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.

15WATT CLASS-B AMPLIFIER

1. The circuit shown here is of a simple Class-B audio amplifier based on opamp TL082, transistors TIP41 and TIP42. LM833 is a dual opamp with high slew rate and low distortion particularly designed for audio applications.
2. This audio amplifier circuit can deliver 15 watt audio output into an 8 ohm speaker at +12/-12V DC dual supply.
3. Both opamps in the IC are used here. IC1a is wired as a buffer and capacitor C3 does the job of input DC decoupling.
4. Ic1b is wired in the inverting mode and it provides negative feedback.
5. Complementary power transistors TIP41 and TIP42 are wired in the Class B push pull scheme and they drives the loud speaker.
6. Diode D1 provides 0.7V bias voltage for the push pull pair and capacitor C2 protects the 0.7V bias voltage across D1 from heavy voltage swings at the IC1b’s output.
   
   
   
   
   
   
   NOTES:
   

   • The audio amplifier circuit must be assembled on a good quality PCB.
   • Use a holder for mounting IC1.
   • Use a +12/-12V dual supply for powering the amplifier.
   • Potentiometer R2 can be used as a volume control.
   • Raising the power supply voltage will increase the output power. Anyway note the following points.
   • TIP42 and 41 can handle only up to 6A.
   • Maximum supply voltage IC1 can handle is +16/-16 V DC.

   
   

BRIDGE AMPLIFIER USING TDA4935

1. TDA4935 is 2x15W high quality audio amplifier IC from Siemens.
2. The IC can be used in stereo mode or bridge mode.
3. In stereo mode it can deliver 15W per channel and in bridge mode it can deliver 30W into a 8 ohm load at 30V supply.
4. TDA4935 requires very few external components and has a wide power supply voltage range.
5. The IC operates in class B mode and has built-in circuitry for over temperature protection and overload protection.

NOTES:

• The circuit must be assembled on a good quality PCB.
• TDA 4935 must be fitted with a proper heat sink.
• The supply voltage can be anything between 8 to 30V DC.
• Capacitors C1, C2, C8 are polyester capacitors.
• Capacitors C3, C4 and C6 are ceramic capacitors while C5 and C9 are electrolytic.

POCKET HEADPHONE AMPLIFIER

1. Here I present a very simple and powerful headphone amplifier using OPA134. In addition to the IC OPA134, the circuit uses only few passive components and can easily generate a lot of sound from even the most inefficient headphones and there will be no compromise for the quality.
2. OPA134 is low noise, low distortion operational amplifier from BURR-BROWN semiconductors and it is exclusively meant for audio applications. The FET based input stage provides the IC with high input impedance and it makes the circuit very flexible in terms of the audio source.
3. You can plug almost all types of sound sources like, mp3 players, iPods, mobile phones etc to the circuit.
   In the circuit IC OPA134 is wired as a non-inverting amplifier. The +/-4.5V split power supply required for the IC is obtained from a 9V PP3 battery using the circuit comprising of components D1. R6, R7.R8, C2 and C3. D1 is an LED which indicates power ON.
4. Switch S1 can be used as a ON/OFF switch .Resistor R2 and capacitor C1 forms a high pass filter with corner frequency around 15KHz.
5. POT R1 can be used as a volume controller. The load resistor R5 will stabilize the virtual ground and prevents any noise or distortion in the output, but the output will be DC coupled.

   
   

   
   

   
   

   
   

   NOTES:-
   

   • Assemble the circuit a good PCB.
   • The circuit can be powered using a 9V PP3 battery.
   • POT R1 can be used as a volume controller.

   
   

TEMPERATURE CONTROLLED LEDS

1. The circuit is nothing but two LEDs (D1 and D2), whose status are controlled by the temperature of the surroundings.
2. The famous IC LM35 is used as the temperature sensor here. Output of LM35 increases by 10mV per degree rise in temperature. Output of LM35 is connected to the non inverting input of the opamp CA3130. 
3. The inverting input of the same opamp can be given with the required reference voltage using POT R2. If the reference voltage is 0.8V, then the voltage at the non inverting input (output of LM35) becomes 0.8V when the temperature is 80 degree Celsius.
4. At this point the output of IC3 goes to positive saturation. This makes the transistor Q1 On and LED D1 glows.
5. Since the base of Q2 is connected to the collector of Q1, Q2 will be switched OFF and LED D2 remains OFF. When the temperature is below 80 degree Celsius the reverse happens.IC1 produces a stable 5V DC working voltage from the available9V DC supply. If you already have a 5V DC supply then you can use it directly.
   
