About The Circuit
Here is the circuit diagram of an audio monitoring system which can be easily setup on a PCB. The main use of such a circuit is to transmit a sound from one location and to reproduce it on the other end with the help of a reciever. In this ciruit both the power supply as well as the signal that is transmitted share the same wire.
As seen in the figure, it must be noted that the terminal A is connected tp A’using the wire of required length. The same procedure must be followed for B to B’. A simple speaker of 8 Ohm and 2 Watts is required as K1. The gain of the transmitter circuit is controlled by the POT R2. while POT R14 is used to control the gain at the reciever end. A 12 Volt battery or 12 Volt DC power supply is enough as the input voltage.
The microphone M1 picks up the audio signals. The signals will then be amplified by the double stage amplifier build along with the transistors Q1 and Q2. The microphone has no special specifications. It can be a general purpose one. The by[pass of all the audio frequencies and noise from the line is done by the capacitor C4. C4 also ensures the pure DC for the circuit. The audio signal output of the amplifier is coupled to the line with the help of the capacitor C6. The audio signal from the line is extracted and is fed to the inverting input of TL 071, which is the IC1 in the circuit. This operation is done by the capacitor C7. The integrated power amplifier IC2 for which an LM 386 IC is used recieves the output of IC1. IC2 also provides all the necessary current gain to drive the speaker. The audio frequencies are isolated along with the noise from the power supply of both the IC’s with the help of capacitor C1.