

Simple Voltage Booster

Here is a simple circuit for boosting 12 V DC to 24 V DC .The circuit is designed straight forward and uses few components.With few modifications the circuit can be used to boost any voltages.

The transistor Q1 and Q2 (D1616)  essentially drives the primary of the transformer.The diodes rectifies the output of transformer to obtain a 24V DC at the output load(here a fan).The capacitors filter away noise and harmonics away from the output.

Simple Voltage Booster Circuit Diagram :



  • The component values are not very specific here.We can use any NPN power transistors like D1616,2N 3055,C2236,SL 100 etc for Q1 and Q2.
  • The transformer can be any center tapped 5A transformer with a  7:1 winding ratio.
  • The diodes can be 1N 914 ones.
  • In fact you can easily assemble the circuit from the components in your electronics junk box.
  • By experimenting on the tranformer winding you can get different boost ratios.
  • For high current (around 5A)  games use 2N 3055 transistor or more powerful Darlington pairs for Q1 and Q2.


Source : circuitstoday.com/voltage-booster-circuit

Simple Garage Stop Light Circuit

A novel use of solar cells makes positioning your car in the garage rather easier than old tyres, a mirror, or a chalk mark. The six solar cells in figure 1 serve as power supply and as proximity sensor. They are commercially available at relative low cost. The voltage developed across potentiometer Pi is mainly dependent on the intensity of the light falling onto the cells. The circuit is only actuated when the main beam of one of the car's headlights shines direct onto the cells from a distance of about 200 mm (8 inches). The distance can be varied somewhat with P,

Simple Garage Stop Light Circuit Diagram :

Simple-Garage-Stop-Light-Circuit Diagram


Under those conditions, the voltage developed across C1 is about 3 V, which is sufficient to trigger relaxation oscillator Ni. The BC547B is then switched on via buffer N2 so that D3 begins to  lfash. Diodes Di and D2 provide an additional in- crease in the threshold of the circuit. The total voltage drop of 1.2 V across them ensures that the  potential at pin I of the 4093 is always 1.2 V below the voltage developed by the solar cells. As the trip  level of Ni lies at about 50 per cent of the supply  voltage, the oscillator will only start when the supply voltage is higher than 2.4 V.

The circuit, including the solar cells, is best constructed on a small veroboard as shown in figure 3, and then fitted in a translucent or transparent man- made fibre case. The case is fitted onto the garage wall in a position where one of the car's headlights shines direct onto it. The LED is fitted onto the same wall, but a little higher so that it is in easy view of the driver of the car. When you drive into the garage, you must, of course, remember to switch on the main beam of your headlights!

15-V 1-A Regulated Power Supply

The supply receives - 20 V from the rectifier/filter which is fed to the collector of the Darlington pnp pass transistor, a TIP105. The base drive to the TIP105 is supplied through resistor R5. The base of the TIP105 is driven from Vz terminal at pin 9, which is the anode of a 6.2-V zener diode that connects to the emitter of the uA723 output control transistor.

Circuit diagram :

15-V 1-A Regulated Power Supply-Circuit Diagram

15-V 1-A Regulated Power Supply Circuit Diagram

The method of providing the positive feedback required for foldback action is shown. This technique introduces positive feedback by increased current flow through resistors R1 and R2 under short-circuit conditions. This forward biases the base-emitter junction of the 2N2907 sensing transistor, which reduces base drive to the TIP105.

Reanimating Probe for AVR μC

AVR device not responding, When this discouraging message appears while you’re programming your Atmel microcontroller, that’s where the problems really begin! The problem is of ten due to incorrect programming of the fuse bits. This is where the unblocking probe comes into play…


Once the whole thing is powered up, all you have to is use one hand to apply the tip of the probe to the microcontroller’s XTAL1 input and then use your other hand to go ahead and program it with your favourite sof t ware. And there, your microcontroller is saved! The electronics are as simple as can be, the aim being to design something cheap and easy to reproduce. It consists of an oscillator generating a rectangular wave at around 500 kHz, built using  a 74HC04. This circuit will also work with a 74HC14, but depending on the make of IC, the frequency of around 500 kHz may vary by around ±50 kHz. This doesn’t affect the operation of the probe.

circuit diagram123w

The unblocking board is connected using a ribbon cable, terminated with two female HE10/10 connectors. The pinout of the HE10/10 connector is the same as used in the majority of circuits, but of course it can be adapted for an HE10/06 connector.

