FIRE ALARM CIRCUIT

1. Here is a simple fire alarm circuit based on a LDR and lamp pair for sensing the fire.The alarm works by sensing the smoke produced during fire.The circuit produces an audible alarm when the fire breaks out with smoke.
2. When there is no smoke the light from the bulb will be directly falling on the LDR.The LDR resistance will be low  and so the voltage across it (below .6V).The transistor will be OFF and nothing happens.
3. When there is sufficient smoke to mask the light from falling on LDR, the LDR resistance increases and so do the voltage across it.Now the transistor will switch to ON.
4. This gives power to the IC1 and it outputs 5V.This powers the tone generator IC UM66 (IC2)  to play a music.This music will be amplified by IC3 (TDA 2002) to drive the speaker.
5. Resistor R6 is meant for protecting the transistor when R4 is turned towards low resistance values .Resistor R2 and R1 forms a feedback network for the TDA2002 and C1 couples the feed back signal from the junction of R1 & R2 to the inverting input of the same IC.
6. The diode D1 and D2 in combination drops 1.4 V to give the rated voltage (3.5V ) to UM66 .UM 66 cannot withstand more than 4V.

NOTES:

• The speaker can be a 32Ω tweeter.
• POT R4 can be used to adjust the sensitivity of the alarm.
• POT R3 can be used for varying the volume of the alarm.
• Any general purpose NPN transistor(like BC548,BC148,2N222) can be used for Q1.
• The circuit can be powered from a 9V battery or a 9V DC power supply.
• Instead of bulb you can use a bright LED with a 1K resistor series to it.

LOW COST SIMPLE AM RECEIVER CIRCUIT

1. A simple AM receiver circuit based on the MK484 AM receiver IC from Rapid Electronics Ltd is shown here. MK4844 is a monolithic integrated circuit that has all the necessary sections of a AM receiver like RF amplifier, detector, AGC etc. The IC is available in TO92 package and requires very few external components.
2. In the circuit L1 and C1 forms a tank circuit. L1 also serves as the antenna coil. The audio signals available at the pin 3 of IC1 are amplified using the amplifier built around transistor Q1.
3. C2 is a bypass capacitor while C3 is a DC decoupler. Capacitor C4 provides negative feedback while R4 biases the transistor Q1.

NOTES:

• Assemble the circuit on a vero board.
• Use a 1.5V cell for powering the circuit.
• Never give more than 2V to the circuit. It will destroy the IC.
• The MK484 can be operated over a range of 150 KHz to 3 MHz.
• For L1 make 90 turns of 36SWG enamelled copper wire on a 1cm diameter card board former. Experiment with thenumber of turns for getting optimum performance. A little trial  and error is needed.

DEW SENSOR CIRCUIT

1. Here is a simple circuit that can be used to switch ON or OFF a device when the dew present in the surrounding atmosphere crosses a set value.The circuit uses a dew sensitive resistive element and a comparator based on LM 358 to perform the above said operation.
2. At normal condition the resistance of dew sensor element will be low and so the voltage drop across it.So the voltage at the non inverting pin of LM358 (IC1) will be less than the voltage at the inverting input of the LM358
3. So the output of the opamp will be low.This keeps the opto-coupler (MCT2E) deactivated.When the dew increases the resistance of the element increases and so do the voltage across it.
4.  Now the voltage at the non inverting pin of LM358 (IC1) will be higher than the voltage at the inverting input of the LM358.So the output of the op amp will be switched to high.
5. This in turn activates the optocoupler.The LED  glows to indicate it. As a result we get an optopcoupler activated and de activated according to the amount of dew in the atmosphere.The output pins of optocoupler pin (5&4) can be used to control the external device.

NOTES:

• The dew sensor is hard to find in market.But it can be easily obtained from a old VCR.Also the type no of the sensor is not so important here.Try with any thing you get.I used one from a old Hitachi VCR.
• LM 358 is a dual opamp.Here only one opamp inside it is used.

ONE TRANSISTOR CODE LOCK

1. This is of course the simplest electronic code lock circuit one can make. The circuit uses one transistor, a relay and few passive components. The simplicity does not have any influence on the performance and this circuit works really fine.
2. The circuit is nothing but a simple transistor switch with a relay at its collector as load. Five switches (S0 to S4) arranged in series with the current limiting resistor R2 is connected across the base of the transistor and positive supply rail.
3. Another five switches (S5 to S9) arranged in parallel is connected across the base of the transistor and ground. The transistor Q1 will be ON and relay will be activated only if all the switches S0 to S4 are ON and S5 to S9 are OFF.
4. Arrange these switches in a shuffled manner on the panel and that it. The relay will be ON only if the switches S0 to S9 are either OFF or ON in the correct combination.  
5. The device to be controlled using the lock circuit can be connected through the relay terminals. Transformer T1, bridge D1, capacitor C1 forms the power supply section of the circuit. Diode D2 is a freewheeling diode. Resistor R1 ensures that the transistor Q1 is OFF when there is no connection between its base and positive supply rail.

