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Recent Posts Testing a UJT Police siren UP DOWN counter circuit Photocell based night light Digital code lock Temperature controlled fan regulator Simple 10W audio amplifier Three transistor audio amplifier TV transmitter circuit

Whistle to beep circuit

Most Commented Long Range FM Transmitter 1 (30) 100 Watt inverter circuit (30) 150 Watt amplifier circuit (22) Infrared motion detector circuit (18) Lead acid battery charger circuit (17) Fire alarm circuit (15) Single Chip FM Radio circuit (14) Automatic LED Emergency Light (12) Remote controll tester circuit (10) Pocket size FM radio station (8) Digital Stop Watch (7) Simple Lamp Dimmer/ Fan Regulator (7) Simple 40W inverter (7) High & Low voltage cut-off with delay& alarm (7) Simple Electronic Combination Lock using IC LS 7220 (6) Melody Generator using IC UM66 (6) Simple Water Level Indicator. (6) Remote controlled appliance switch circuit (6) Low cost fire alarm circuit. (6) UJT RELAXATION OSCILATOR (6) Make PCB using a laser printer-Printed Circuit Boards (6) 9 V regulator using 7809 (5) USB Lamp circuit. (5) Lamp Flasher using NE 555 (5) 2X15 Watt stereo amplifier using TDA4935 (5)

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CIRCUITS TODAY – PROJECT CONTEST-2009

BEST PROJECT will be awarded with an APPLE I-PHONE !

July 23rd, 2009

Testing a UJT By john

UJT (Uni junction transistor) can be easily tested by using a digital multimeter.The three steps for testing the health of a UJT are as follows.

1. Measuring the resistance between B1 and B2 terminals.

Set your digital multimeter in resistance mode.Connect the positive lead of multimeter to the B1 terminal and negative lead to the B2 terminal.The multimeter will show a high resistance ( around 4 to 10K ). Now connect the positive lead to B2 terminal and negative lead to B1 terminal.Again the multimeter will show a high resistance (around 4 to 10K ).Also both the readings will be almost same.

2.Reverse biasing the emitter junction.

Set the digital multimeter in resistance mode.Connect  negative lead of the multimeter to the emitter and positive lead to the B1.The multimeter will show a high resistance (around 100’s of K’s).Now connect the negative lead once again to the emitter and positive lead to B2.Again the meter will show a high resistance.In both cases the reading will be almost same.This test is almost like reverse biasing a diode.

3.Forward biasing the emitter junction.

Set the digital multimeter in resistance mode.Connect the positive lead to the emitter and negative lead to B1.The multimeter will show  a low resistance (around few 100 ohms).Now connect the positive lead once again to the emitter and ngative lead to the B2 terminal.Again the multimeter will show a low resistance reading (around few 100 ohms).In both cases the reading will be almost same.This test is almost like forward biasing a diode.

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Posted in Testing Components | No Comments July 22nd, 2009

Police siren By john

Description.The circuit given here produces an alarm similar to the police siren. When you press the push button switch S2 capacitor C1 will charge and this will make the transistor Q1 to ON slowly. When the switch S1 is released the C1 will discharge and the transistor Q1 will become OFF slowly. When the Q1 is switched ON, its collector voltage falls and makes the transistor Q2 ON. The capacitor C2 will be charged almost to full supply voltage. This results in an increase in the collector-emitter voltage of Q2.This change in voltage is coupled to the base of Q1 via the capacitor C2.As a result the transistor Q1 comes slightly out of saturation. As a result the collector voltage of Q1 drops and makes the Q2 more OFF. This action continues until both transistors become OFF. Then the capacitor C2 discharges, and transistor Q1 will be switched ON again to start a new cycle. When the capacitor C1 is charged the tone will rise and when the capacitor C1 is discharging the tone will fall.Circuit diagram with Parts list.

Notes.

The circuit can be assembled on a vero board. The circuit can be powered from 9V DC. Switch S1 can be used as a power ON/OFF switch. K1 can be an 8 Ohm loud speaker.

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Posted in Bells & Tone generator circuits | No Comments July 21st, 2009

UP DOWN counter circuit By john

Description.This is the circuit diagram of a very simple up down counter that can be used of a large number of applications. The circuit is based on the IC CD40110BE which is a CMOS decade up/down counter. For ICs are used here. Common cathode seven segment display is connected to the 7-segment output of each IC.Display connected to the IC1 represents the lowest number and display connected to IC4 represents the largest number. Synchronous counting is achieved by connecting BORROW and CARRY pins of the preceding stage IC to the CLK DOWN and CLK UP of the next stage IC. For UP counting trigger pulse must be given to the pin7 of IC1 and for down counting trigger pulse must be given to the pin9 of IC1.The RESET pins of all IC are shorted and they have to be connected to ground during normal operation. Connecting the RESET pins to positive supply using the  switch S1 resets the counter.