The first connector is connected to the board to be unblocked, which allows powering of the electronics. The second connector is connected to the ISP programmer (STK200 compatible). The contact at the crystal is made using a needle, to ensure contact even through a board that has been varnished. There’s no need to unsolder the crystal for this operation.

The PCB design in Eagle format is available from : www.elektor.com

Author : P. Rondane - Copyright : elektor

Simple Relay Actuating Circuit with Millivolt Sensitivity

Common electromagnetic relays with coil voltage of 5V, 6V or 12V can operate power circuits in the range of 250 to 300 volts and up to 20A. The actuating current of these relays is in the range of 50 to 100 mA. In many applications, such as temperature-controlled equipment, energisation/de-energisation of 5V, 6V or 12V relays for ‘on’/‘off’ control of the load is to be achieved by a few millivolts.

Circuit diagram :

Simple Relay Actuating Circuit Diagram

Simple Relay Actuating Circuit Diagram

Here is a simple relay actuating circuit that can be used to energise relays with a few millivolts as the input. As shown in the figure, the circuit is built around op-amp µA741(IC1) followed by an emitter follower using transistor 2N3440 (T1) and a few discrete components. You can change the sensitivity required (when the input is in millivolts range) to energise the relay by changing the value of resistors R1, R2 and R3. In all the cases, values of R4, R5 and R6 remain unchanged.

Resistor R8 is basically a current-limiting resistor used in series with the relay coil. This resistor is not required with a 12V, 200-ohm relay. This circuit can be used to drive 24V relays, by changing the supply voltage to ±18V. To get the best results, using preset VR1 adjust the offset at pin 4 of the op-amp to as low a value as possible.  The working of the circuit is simple. When a few millivolts (say, 200 mV) are applied at the input terminal shown in the diagram, the output of IC1 at pin 6 goes high and transistor T1 conducts to energise relay RL1. As a result, the load gets power supply through normally-open (N/O) contacts of the relay.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Connect the relay carefully so that the contact can handle the load current.

Do not use resistor R8 with 9V or 12V relays. In this case, connect the junction of the anode of D2 and the coil of the relay to ground.

Author : DR G.N. Acharya  - Copyright : EFY

High Level Wideband RF Preamplifier

A linear RF amplifier can be made in two ways: (1) with the aid of a linear active element, or (2) with a non-linear element operating with negative feed-back. This circuit is of the second kind, using an RF power transistor as the active element. Feedback is also required to ensure correct termination (50 Q) of the aerial, since bipolar transistors normally exhibit a low input impedance. Also, the noise figure is not increased because virtually no signal is lost.

Circuit diagram :

High Level Wideband RF Preamplifier-Circuit Diagram

High Level Wideband RF Preamplifier Circuit Diagram

The common-base amplifier is based on a UHF class A power transistor Type 2N5109 from Motorola. The feedback circuit is formed by RF transformer Th. The input and output impedance of the preamplifier is 50 4 for optimum perform-ance. Network R3-C5  may have to be added to  preclude oscillation outside the pass-band, which  ranges from about 100 kHz to 50 MHz. The gain is  approximately 9.5 dB, the noise figure is between 2  and 3 dB, and the third-order output intercept point  is at least 50 dBm.

The input/output transformer is wound on a Type FT37-75 ferrite core from Micrometals. The input winding is 1 turn, the output winding 5 turns with a tap at 3 turns.

Simple Smooth Flasher

Ordinary LED flashers turn the LED on and off abruptly, which can get a little irritating after a while.

Circuit diagram :

Simple Smooth Flasher-Circuit Diagram

Simple Smooth Flasher Circuit Diagram

The circuit shown here is  more gentle on the eyes: the light intensity changes very slowly and sinusoidally, helping to generate a relaxed mood. The circuit shows a phase-shift oscillator with an adjustable current source at its out-put. The circuit is capable of driving two LEDs in series without affecting the current. The frequency is set by three RC networks, each of which consists of a 100 µF capacitor and a 22 kΩ resistor.

Operation is largely independent of supply voltage, and the average LED current is set at about 10 mA. The circuit adjusts the voltage across the emitter resistor so that it matches the base voltage of the first transistor (around 0.6 V). The phase shifting network gives rise to the oscillation around this average value. In the prototype of this circuit we used an ultra-bright red LED.