NOTES:

• This circuit can be assembled on a Vero board.
• Switch S1 is the lock’s power switch.
• The no of switches can be increased to make it hard to guess the combination.
• Transistor 2N2222 is not very critical here. Any low or medium power NPN transistor will do the job.

MAGNETIC PROXIMITY SWITCH

1. Here is the circuit diagram of a magnetic proximity switch that finds a lot of applications in many fields.
2. The circuit is based on a magnetic reed switch(S1) as the proximity sensor.
3.  A monostable multivibrator based on NE555 (IC1) and a toggle flip flop based on CD4013 (IC2) does the rest of the circuit.
4.  When a magnet is reached in proximity of S1 it closes to give a negative trigger at pin 2 of IC1.The output of IC1 goes high for a time determines by R2 and C2.
5.  This clocks the IC2 wired as a toggle flip flop.The output (pin 1 ) of IC2 goes high and the transistor Q1 is biased to ON.
6.  This clocks the IC2 wired as a toggle flip flop.The output (pin 1 ) of IC2 goes high and the transistor Q1 is biased to ON.

NOTES:

• Switch S1 can be a general purpose magnetic reed switch.
• The equipment to control can be connected using NC,NO and C points of the relay according to the application.
• Use a 12 regulated power supply for powering the circuit.

BATTERY ELIMINATOR CIRCUIT

1. Here is the circuit diagram of a battery eliminator circuit that can be used as a replacement for 9V PP3 batteries.
2. The circuit given here can be used to power any device that operates from a 9V battery. The transformer T1 steps down the mains voltage and bridge D1 performs the job of rectification.
3.  Capacitor C1 is a filter. IC LM317T is the regulator here.
4.  The value of R1, R2 and R3 are so selected that the output voltage of IC1 will be steady 9 volts.

NOTES:

• Assemble the circuit on a good quality PCB.
• Transformer T1 can be a 230V primary, 9V secondary, 1.5A step down transformer.
• If 1A Bridge is not available, then make one using four 1N 4007 diodes.
• Do not connect loads that consume more than 1.5A to this circuit.
• A heat sink is recommended for IC1.

3 LED BATTERY MONITOR

1. This is the circuit diagram of a 3 LED bar graph type battery monitor circuit that is ideal for monitoring the voltage level of an automobile battery.
2. When battery voltage is 11.5V or less transistor Q1 will be On and the LED D1 will be glowing.
3.  When battery voltage is between 11.5 and 13.5V, the transistor Q2 will be On and the LED D2 will be glowing.
4.  When battery voltage is above 13.5V the transistor Q3 will be On and the LED D7 will be glowing.

NOTES:

• Assemble the circuit on a general purpose PCB.
• The battery to be monitored can be connected between the terminals namely A and B.
• It is always better to use LEDs of different colour.

12V-15A VOLTAGE REGULATOR CIRCUIT

1. Here is the circuit diagram of a powerful 12V regulator that can deliver up to 15 A of current.
2. The common voltage regulator IC 7812(IC1) is used to keep the voltage at steady 12V and three TIP 2599 power transistors in parallel are wired in series pass mode to boost the output current.
3. The 7812 can provide only up to 1A and rest of the current is supplied by the series pass transistors.
4. The 15A bridge B1 does the job of rectifying the stepped down AC input. The capacitor C1, C2 and C3 act as filters.
5. The 1A fuse F1 protects the IC1 from over current in case if the pass transistors fail. The 15A fuse F2 protects the entire circuit (especially the pass transistors) from over current.

NOTES:

• Assemble the circuit on a good quality PCB.
• The T1 can be a 230V AC primary, 18V secondary, 15A type transformer.
• The B1 can be a 15A bridge.
• If 15A Bridge is not available, make one using four RURG1520CC diodes.
• The IC1 and transistors must be mounted on heat sink.

CAR STEREO AMPLIFIER CIRCUIT

1. Here is the circuit of a car stereo amplifier based on TDA1553. TDA1553 is a monolithic Class-B audio amplifier which contains 2 x 22 watt amplifiers in bridge tied load configuration.
2. The amplifier operates from 12V DC and is developed intentionaly for car audio applications.
3. The IC also has a load of good features like short circuit protection, load dump protection, reverse polarity protection, loud speaker protection etc.
4. In the circuit , C5 and C4 are input decoupling capacitors while C3 sets the delay time for loud speaker protection.
5. C1 and C2 are power supply filter capacitors.

   
   

   
   

   
   

   
   

   NOTES:
   

   • Assemble the circuit on a good quality PCB.
   • Use 12V DC for powering the circuit.
   • The circuit can deliver 22W per channel into 4 ohm speakers.
   • Fit the IC with a proper heat sink.

