Wiring Schematic Diagram

Most good bench power supplies include a form of overvoltage protection, but for those power supplies or for other applications where over voltage protection is required, a simple over voltage crowbar circuit can be built. It uses just four components: a silicon controlled rectifier or SCR, a zener diode, a resistor and a capacitor.

The SCR over voltage crowbar or protection circuit is connected between the output of the power supply and ground. The zener diode voltage is chosen to be slightly above that of the output rail. Typically a 5 volt rail may run with a 6.2 volt zener diode. When the zener diode voltage is reached, current will flow through the zener and trigger the silicon controlled rectifier or thyristor. This will then provide a short circuit to ground, thereby protecting the circuitry that is being supplied form any damage.

However it is necessary to ensure that the power supply has some form of current limiting. Often a fuse is ideal because the SCR will be able to clamp the voltage for long enough for it to blow. The small resistor, often around 100 ohms from the gate of the thyristor or SCR to ground is required so that the zener can supply a reasonable current when it turns on. It also clamps the gate voltage at ground potential until the zener turns on. The capacitor is present to ensure that short spikes to not trigger the circuit. Some optimisation may be required in choosing the correct value although 0.1 microfarads is a good starting point.

This circuit is used to indicate the amount of water present in the overhead tank. Not only that, but this circuit also gives an alarm when the tank is full. CD4066 is used to indicate the level of the water through LEDs.

The circuit uses the widely available CD4066, bilateral switch CMOS IC to indicate the water level through LEDs. When the water is empty the wires in the tank are open circuited and the 180K resistors pulls the switch low hence opening the switch and LEDs are OFF. As the water starts filling up, first the wire in the tank connected to S1 and the + supply are shorted by water. This closes the switch S1 and turns the LED1 ON. As the water continues to fill the tank, the LEDs2 , 3 and 4 light up gradually. The no. of levels of indication can be increased to 8 if 2 CD4066 ICs are used in a similar fashion. When the water is full, the base of the transistor BC148 is pulled high by the water and this saturates the transistor, turning the buzzer ON. The SPST switch has to be opened to turn the buzzer OFF. Remember to turn the switch ON while pumping water otherwise the buzzer will not sound!

This is a volt meter circuit based on LM324 OpAmps chip. A bargraph generator is nothing more than a slack-handful of OpAmps, all thrown into a single chip. It is cheaper to use the LM324 and put in the few extra components needed.

R1 to R8 down the left all form a voltage divider from the 7.5v Zener diode. The resistors give me taps of 0.5v, 1.5v, 2.5v, 3.5v, 4,5v, 5.5v, and 6.5v. The top of the zener gives me the eighth voltage: 7.5v. These reference voltages are all connected to the negative-acting input of each OpAmp, and all the positive-acting inputs are gathered together for a common input. With 0v on the +ve inout, the output voltage of avery OpAmp is 0v.

If the input voltage lies between 0v and 0.5v then no LED lamps will light. If the input voltage rises to between 0.5v and 1.5v (1v +/-0.5v) then the first LED will light when the first OpAmp +ve input exceeds the -ve input threshold. In this way the LEDs all form a nice bar-graph display in 1v steps, +/- half a volt.

The A-B-C links should be in the A-B position for a normal bar-graph display, when each LED succesively lights as the voltage rises. With +4v input there will be four LEDs lit. If you fit the links in the A-C position then only one LED will light for each voltage step, to give a moving dot display. The top (last) link should always be in the A-B position for both modes.


This Op-Amp Voltmeter Circuit From www.sm0vpo.com:800

If you want a 1.5V to 5V Step-Up Converter here is a simple circuit using LT1073 from Linear Technology. The LT1073 is a versatile micro-power DC / DC converter. The circuit requires few external components to deliver a fixed output of 5V.

1.5V to 5VDC Converter Circuit by LT1073

LT1073 Pinout

The very low minimum supply voltage of 1V allows the use of the LT1073 in applications Nowhere the primary power source is a single cell. An on-chip auxiliary gain block cans function as a low battery detector or linear post-regulator.The LT1073 is good idea, it Used for dc to dc converter.Average current drain of the LT1073-5 Used as shown in the Typical Application Circuit below is just 135mA unloaded, making it ideal for applications Nowhere Long battery life is Important. The circuit shown cans deliver 5V at 40mA from an input as low as 1.25V and 5V at 10mA from a 1V input.