Author : Burkhard Kainka - Copyright: Elektor

Security Door Electronic Key

It is a relatively simple circuit of electronic lock of safety with code of 7 digits. It should is given attention in the time that will be stepped the keys, that shape code and it does not exist it delays. With the right step of keys and if code is right then is activated exit Q7 for roughly 4 seconds, driving the transistor Q2, which with the line can drive one relay, for the opening of door, or any other circuit.

Circuit diagram :

security-door-electronic-key Circuit diagram

Security Door Electronic Key Circuit Diagram

With LED D we can have optical clue of activation. The code of circuit, as it has been given have been:1704570 but can change, if we change the connections between in the exits of IC1 and the switches.

Parts List :

  • R1-7=4.7Kohm
  • R8=15Kohm
  • R9=1Mohm
  • R10-13=10Kohm
  • R11=100ohm
  • R12=220Kohm
  • R14=1.2Kohm
  • C1-3=100nF 100V
  • C2=4.7uF 25V
  • D1-2=1N4148
  • D3=RED LED 3mm
  • IC1=4022
  • Q1=BS170
  • Q2=BD679
  • S1-10=Push button or keyboard

Lighting Governor Using 555 Timer IC

This circuit is very handy as a timer circuit for a lamp, for lighting a staircase, for example, but can also be used as indicator for the front doorbell. A significant advantage of this circuit is that the circuit draws almost no current when in the inactive state. The circuit is activated with push button (S1), after which IC5 (a 555 timer IC) starts to count down the set time. During this  time the triac continues to conduct and the lamp is turned on. The ‘on’ time of the  lamp is on is determined by the combination of R1 and C2 and can be changed as  required by your application or personal  preference. R2 and C3 have been added because the  555 expects a ‘negative’ pulse at its trigger  input. When the power supply is turned on,  C3 holds the TR input of the 555 Low for a short time, which triggers the timer IC.

Circuit diagram :

Lighting Governor -Circuit Diagram

Lighting Governor Circuit Diagram

Depending on the exact type (brand) of  555, the value of C4 (330 nF) may have to be changed to ensure a high enough  power supply voltage when in the active  state. Note also that you shouldn’t use a  ‘too heavy’ version of the triac. The circuit will drive at the most just a little more than 5 mA into the gate of the triac. The circuit worked properly when tested with a TIC206  and the slightly bigger TIC216.

When selecting push button S1, take into  account the switching current of the lamp. The switch must be able to handle that  safely. In the event of a defective part, a 15-V zener diode is connected across the power supply for protection (D3). R6 and R7 have been added so that C4 will be discharged. In this way no dangerous voltage can remain when the circuit is unplugged. When large values for C2 are used, such  as the 470 µF shown here, a good quality  capacitor is required for C4. Any potential  leakage resistance will then have no influence on the set time. Because of an inferior  capacitor in our prototype the time was  considerably longer than expected.

Author : Peter Jansen - Copyright : Elektor

Touch-Free Timer Switch Circuit

This type of infrared proximity circuit is widely used as an electric switch where physical contact is not desired for hygiene purpose. For example, we commonly see use of infrared proximity sensors on public drinking fountains and in public washrooms. The simple circuit presented here can be operated by moving your hand in front of it. This is achieved by detecting the infrared light reflected by your hand onto a receiver device.

Circuit diagram :

Touch-Free Timer Switch-Circuit diagram

Fig. 1: Touch-Free Timer Switch Circuit Diagram

Fig. 1 shows the circuit of the touch-free timer switch. It has two sections: transmitter and receiver. The IR transmitter is built around timer LMC555 (IC1), which is wired as an astable multivibrator. The multivibrator produces 38kHz pulses (at low duty cycle) that drive an infrared LED (LED1). This frequency can be tuned using a 10-kilo-ohm preset (VR1). A 220-ohm series resistor (R3) ensures that the current consumption of the IR transmitter is not out of arrangement.

The receiver section is built around IR receiver module TSOP1738 (IRX1), timer LMC555 (IC2) and a few discrete components. The TSOP1738 is an integrated miniaturised receiver for infrared remote control systems. Everything required for IR signal processing, including the PIN diode and preamplifier, are assembled on a lead frame and the epoxy package is designed as an IR filter.