   
   

AUDIO LEVEL METER CIRCUIT

1. A simple and low cost audio level meter circuit that can be used to measure the audio level of your sound source.This circuit is a valuable tool for those who are interested in audio circuits.
2. The circuit is designed with a flat frequency response in the range of  20Hz to  50Khz.
3. nput sensitivity is 100mV for a FSD on a 100uA ammeter.The circuit is build based on on two common emitter amplifiers, the first stage has a preset resistor R3 (1K) which may be adjusted for a FSD. The last stage is biased to operate at about 1/2  the supply voltage for maximum AC voltage deflection.
4. C2&C2  (10 uF) acts as a filters through which audio frequencies  are passed .  The full wave bridge rectifier converts the signal to a varying dc voltage
5. The meter will show this voltage as the output reading which is proportional to the input voltage level.

   
   

NOTES:

• The meter reading is instantaneous and it will not provide you with a peak to peak reading.
• To calibrate the meter, provide a  1Khz 100mV sine wave at the input and  adjust R3 to get a full scale reading on the meter.
• Use a well regulated & filtered 12 V supply for powering the circuit.

LED RAMPING CIRCUIT

1. In this circuit the intensity of LED will vary in a ramping fashion. 
2. The circuit consists of three ICs: Two 555 timer ICs and one LM393 op-amp.
3. IC1 and IC2 are wired as oscillators to produce 10 KHz and 1 Hz frequencies respectively.
4. These two frequencies are given to the inputs of the op-amp LM393.LM393 is wired as a comparator and its output will be a PWM signal
5. This PWM signals controls the FET Q1 to drive the LED. The LED will rise from OFF state to full brightness slowly and then slowly fades to OFF state and this operation repeats.
6. The resistor R4 controls current through the LED.

   
   

NOTES:

• Assemble the circuit on a general purpose PCB.
• The ICs must be mounted on holders.
• The power supply for this circuit can be anything between 3 to 12V.
• I recommend 6V for this circuit.
• The effect can be varied by changing the values of C1, C2, R1 or R2.

SINGLE TRANSISTOR AUDIO MIXER

1. This very simple audio mixer circuit uses only one transistor.
2. The base emitter junction of the transistor is biased by the diodes D1 and D2.
3.  The signals to be mixed are directly coupled to the base of Q1.
4. Each input lines are current limited by using a 33K resistor. With the used component values the collector current is around 1mA.
    
    
    
    
   NOTES:
   

   • The circuit can be assembled on a Vero board.
   • The circuit can be powered from 15V DC.
   • Gain of the individual channels can be adjusted by adding a 10K POT in series to the lines.

ELECTRONIC MOSQUITO REPELLER

1. Here is the circuit diagram of an ultrasonic mosquito repeller.
2. The circuit is based on the theory that insects like mosquito can be repelled by using sound frequencies in the ultrasonic (above 20KHz) range.
3. The circuit is nothing but a PLL IC CMOS 4047 wired as an oscillator working at 22KHz.
4. A complementary symmetry amplifier consisting of four transistor is used to amplify the sound.
5. The piezo buzzer converts the output of amplifier to ultrasonic sound that can be heard by the insects.

   
   
   
   
   
   
   
   
   NOTES:
   

   • Assemble the circuit on a general purpose PCB.
   • The circuit can be powered from 12V DC.
   • The buzzer can be any general purpose piezo buzzer.
   • The IC1 must be mounted on a holder.

SIMPLE SOUND GENERATOR CIRCUIT

1. This is a real scream generator circuit suitable for any purpose like alarm or car horn.
2.  The circuit is based on two transistors Q1 and Q2.
3. When you press the switch S2 the siren starts up moving to a high frequency.
4. When the switch is released the tone slips down until you shift it up again by pushing the switch S2.

   NOTES:

• Adjustment of tone quality can be obtained by different values for C2.
• If the alarm oscillates before S2 is pressed.The transistor is leaky, replace it.
• S1 can be used as a power switch.

FAN SPEED CONTROLLER USING LM2941

1. The circuit diagram shown here is of 12V DC fan speed controller using the IC LM2941CT which is a low drop out 1A voltage regulator.
2. The IC has a dropout voltage as low as 0.5 and has also many useful features like power supply reverse protection, thermal protection, short circuit protection etc. The maximum output current the IC can source is 1A.
3. The 12V DC supply is connected between the Vin (pin4) and ground (pin3) of the IC.
4. he load, which is the fan, is connected across the Vout (pin5) and ground (pin3) of the IC. The network comprising of potentiometers R1, R2 and resistor determines adjust current (Iadj) of the IC. By varying the Iadj using the POT R2 we can adjust the output voltage of the IC and hence the fan speed.
    