Circuit diagram with Parts list.

Notes.

The circuit can be powered from a 9V PP3 battery. All 7 segment displays must be common cathode type. Resistors R1 to R28 must be 680 Ohm ones. R29 and R30 can be 10K ones. Switch S1 can be  a SPDT switch.

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Posted in Counter Circuits | No Comments July 9th, 2009

Photocell based night light By john

Description.Many automatic night light circuits had been published here. This one uses a photocell for detecting the light intensity. At full light the resistance of the photocell will be few ten ohms and at darkness it will rise to several hundred ohms. The IC1 uA741 is wired as a comparator here. At darkness the resistance of photocell increases and so the voltage at the inverting input of the IC1 will be less than the reference voltage at the non inverting input. The output of the IC1 goes to positive saturation and it switches ON the transistor to activate the relay. By this way the lamp connected through the relay contact glows. The diode D1 works as a freewheeling diode.

Circuit diagram.

Notes.

The circuit can be assembled on a Vero board. Use 9V DC for powering the circuit. POT R7 can be used to adjust the sensitivity of the circuit. The relay K1 can be a 9V, 200 Ohm SPDT type. L1 can be a 230V,60W lamp. R8 can be a ORP 12 photocell.

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Posted in Light Related, Lighting Circuits | 1 Comment July 9th, 2009

Digital code lock By john

Description.This is a simple but effective code lock circuit that has an automatic reset facility. The circuit is made around the dual flip-flop IC CD4013.Two CD 4013 ICs are used here. Push button switches are used for entering the code number. One side of all the push button switches are connected to +12V DC. The remaining end of push buttons 2,3,6,8 is connected to clock input pins of the filp-flops. The remaining end of other push button switches are shorted and connected to the set pin of the filp-flops.The relay coil will be activated only if the code is entered in correct sequence and if there is any variation, the lock will be resetted. Here is correct code is 2368.When you press 2 the first flip flop(IC1a) will be triggered and the value at the data in (pin9) will be transferred to the Q output (pin13).Since pin 9 is grounded the value is “0” and so the pin 13 becomes low. For the subsequent pressing of the remaining code digits in the correct sequence the “0” will reach the Q output (pin1) of the last flip flop (IC2b).This makes the transistor ON and the relay is energised.The automatic reset facility is achieved by the resistor R11 and capacitor C2.The positive end of capacitor C2 is connected to the set pin of the filp-flops.When the transistor is switched ON, the capacitor C2 begins to charge

and when the voltage across it becomes sufficient the flip-flops are resetted. This makes the lock open for a fixed amount of time and then it locks automatically. The time delay can be adjusted by varying the values of R11 and C2.

Circuit diagram with Parts list.

Notes.

Assemble the circuit on a good quality PCB. The circuit can be powered from 12V DC. Mount the ICs on holders. The L1 can be a 12V, 200 Ohm SPDT relay. Capacitor C1 should be tantalum type. The C1 and C2 must be rated at least 25V.

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Posted in Electronic Keys & locks | No Comments July 8th, 2009

Temperature controlled fan regulator

By john

Description.This fan regulator circuit will automatically control the speed of your fan according to the temperature. Two thermistors (R1 and R2) are used to sense the temperature. The circuit works almost like the published here previously. Here the potentiometer is replaced by the thermistors. When the temperature is increasing the resistance of NTC thermistor ( R1) decreases and at the same time the resistance of PTC thermistor (R2) increases. At the same time, rate of change of the resistance will be different for R1 and R2.This action

is similar to a potentiometer used in a conventional Triac based fan regulator. When the resistance is varied the firing angle of the triac changes and so do the speed of the fan.

Circuit diagram.

Notes.

The circuit can be assembled on a Vero board. L1 can be a 70uH inductor. C1 and C2 must be rated at least 200V. C3 must be rated at least 600V. Triac BT136 can safely handle up to 4A load current. The two thermistors must be mounted as close as possible.

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Posted in Power Controller Circuits | 1 Comment July 8th, 2009

Simple 10W audio amplifier

By john

Description.The circuit given below is of a simple audio amplifier that can deliver 12W to an 8 Ohm speaker. Op amp IC TL081 is used as the preamplifier here. Actually any opamp with matching power supply ratings can be used instead of TL081.Transistors Q1 and Q2 (TIP125 and TIP120) comprises the power amplifier stage. Output is taken from the collector junction of two transistors.

Circuit diagram.

Notes.