This is a circuit which built to one of my audio amplifier projects to control the speaker output relay. The purpose of this circuit is to control the relay which turns on the speaker output relay in the audio amplifier.

Turn On Relay Delay Circuit for power amplifier

The idea of the circuit is wait around 5 seconds ofter the power up until the speakers are switched to the amplifier output to avoid annoying "thump" sound from the speakers. Another feature of this circuit is that is disconnects the speaker immediately when the power in the amplifier is cut off, so avoiding sometimes nasty sounds when you turn the equipments off.

This circuit is not the most accurate and elegant design, but it has worked nicely in my small home-built PA amplifier. This circuit can be also used in many other applications where a turn on delay of few seconds is needed. The delay time can be increased by using bigger C2 and decreased by using a smaller C2 value. Note that the delay is not very accurate because of simplicity of this circuit and large tolerance of typical electrolytic capacitors.

This circuit can clearly show the level of the supply voltage as long as the indicator has good 12 volts at its input, LED1 gives steady, uninterrupted yellow light. If the input voltage falls below 11 V, LED1 will start to blink and the blinking will just get slower and slower if the voltage drops further - giving very clear and intuitive representation of the supply's status. The blinking will stop and LED1 will finally go out at a little below 9 volts. On the other hand, if the input voltage rises to 13 V, LED2 will start to glow, getting at almost full power at 14 V. The characteristic voltages can be adjusted primarily by adjusting the values of R1 and R4. The base-emitter diode of T2 basically just stands in for a zener diode.

Voltage Indicator Circuit For 12V Power suplay

The emitter-collector path of T1 is inversely polarized and if the input voltage is high enough - T1 will cause oscillations and the frequency will be proportional to the input voltage. The relaxation oscillator ceases cycling when the input voltage gets so low that it no longer can cause breakdown along the emitter-collector path. Not all small NPN transistors show this kind of behavior when inversely polarized in a similar manner, but many do. BC337-40 can start oscillations at a relatively low voltage, other types generally require a volt or two more.

This is an interesting 12V Battery Monitor circuit of a low power electronic dc voltmeter circuit that can be used with car electric systems that run on 12 volt batteries.

In the circuit having a voltage quad comparator (LM339) is used as a simple bar graph meter to indicate the state of charge 12-volt lead-acid battery acid. A 5 volt reference voltage is in each of the (+) inputs of four comparators and the (-) inputs are connected, each point is connected to a voltage divider. The LED lights up when the voltage at the negative (-)-input exceeds the reference voltage. Calibration can be by adjusting the 2K potentiometer so that all four LEDs illuminate when the battery voltage is 12.7 volts indicates a full charge with no load on the battery is done. At 11.7 volts, the LED should be made, and shows an empty battery. Each LED is a change of about 25% charge or 300 millivolts, so that 3 LEDs indicate 75%, 2 LEDs indicate 50%, etc.

A light dimmer circuit is a means of controlling the "brightness" level of a lamp, in this circuit we will use a 555 timer to control the brightness level of a low voltage incandescent bulb of up to 60 watts.

The circuit works by pulse-width modulating the 12V supply to the lamp. The 555 timer is configured as a "variable cycle", astable oscillator running some where around 300 Hz.

Potentiometer 47K from minimum to maximum varies the values accordingly the duty cycle of the 555 timer controlling light dimmer circuit.

The power mosfet used here would be a TO-220 type such as MTP3055E or similar. Note the need for a TO-220 type heat sink for full rated loads.

The simple and easy to build on/off LED fade effect circuits included here will specially interest the new electronic hobbyists. Just like the name of the circuit, the light intensity of LEDs in this circuit will fade from high intensity to low intensity and then off.

C 30 uF and R 100k (blue color) control the speed of the fading. You can replace the 100K ohm resistor with 100k linear potentiometer so you can adjust the speed any time. You can make the circuit even simplier by leaving out other of the transistors and those leds that are connected to it. Good for power led pulsing etc.

Op-Amp LM324 Pinout

LM324 is an OP-Amp amplifier. There 4 op amp, and we just use 2 op amp in above circuit diagram. You may use the other 2 op-amp to build another similar circuit. Transistor is BC547 and the led is ultra bright red with 220 ohm resistor in series.