    
    
    
   NOTES:

   • The circuit can be powered from 12V DC.
   • The maximum possible load current is 1A.
   • A heat sink is recommended for the IC.
   • POT R1 can be used to adjust the minimum fan speed.
   • POT R2 can be used to adjust the fan speed.
   
   
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
   
   

   
   

NIGHT SECURITY LIGHT

1. Here is a simple circuit switches on a light around 2 hours after midnight, the time at which most of the robberies taking place.
2. This simple circuit is build around a CMOS IC 4060 to obtain the required timing.
3.  During day time the LDR has low resistance and keeps the pin 12 of the IC1 high, preventing the IC1 from oscillating. When it is dark the LDR resistance becomes high and the pin 12 of IC1 becomes low and the IC starts oscillating, which indicated by the flashing of LED D3.
4. The values of the timing components R1, R2, C4 are so selected that the out put pin3 of IC1 goes high after 8 hours.
5.  That means the high output drives the triac to switch on the lamp around 2’O clock. At morning, the LDR resistance drops and the pin 12 of IC1 goes high and stops the oscillation, making the lamp OFF. The switch S1 can be used to manually ON the lamp. The capacitor C2 prevents false triggering.
   
   

NOTES:

• Assemble the circuit on a good quality PCB or common board.
• The LDR can be general purpose LDR.
• The light sensitivity can be adjusted using the preset R6.
• The IC1 must be mounted on an IC holder.

MISSING PULSE DETECTOR CIRCUIT USING NE 555

1. An NE555 timer IC connected as shown here can detect a missing pulse or abnormally long period between two consecutive pulses in a train of pulses. Such circuits can be used to detect the intermittent firing of the spark plug of an automobile or to monitor the heart beat of a sick patient.
2. The signal from the pick up transducer is shaped to form a negative going pulse and is applied to pin 2 of the IC which is connected as a mono stable. 
3. As long as the spacing between the pulse is less than the timing interval,the timing cycle is continuously reset by the input pulses and the capacitor is discharged via T1.
4. A decrease in pulse frequency or a missing pulse permits completion of time interval which causes a change in the output level.

   
   

NOTES:

• Assemble the circuit on a good quality PCB or common board.
• The circuit can be powered  from a 9V battery or 9V DC power supply.
• The IC1 NE555 could be mounted on a holder.

SPEAKER TO MICROPHONE CONVERTER CIRCUIT

1. This circuit is a simple approach for converting a loud speaker into a microphone.
2. When the sound waves fall on the diaphragm of a speaker, there will be fluctuations in the coil and there will be a small proportional induced voltage. Usually this induced voltage is very low in magnitude and useless. Here in the circuit the low voltage is amplified using transistors to produce a reasonable output.
3.  The transistor Q1 is wired in common base mode and produces the required voltage gain. The transistor Q2 is wired as an emitter follower to produce enough current gain.
4. The voice quality of this circuit will not be as much as a conventional microphone but quite reasonable quality can be obtained. To set up the circuit, keep the preset R2 at around 10 Ohms and connect the battery. Now adjust R2 to obtain the optimum sound quality.
   
   

NOTES:

• Assemble the circuit on a general purpose PCB.
• Power the circuit from a 9 V PP3 battery.
• A 3 inch speaker can be used as K1.
• All capacitors must be rated at least 15V.
• An 8 Ohm speaker or head phone can be connected at the output to hear the picked sound.

MICRO AMPERE METER CIRCUIT USING uA 741

1. The micro ampere meter shown here is basically a DC millivolt meter.The circuit gives full scale deflection for 0.1V input.The current to be measured is passed through a known resistance R and the voltage drop across it is measured
2. Here the IC 1 uA 741 op-amp is wired as a non inverting amplifier.

   
   

NOTES:

• Assemble the circuit on a good quality PCB or common board.
• Use a +9V/-9V dual power supply for powering the circuit.
• The IC 1 ,uA 741 must be mounted on an IC base.
• The table given below shows the relationship between different values of R and the current that will give full scale deflection.

METAL DETECTOR CIRCUIT

1. This is the circuit diagram of a low cost metal detector using a single transistor circuit and an old pocket radio.This is nothing but colpitts oscillator working in the medium band frequency and a radio tuned to the same frequency.
2. First the radio and the circuit are placed close.Then the radio is tuned so that there is no sound from radio.
3. In this condition the radio and the circuit will be in same frequency and same frequencies beat off to produce no sound.
4.  When the metal detector circuit is placed near to a metal object the inductance of its coil changes , and so do the frequency of oscillations.Now the two frequency will be different , there will be no canceling and radio produces a hissing sound.The metal is detected.

NOTES: 

• To make L1 make 60 turns of 36SWG enameled Copper wire on a 1 cm PVC tube.

• Powering the circuit using a adapter rather than a battery induces noise. It is always good to power radio projects from battery.