When a short IR burst is received by IRX1 (as you wave your hand in front of the switch), the demodulated pulses are fed to the trigger input (pin 2) of the second LMC555 (IC2). This, in turn, triggers the monostable wired around IC2 and its output pin 3 goes high for a period determined by the 2.2-mega-ohm potentiometer and capacitor C5. This turns off the standby indicator (LED1) and transistor T1 conducts to drive the 5V relay (RL1). LED1 enables you to locate the switch in the dark. AC mains supply to the load to be switched-on is routed through the pole and normally-opened contacts of RL1 as shown in the diagram. The circuit works off regulated 5V DC.

Pin configurations of TSOP1738, IR LED and BC547

Fig. 2: Pin configurations of TSOP1738, IR LED and BC547

Fig. 2 shows the pin configurations of TSOP1738, IR LED1 and transistor BC547. Assemble the circuit on a general-purpose PCB and enclose in a small plastic cabinet. Fit IR LED1 with a reflecting hood at a recessed position on the front panel of the enclosure. The dome-shaped face of the TSOP1738 should stick out from the front panel. Fit the time-control potentiometer (VR2) in an appropriate position. Finally, fit the standby indicator LED1 inside a suitable LED holder such that it slightly protrudes from the front panel. To prevent unwanted reflection of the IR beam, the finished unit should be mounted such that it does not face a nearby wall.

 Suggested enclosure

Fig. 3: Suggested enclosure

Using high-precision linear potentiometer VR2 and capacitor C5 (100µF), the time length can be set from nearly 1 second to 120 seconds. Attach a small paper dial on the front panel of the enclosure and mark various positions of the control knob of VR2 as shown in Fig. 3. The accuracy of the timer depends mainly upon the quality (and value) of timing capacitor C5. In practice, most electrolytic capacitors are rated on the basis of minimum guaranteed value and the real value may be higher.

Author :T.K. Hareendran - Copyright: EFY

Simple Automatic Street Light

There have been lot of problems in street lights. Major problem in some places is every evening a person has to come and switch ON the street light and it should be again switched off in morning. Yes, this may not be the situation in everywhere but exists in many places. So this problem can be overcome by using a simple circuit. Below shown circuit will be automatically switched ON and OFF during night and morning times respectively.

Circuit diagram :

Automatic-street-light Circuit diagram

Simple Automatic street light Circuit Diagram

In above circuit R1 can be used to adjust the sensitivity. And the working of the circuit is very simple. The LDR will have very low resistance during day time so the transistor Q1 will be in OFF condition. And during night time the resistance will be very high so automatically the transistor Q1 will be ON. The Q1 is PNP transistor and the emitter of Q1 is given to base of Q2. So the Q2 transistor will be ON only if the transistor Q1 is ON. The TRIAC is used in the circuit to make is circuit complete. As the TRIAC will allow voltage to pass from either directions only when there is a certain threshold voltage in gate terminal. And the gate of TRIAC is controlled by transistor Q2.

So totally the lamp will be ON during night time and will be again switched off during day light. To change the sensitivity of the circuit to light adjust R2.

4 X 15 Watt Mini Power Amplifier

A lot of electronic circuits in the domain of audio amplifiers are already been published here. This circuit is a little different because it is a four channel amplifier. Each channel of this amplifier can deliver an output of 15Watts into a 4 ohm speaker. The amplifier can be operated from a single 12V DC supply and this makes it possible to use this amplifier in car audio applications too.

Circuit diagram :

4X15W-amplifier-circuit Diagram

4 X 15 Watt Mini Power Amplifier

The circuit is based on the 15W BTL X 2 channel audio power amplifier IC TA8215 from Toshiba. Even though chip is specifically designed for car audio applications it can be also used for home audio applications. Two TA8215 ICs are used here in order to obtain a 4 channel amplifier system. The circuit is designed almost exactly as per the application diagram in the ICs datasheet. Pins 7 and 19 are the Vcc pins of the ICs internal integrated power amplifier stages and these pins are connected to the positive supply. Pin 9 is the Vcc pin for ICs internal preamplifier and it is also connected to the positive supply. Pins 13 and 14 are the internal power amplifiers ground pins and they are tied together and connected to the ground.