The circuit can be assembled on a Vero board. Heat sinks are recommended for the transistors. A +15V/-15V dual DC power supply can be used to power the circuit. It is better to mount IC1 on a holder.

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Posted in Audio Circuits, Power Supplies | No Comments July 8th, 2009

Three transistor audio amplifier

By john

Description.This is the circuit diagram of a simple three transistor audio amplifier that can deliver around 100mW power to a 25 Ohm speaker. The diodes D1 and D2 provides a constant bias voltage for the transistors Q1 and Q2.The transistor Q1 works as a preamplifier. Transistors Q2 and Q3 drives the speaker. The type no of the transistor is not very crucial here. You can use any NPN transistor for Q1, Q3 and any PNP transistor for Q2.Any way the minimum collector current capacity of the transistors must not be less than 100mA.The circuit will work well with an 8 ohm speaker too, but the volume will be a little less.

Circuit diagram.

Notes.

The circuit can be assembled on a Vero board. A 9V PP3 battery can be used to power the circuit. Loud speaker K1 can be a 25 Ohms one. Capacitor C1 and C2 must be rated at least 15V.

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Posted in Audio Circuits | No Comments July 2nd, 2009

TV transmitter circuit

By john

Description.The TV transmitter given here uses UK standard 1 FM modulation for sound and PAL for video modulation. The audio signal to be modulated is pre-amplified using the transistor Q1 and associated components. The transistor Q2 has two jobs: production of carrier frequency and modulation. The pre-amplified audio signal is fed to the base of transistor Q2 for modulation. Capacitor C5 and inductor L1 forms the tank circuit which is responsible for producing the carrier frequency. The video signal is fed to the emitter of transistor Q2 via POT R7 for modulation. The modulated composite signal (audio+video) is transmitted by the antenna A1.

Circuit diagram with Parts list.

Notes.

Assemble the circuit on a good quality PCB. Inductor L1 can be made by making 4 turns of 24SWG enameled copper wire on

a 6mm dia: plastic former. T1 can be a radio frequency transformer with built in capacitor. (Can be found on

old transistor radio boards). Antenna A1 can be a 1M long copper wire. (Experiment with the length to get

optimum performance). This transmitter is working in VHF band somewhat between 50 – 210MHz. This transmitter is compatible only with PAL B and PAL G systems.

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Posted in Television Related, Transmitters | 5 Comments June 30th, 2009

Whistle to beep circuit

By john

Description.This simple circuit produces a beeping sound that lasts for around 3 seconds whenever you make a whistle. The CMOS Hex inverter CD4049 is the heart of this circuit. Out of the six inverters in CD4049, U1a is wired as an audio amplifier which amplifies the signal picked up by the microphone M1.The U1b is wired as a band pass filter with center frequency around 2KHz.The filter is necessary in order to pass the frequency corresponding to whistling sound and suppress all other frequencies .If the filter is not there, the circuit could easily get false triggered.U1d is wired as a 3S delay monostable multivibrator.The output U1d drives the astable multivibrator formed by U1e and U1f.The astable multivibrator is operating around 4Hz.The combined effect is a intermittent beeping sound that lasts for around 3S.Transistor Q1 is used to drive the buzzer B1.

Circuit diagram with Parts list.

Notes.

Assemble the circuit on a good quality PCB. The circuit can be powered from a 3V battery. IC U1 is a CMOS CD4049 Hex inverter. M1 can be an electret microphone. B1 can be a 3V piezo buzzer. Mount the IC on a holder. The duration of beeping can be adjusted by varying the components C4 and R9.

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Posted in Bells & Tone generator circuits, Fun & Game | No Comments « Previous Entries |

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Single Chip FM Radio circuit

By admin

Description.

Here is a compact low cost FM radio circuit using IC7400. This circuit is designed as per the data sheet and the result is excellent.Ideal for all category of electronic enthusiasts.

The TDA7000 is a monolithic integrated circuit for mono FM portable radios, where a minimum on peripheral components is crucial. The IC TDA 7000 has a Frequency-Locked-Loop system with an intermediate frequency of 70 kHz. The intermediate frequency selectivity is achieved by active RC filters. The only function which needs alignment is the resonant circuit for the oscillator, thus selecting the reception frequency. Spurious reception is avoided by means of a mute circuit, which also eliminates too noisy input signals. Special steps are taken to meet the radiation requirements.

Circuit Diagram with Parts List.

Notes

For L1 and L2 wind 5 turns of 0.6 mm enameled Copper wire on a 4 mm dia plastic former.

For antenna use a 50mm long insulated copper wire. IC TDA 7000 can withstand up to 10 V supply voltage.But I recommend 6V. Use an 8 Ohm speaker or Headphone at the audio output.