   
   
   
   
   
   NOTES:-
   

   • The circuit can be assembled on a Vero board.
   • IC3 must be mounted on a holder.
   • The temperature trip point can be set by adjusting POTR2.
   • Type no of Q1 and Q2 are not very critical. Any general purpose NPN transistors will do it.

HUMIDITY TESTER

1. A variety of humidity tester circuits are available, but this  is a circuit which is as simple as possible.Using only a transistor ,LED and few resistors ,this circuit can be used to check the humidity level of materials like soil , paper etc. 
   
2. When the humidity in a substance increases the current conducted through  it also increases .
3.  This is the working principle.If there is required humidity, the current through R3 will be sufficient to produce a voltage drop across R3 which is sufficient enough(0.7V)  to switch on the transistor and LED glows.R1 is the current limiting resistor for LED.R1 protects the transistor from accidental shorting of the probes.

   
   

   
   

   
   

   NOTES:-
   

   • The probes can be anything like iron nail.pin paper clip etc.
   • To calibrate the circuit for particular density,insert the two probes to the required material and adjust R3 so that the LED glows.The LED will glow whenever the humidity of the testing sample becomes equal to this humidity level.
   • A 3V battery can be used to power the circuit.
   • Another general purpose NPN transistors like BC107,BC148,2N2222 etc can be also used for Q1.

   
   

USB LAMP CIRCUIT

1. Here is a simple USB powered lamp that can be used to light your desktop during power failures. The circuit operates from the 5 Volt available from the USB port.
2.  The 5V from the USB port is passed through current limiting resistor R2 and transistor Q1. The base of transistor Q1 is grounded via R1 which provides a constant bias voltage for Q1 together with D2.
3. The diode D1 prevents the reverse flow of current from battery.C1 is used as a noise filter.Two white LED’s are used here for the lamp, you can also use a 2 V torch bulb instead of LED’s. LED D3  indicates connection with USB port.
    

 

 

NOTES:-

• USB port is only able to provide up to 100 mA current.So don’t overload the circuit with more no of LED’s.
• Before wiring the circuit confirm the positive and ground leads of USB by a multimeter.
• Switch S1 can be used to turn on the lamp.

MULTI CHANNEL AUDIO MIXER USING LM 3900

1. A simple multi-channel audio mixer circuit using LM3900 quad amplifier is given below.
2. The circuit consists of 4 channel quad amplifier (LM3900). Two mic audio inputs and two direct line     inputs are available in this circuit.
3.  By adding the same circuit parallel with this, you can increase the number of inputs according to the applications. Each input is connected to the inverting terminal of LM3900.
4. The built in amplifier of each section amplifies every audio input separately and is fed to the output terminals. The output terminal from each channel is connected to a single output line with a resistance not greater than 680K and produces a mixed audio at the output with very low noise.
5.  This audio mixer circuit doesn’t use a low impedance input to mix ideal sources. Capacitors C1 to C4 are the decoupling capacitors for the corresponding channels. C5 is the output decoupling capacitor.
    
    
    

   The main features of this audio mixer circuit as compared to the other audio mixers are given below.

   1)      Wide range of supply voltage input (4volt to 32volt)

   2)      Dual voltage supplies also adoptable (+/- 2.2volt to +/-16volt)

   3)      Low input biasing current(30nA)

   4)      Providing very high open loop gain(70dB)

   5)      Output short circuit protection

   6)      Simple and compatible design

   7)      Low distortion

   8)      Good frequency response

   
   

   
   

   NOTES:- 

   • Assemble the circuit on a good quality PCB.
   • The circuit can be powered from anything between 5 to 30V DC.
   • The power supply must be well regulated and free from any sort of noise.
   • LM3900 must be mounted  on a holder.
   • VR1 to VR4 can be used for adjusting the volume level of the corresponding channels.
   • All fixed resistors are 1/4 watt carbon film type.
   • If the power supply circuit is far from the mixer circuit, then a 100uF/50V electrolytic capacitor must be connected from the positive supply rail to the ground.