The internal preamplifier’s ground pin (pin5) is connected to the common ground through a 10 Ohm resistor which makes the input ground separated from the common ground by a resistance of 10 ohms and this improves the noise rejection. The 100uF capacitor works as a power supply de-coupler. The resistor networks connected to the output lines of each amplifier improves the high frequency stability. The variable resistors (R3, R4, R12 and R13) works as the volume controller for the corresponding channels.

Notes :

  • Assembling the circuit on a good quality PCB is a must for obtaining optimum sound quality.
  • Use 12V DC for powering the circuit.
  • The ICs must be fitted with adequately sized heat sinks.
  • R3, R4, R12 and R13 serves as volume controllers.
  • K1 to K4 can be 4 Ohm, 20W speakers.
  • This amplifier circuit can be used in a variety of applications such as car audio systems, home theater systems, personal audio systems, public address systems etc.

A Simple Stereo Amplifier with Voltage Regulators

A simple stereo audio amplifier is built around two 7905 negative-voltage regulators (IC1 and IC2) and a few discrete components. The circuit will also work with other 79XX regulators if appropriate power supply is used. Regulator IC 7905 works as an amplifier for the voltages applied to common pin2 (Ground or GND). Also check the LM317 audio amplifier, another interesting circuit.

The minimal voltage drop over the standard 7905 is around 2V and it depends on the output current. Feedback resistors in the IC set the gain of the channel internally. The amplifier is a class-A audio amplifier. The minimal applicable value of R3 for the regulator 7905 is 8.2 to 10 ohms per 5W.

Circuit diagram :

A simple Stereo Amplifier with Voltage Regulators

1W Stereo Amplifier with Voltage Regulators Circuit Diagram

If the required output current for LS1 is below 100 mA, the value of resistor R3 can be 33 to 51 ohms per watt. The circuit works with any load resistance (R3 in parallel with LS1 as the load) under the condition that the regulator is not overloaded with current and power dissipation. However, it is preferable to use a loudspeaker with a high resistance (8 ohms, 16 ohms or more). The amplifier works well with low-impedance headphones having a resistance of 24 to 32 ohms. The voltage difference between the ground pin of 7905 and the output pin is fixed internally.

S2 is the on/off switch. Switch S1 is for mono/stereo selection. When switch S1 is closed, the amplifier works as a two-way mono amplifier. If S1 is open, the amplifier works as a stereo amplifier. If no input signal is applied, the DC voltage on the output of the regulator 7905 should be around –5V, which depends to some extent on the value of VR1. The maximum output current of 7905 can be up to 1A and the maximum power dissipation is up to 15W. Mount the regulator IC 7905 on a heat-sink with thermal resistance below 15°C/W.

Copyright : EFY

Brake Lights Monitor

The circuit described below monitors your car's  brake lights, and indicates by a light emiting diode  whether they both function correctly. In that sense, it can save you money by preventing your being fined for driving with defective brake lights, and it also leads to increasing road safety.

Circuit diagram :

Brake Lights Monitor-Circuit Diagram

Brake Lights Monitor Circuit Diagram

The monitor depends inevitably on the voltage drop across the supply lines to the two lamps. For the circuit to work correctly, that drop needs to be greater than 0.6 V. If this is not so, the drop must be increased by adding a 5 V diode in series with each lamp. Transistor Ti and T2 in figure 1 form a  Schmitt trigger, which reacts to the voltage drop across the supply lines to the two brake lights. This reaction manifests itself in Di lighting via T3. If  one of the brake lights is faulty, the switch-on cur- rent drawn by the other lamp will cause Di to light  briefly when the brake pedal is pressed. If both  brake lights are defective, Di will not light at all.  All three possible states of the brake lights are thus  indicated.

The hysteresis of the trigger, and, therefore, the sensitivity of the circuit, can be adjusted within narrow limits with Pi. The preset is best adjusted with one lamp out of action in a manner which makes Di light briefly as described above.

If you find it disturbing that Di lights every time  you brake, the operation can be reversed by replacing the BC557B in the T3 position by a BC547B  (n-p-n). The collector of T3 is then connected to the positive supply line, and the emitter to R6. On the printed circuit board this means that the flat edge  of T3 must be turned the other way. A second base  connection has also been provided on the PCB.  Note, however, that this configuration no longer makes it possible to ascertain whether one or both brake lights are faulty, i.e., when the LED lights, one or both lamps need replacing.

The printed circuit board is not available ready  made. In figure 1, Si is the brake pedal switch, and Lai and La2 are the brake lights.