This is a very simple 1.5V LED Flasher circuit based LM3909. It is known that LM3909 is an oscillator, a monolithic one, which is designed to flash Light Emitting Diodes (LED). According to the datasheet, it may deliver pulses of 2 more volts while operating on a supply of 1.5V by using the timing capacitor.

The LM3909 features low cost, low voltage and low current drain, with a bright-high current LED pulse and wide temperature range. It can be applied on applications such as flashlight finder, siren for toy fire engine, sawtooth generators, emergency locators, warning indicators and etc.

LED1    : Red LED
C1      : 100uf/16V
IC1     : LM3909
Battery_:_____1.5V AAA 

The Following diagram is the Pulse (Clock) Generator circuit diagram of the which the build based on NAND Gate Logic IC. You May use IC 7400 or 4011 for this circuit. This circuit is very useful while checking / operating counters, stepping relays and other digital circuits. It avoids the procedure of setting a switch for the required number of pulses.

NAND Gate Pulse Generator Circuit

7400 NAND Gate Pinout

The following diagram is the circuit diagram of Treble Booster. This circuit can be used with an electric guitar (and also electronic instruments) to boost the higher order harmonics and give a more brilliant sound.

A circuit of this type gives a fairly flat response at bass and most middle audio frequencies, with the upper-middle and lower treble frequencies being given a substantial amount of boost. In this circuit has can to improve the treble about 10000 Hz that 40 dB. By you can change C1 be other value as a result.

In this fire alarm circuit, a thermistor works as the heat sensor. When temperature increases, its resistance decreases, and vice versa. At normal temperature, the resistance of the thermistor is approximately 10 kilo-ohms, which reduces to a few ohms as the temperature increases beyond 100°C. This phenomenon is employed here for sensing the fire.

The NE555 is configured as a free running oscillator at audio frequency. The transistors T1 and T2 drive IC1. The output of IC1 is couples to base of transistor SL100, which drives the speaker to generate alarm sound. The frequency of NE555 depends on the values of resistances R5 and R6 and capacitance C2. When thermistor becomes hot, it gives a low-resistance path for the positive voltage to the base of transistor T1 through diode D1 and resistance R2.

This photodiode alarm circuit can be used to give a warning alarm when someone passes through a protected area. The circuit is kept standby through a laser beam or IR beam focused on to the Photodiode. When the beam path breaks, alarm will be triggered.

Photodiode Alarm Circuit

Simple IR transmitter circuit

The circuit uses a PN Photodiode in the reverse bias mode to detect light intensity. In the presence of Laser / IR rays, the Photodiode conducts and provides base bias to T1. The NPN transistor T1 conducts and takes the reset pin 4 of IC1 to ground potential. IC1 is wired as an Astable oscillator using the components R3, VR1 and C3. The Astable operates only when its resent pin becomes high. When the Laser / IR beam breaks, current thorough the Photodiode ceases and T1 turns off. The collector voltage of T1 then goes high and enables IC1. The output pulses from IC1 drives the speaker and alarm tone will be generated.

A IR transmitter circuit is given which uses Continuous IR rays. The transmitter can emit IR rays up to 5 meters if the IR LEDs are enclosed in black tubes.

Here is Simple Pulse Generator circuit Based IC invertig CD4049. This circuit use only 1 active component and very..very few passives components around. It use digital cmos CD4049 Hex Inverting Buffer IC. The pulse generators can also produce a 50% duty cycle square wave. It may use digital techniques

Simple Pulse Generator circuit

CD4049 Pinout

Here is a 60 Hz clock pulse generator circuit provides a clean, stable square wave and it will operate on anywhere from 6 to 15 volts. The IC and color-burst crystal are the kind used in TV receivers. The 3.58 MHz output makes a handy marker signal for shortwave bands.

This is a 1Hz pulse generator circuit using the timer IC 555 which is wired as an Astable Multivibrator. The output pulses can be indicated visually by the LED. An Astable Multivibrator, often called a free-running Multivibrator, is a rectangular-wave generating circuit. This circuit does not require any external trigger to change the state of the output, hence the name free-running. This circuit can be used in applications that require clock pulses

1Hz Pulse Generator Circuit

We can set the 555 to work at the desired frequency by selecting the right combination of resistances & capacitance.

