3 INPUT MICROPHONE PREAMPLIFIRE

1. The circuit given here is of a three input mic mixer cum preamplifier using the IC LM348. The LM348 is a high gain, internally compensated quad operational amplifier with a class AB output stage.
2. The IC has very low input supply current drain (0.6mA/ opamp) and operates from a dual power supply.
3. Out of the four op-amps inside the IC LM348, IC1a, IC1b and IC1c are wired as non inverting amplifiers and they serve as the input amplifiers for the corresponding mic channels. 
4. The output of these three amplifiers are tied together and connected to the inverting input of the IC1a which is wired as an inverting amplifier.IC1a mixes the signals from each channel and also works as the output stage.
   
   
   
   
   
   
   
   
   NOTES:
   

   • Assemble the circuit on a good quality PCB.
   • Use +12/-12V DC dual supply for powering the circuit.
   • The power supply must be well regulated and filtered.
   • POT R5 to R7 can b e used for adjusting the gain of individual channels.

   
   
   
   

STEREO PREAMPLIFIER WITH TONE CONTROL

1. Here is the circuit diagram of an excellent stereo preamplifier with tone control using the IC TDA1524 from Phillips.
2. The IC requires very few external components, has very low noise and has a wide power supply voltage range.
3. POTs R1 to R4 can be used for controlling the volume, balance, bass and treble respectively.
   
4. 
   
   LED is D1 is a power ON indicator and R1o is its current limiting resistor.

   
   

   
   

   
   

   
   

   NOTES:
   

   • Assemble the circuit on a good quality PCB.
   • Switch S2 is the ON/OFF switch.
   • Swich S1 can be used to select linear or contour mode.
   • Supply voltage can be anything between 8 to 16V .Here I used 12V DC.

   
   

   
   

DARK DETECTOR

1. The dark detector circuit shown here can be used to produce an audible alarm when the light inside a room goes OFF.
2. The circuit is build around timer IC NE555.A general purpose LDR is used for sensing the light. 
3.  When proper light is falling on the LDR its resistance is very low.
4. When there is no light the LDR resistance increases.
5.   At this time the IC is triggered and drives the buzzer to produce an alarm sound.
6.  If a transistor and relay is connected at the output (pin3) of IC1 instead of the buzzer, electrical appliances can be switched according to the light.
   
   
   
   
   
   
   NOTES:
   

   • The LDR ,R4 can be any general purpose LDR.
   • The circuit must be assembled on  a good quality PCB or  common board.
   • The circuit can be powered from a 9V PP3 battery.
   • The POT,R3 can be sued as a volume controller.
   • Mount the IC1 on a holder.It will make replacements easy.

   
   

PARKING SENSOR CIRCUIT

1. This simple circuit can be used as an aid for sensing the distance between the rear bumper of the car and any obstacle behind the car. The distance can be understood from the combination of the LEDs (D5 to D7) glowing. At 25cm D7 will glow, at 20 cm D7&D6 will glow and at 5cm D7, D6 and D5 will glow. When the obstacle is beyond 25 cm none of the above LEDs will glow.
2.  Two ICs are used in the circuit. The IC1 (NE555) is wired as an astable multivibrator for driving the IR Diode D1 to emit IR pulses. The operating frequency of the transmitter is set to be 120Hz.
3.  The IR pulses transmitted by D1 will be reflected by the obstacle and received by the D2 (IR photo diode).The received signal will be amplified by IC2a.The peak of the amplified signal will be detected by the diode D4 and capacitor C4.R5 and R6 compensates the forward voltage drop of D4.The output voltage of the peak detector will be proportional to the distance between car’s bumper and obstacle.
4. The output of peak detector is given to the inputs of the other three comparators IC2b,IC2c and IC2d inside the IC2(LM324).The comparators switch the status LEDs according to the input voltage their inverting inputs and reference voltages at their non inverting inputs. Resistances R7 to R10 are used to set the reference voltages for the comparators.
   
   

   
   

   
   

   NOTES:
   

   • Assemble the circuit on a good quality PCB or common board.
   • The D1 & D2 must be mounted close (~2cm) to each other, looking in same direction.
   • The D1 can be a general purpose IR LED.
   • The D2 can be general purpose IR photo diode with sun filter.
   • The transmitter as well as receiver can be powered from the car battery.
   • For proper working of the circuit, some trial and error is needed with the position of D1 and D2 on the dash board.
   • All capacitors must be rated 25V.
   • The ICs must be mounted on holders.