Best Automatic Car Alarm

Even the best car alarm is useless if you forget to set it upon leaving your car, whence this circuit. The relay has a make and a break contact: the  former is necessary to delay the switching in of the  alarm after you have got out of your car, and the  latter serves to switch on the car alarm proper. Immediately on re-entering your car, you must press the hidden switch, Si. This causes silicon-controlled rectifier Thi to conduct so that the relay is energized. At the same time, the green LED lights to indicate that the alarm is switched off. 

Circuit diagram :

Best Automatic Car Alarm-Circuit Daigram

 Best Automatic Car Alarm Circuit Diagram

As soon as the ignition is switched off, T, is off, T2  is on, and the buzzer sounds. At the same time,  monostable IC1 is triggered, which causes T3 to  conduct and the red LED to light. The silicon- controlled rectifier is then off, and D4 is reverse  biased, but the relay remains energized via its make  contact for a short time, preset by Pi As soon as this  time has lapsed, the relay returns to its quiescent  state, and the alarm is set via the break contact. The  delay time can be set to a maximum of about 1 minute.

Battery Charger with Constant Current

T here are many ways of battery charging but constant-current charging, in particular, is a popular method for lead-acid and NiCd batteries. In this circuit, the battery  is charged with a constant current that  is generally one-tenth of the battery capacity in ampere-hours. So for a  4.5Ah battery, constant charging current would be 450 mA.

This battery charger has the following features:

  1. It can charge 6V, 9V and 12V batteries. Batteries rated at other voltages can be charged by changing the values of zener diodes ZD1 and ZD2.
  2. Constant current can be set as per the battery capacity by using a potmeter and multimeter in series with the battery.
  3. Once the battery is fully charged, it will attain certain voltage level (e.g.13.5-14.2V in the case of a 12V battery),  give indication and the charger will switch off automatically. You need not remove the battery from the circuit.
  4. If the battery is discharged be-low a limit, it will give deep-discharge indication.
  5. Quiescent current is less than 5 mA and mostly due to zeners.
  6. DC source voltage (V CC ) ranges from 9V to 24V.
  7. The charger is short-circuit protected.

D1 is a low-forward-drop schottky  diode SB560 having peak reverse volt-age (PRV) of 60V at 5A or a 1N5822 diode having 40V PRV at 3A. Normally, the minimum DC source voltage should be ‘D1 drop+Full charged  battery voltage+V DSS + R2 drop,’ which is approximately ‘Full charged battery voltage+5V.’ For example, if we take  full-charge voltage as 14V for a 12V  battery, the source voltage should be 14+5=19V.

Circuit diagram : 

Battery Charger with Constant Current-Circuit Diagram

Battery Charger with Constant Current Circuit Diagram

For the sake of simplicity, this constant current battery charger circuit is divided into three sections: constant-current source, overcharge protection and deep-discharge protection sections.

The constant-current source is built around MOSFET T5, transistor  T1, diodes D1 and D2, resistors R1, R2,  R10 and R11, and potmeter VR1. Diode  D2 is a low-temperature-coefficient, highly stable reference diode LM236-5. LM336-5 can also be used with reduced operating temperature range of 0 to  +70°C. Gate-source voltage (V GS )of T5 is set by adjusting VR1 slightly above 4V. By setting V GS , charging current can be fixed depending on the battery capacity. First, decide the charging current (one-tenth of the battery’s Ah capacity) and then calculate the nearest standard value of R2 as follows: R2 = 0.7/Safe fault current R2 and T1 limit the charging current if something fails or battery terminals get short-circuited accidentally.

To set a charging current, while  a multimeter is connected in series with the battery and source supply is present, adjust potmeter VR1 slowly  until the charging current reaches its  required value. Overcharge and deep-discharge protection have been shown in dotted areas of the circuit diagram. All components in these areas are subjected to a maximum of the battery voltage and not the DC source voltage. This makes the circuit work under a wide range of source voltages and without any influence from the charging current value. Set overcharge and deep-discharge voltage of the battery using potmeters VR1 and VR2 before charging the battery.

In overcharge protection, zener diode ZD1 starts conducting after  its breakdown voltage is reached,  i.e., it conducts when the battery voltage goes beyond a prefixed high level. Adjust VR2 when the battery is fully charged (say, 13.5V in case of a 12V battery) so that V GS of T5 is set to zero and hence charging current stops flowing to the battery. LED1  glows to indicate that the battery is fully charged. When LED1 glows, the internal LED of the optocoupler also glows and the internal transistor con-ducts. As a result, gate-source voltage  (V GS ) of MOSFET T5 becomes zero and  charging stops.