Frequency = 1.44 / {(R1 + 2R2) * C1}

This circuit will of turn on / off 12V DC fan Pls temperatures of above normal temperatures. You can set the turn on temperature by adjust VR1. This circuit use an NTC Thermistor, the which means Pls Surrounding the temperature decreases the resistance of this thermistor will of increase is. If the temperature increate So the voltage at pin 3 on LM311 will from decreated. The resistance of the NTC is about 10K at 25'c.

circuit diagram of Automatic DC fan
Controller by Thermistor

This circuit energizes the relay when the temperature rises above the preset level. The value of the thermistor is not critical. The important thing is the voltage on pins 5 & 6. Any value thermistor should work satisfactorily. But you may need to change the value of R1 - to achieve the desired range of adjustment.

A simple 7555 buzzer circuit that will activate when a preset temperature is reached. Please note that there is no hysteresis in this circuit, so that if the temperature changes rapidly, then the buzzer alarm may activate rapidly

The Astable multivibrator using the low power CMOS timer IC 7555 which is the low power version of the popular 555 IC. The reset pin 4 of IC1 is used to activate the alarm. The astable will work only if the reset pin 4 becomes high. The reset pin is connected to the positive rail through the 10 K NTC thermister. The thermister offers high resistance in cold and its resistance becomes low to few ohms when the temperature in its vicinity increases. So when the temperature is low reset pin of IC1 remains low and astable is in off position and buzzer remains silent. When the temperature near the thermister increases, its resistance decreases and provides voltage to the reset pin of IC1 and the astable starts working.

This FM wireless microphone transmitter can transmit speech over a short range. It can be used as a simple cordless microphone. This circuit has good frequency stability and has range over 1 Km (under good conditions). This project features RF amplifier buffer (10dB gain), an AF preamplifier to boost the modulation and good microphone sensitivity. You can use it for guitars and remote control system.

This circuit quite simple to build. The two BC547 transistors can be replaced with any small-signal NPN transistor, such as the 2N2222. L1 is 3.25 turns in spiral form and is an integral part of the PCB foil pattern. The final stage is a BC557 PNP general purpose device. If you use different devices then you should select the 1M0 resistor for 5-volts DC at the collector of the first transistor. Select the 47K resistor for 3 – 4 volts on the collector of the third transistor.

 The inductance L is the critical part and should be handmade. Use an AWG24 (0.5mm) enameled copper wire and make 2-4 loose turns with about 4-5mm diameter. Try to loosen or tighten the coil, or trim the variable capacitor, and try to receive the signal on around 90MHz. Don’t forget to tighten the coil using a glue to fix the coil, avoiding mechanical deformation that change its inductance value.

This circuit uses three bipolar transistors to accomplish the Same result (previous post) with the touch contact referenced to the negative or ground end of the supply. The 12volt 20W lamp will be turn on when the contacts are touched with skin resistance about 2M or less. This circuits are available also for other applications, for additional current the lamp could be replace with a 12V relay and diode across the coil.

Since the base of a bipolar transistor draws current and the current gain is usually Less than 200, three transistors are needed to raise the microamp current levels through the touch contacts to a couple amps needed by the light.

Here is a circuits light a 12V/20 watt lamp when the contacts are touched and the skin resistance is about 2 Megs or less. This circuit uses a power MOSFET (IRF510) which turns on when the voltage between the source and gate is around 6 volts. The gate of the MOSFET draws no current so the voltage on the gate will be half the supply voltage or 6 volts when the resistance across the touch contacts is equal to the fixed resistance (2 Megs) between the source and gate.

Drain to Source Voltage        : 100 V
Drain to Gate Voltage          : 100 V
Continuous Drain Current (ID)  : 5.6 A
Pulsed Drain Current (IDM)     : 20 A
Gate to Source Voltage (VGS)   : ±20 V
Maximum Power Dissipation (PD) : 43 W 

When the wiper of the potentiometer is at the uppermost position, the capacitor will charge quickly through both 1K resistors and the diode, producing a short positive interval and long negative interval which dims the lamp to near darkness. When the potentiometer wiper is at the lowermost position, the capacitor will charge through both 1K resistors and the 50K potentiometer and discharge through the lower 1K resistor, producing a long positive interval and short negative interval which brightens the lamp to near full intensity. The duty cycle of the 200 Hz square wave can be varied from approximately 5% to 95%. The two circuits below illustrate connecting the lamp to either the positive or negative side of the supply.