Normally, zener diode ZD2 con-ducts to drive transistor T3 into conduction and thus make transistor T4  cut-off. If the battery terminal voltage  drops to, say, 11V in case of a 12V battery, adjust potmeter VR3 such that  transistor T3 is cut-off and T4 conducts. LED2 will glow to indicate that the battery voltage is low.

Values of zener diodes ZD1 and  ZD2 will be the same for 6V, 9V and 12V batteries. For other voltages, you need to suitably change the values of ZD1 and ZD2. Charging current provided by this circuit is 1 mA to 1 A, and no heat-sink is required for T5. If the maximum charging current required is  5A, put another LM236-5 in series with  diode D2, change the value of R11 to 1 kilo-ohm, replace D1 with two SB560 devices in parallel and provide a good heat-sink for MOSFET T1. TO-220 package of IRF540 can handle up to 50W.

Assemble the circuit on a general-purpose PCB and enclose in a box after setting the charging current, overcharge voltage and deep-discharge voltage. Mount potmeters VR1, VR2 and VR3 on the front panel of the box.

Author : Monoj Das - Copyright : EFY

Current Monitor And Alarm Circuit

These circuits are intended for remote monitoring of the current consumption on the domestic mains line.

Fig 1 Current Monitor And Alarm-Circuit Daigram

The circuit in Fig. I lights the signal lamp upon detecting a mains current consumption of more than 5 mA, and handles currents of several amperes with appropriate diodes fitted in the D, and D2 positions. Transistor Ti is switched on when the drop  across D,-D2 exceeds a certain level. Diodes from  the well-known I N400x series can be used for currents of up to I A, while lN540x types are rated for up to 3 A. Fuse F, should, of course, be dimensioned to suit the particular application.

A number of possible transistor types have been stated for use in the Ti position. Should you consider using a type not listed, be sure that it can cope with surges up to 700 V. As long as Ti does not con- duct, the gate of the triac is at mains potential via  C,, protective resistor R2 and diode Da, which  keeps C, charged. When Ti conducts, alternating current can flow through the capacitor, and the triac is triggered, so that Lai lights.

Fig 2 Current Monitor And Alarm-Circuit Diagram

The circuit in Fig. 2 is a current triggered alarm. Rectifier bridge D4-D7 can only provide the coil voltage for Re, when the current through Di-D2 exceeds a certain level, because then series capacitor C, passes the alternating mains current. Capacitor C, may need to be dimensioned otherwise than shown to suit the sensitivity of the relay coil. This is readily effected by connecting capacitors in parallel until the coil voltage is high enough for the relay to operate reliably.

Finally, an important point: Many points in these circuits are at mains potential and therefore extremely dangerous to touch.

Solar Lamp using the PR4403

The PR4403 is an enhanced cousin of the PR4402 40 mA LED driver. It has an extra input called LS which can be taken low to  turn the LED on. This makes it very easy  to build an automatic LED lamp using a  rechargeable battery and a solar module. The LS input is connected directly to the solar cell, which allows the module to be  used as a light sensor at the same time as  it charges the battery via a diode. When  darkness falls so does the voltage across  the solar module: when it is below a thresh-old value the PR4403 switches on. During  the day the battery is charged and, with  the LED off, the driver only draws 100 µA.

Circuit Diagram :

Solar Lamp-Circuit Diagram

Solar Lamp using the PR4403 Circuit Diagram

At night the energy stored in the battery is released into the LED. In contrast to similar designs, here we can make do with a single  1.2 V cell. The PR4403 is available in an SO-8 pack-age with a lead pitch of 1.27 mm. The  other components are a 1N4148 diode (or a Schottky 1N5819) and a 4.7 µH choke. Pin 2 is the LS enable input, connected directly to the solar module. According to the datasheet, it is possible to connect a series resistor at this point (typ. 1.2 M) to increase the effective threshold voltage. The LED will then turn on slightly earlier in the evening before it is not completely  dark. Pins 3 and 6 of the device must be connected together and together form the output of the circuit.

Author : Burkhard Kainka - Copyright : Elektor