Full duplex Intercom

Its another technical weekend for Kaushani and me. After burning our grey cells in debugging the decade counter 7-segment display circuits of the first years in 'Innovent' ( organised by IEENC Manipal,MIT) we decided to come up with a new weekend project (Wep) and this time its a 'DUPLEX INTERCOM' . With a 6-12 Volts power supply its a very simple DIY hobby circuit using TDA7052 Audio power amplifier (manufactured by Philips) which gave us a gain of about 40db.




The very interesting part of this circuit is it uses very simple technique to minimise the 'Larsen Effect' which i'll explain in a bit. First of all, there has to be (yes, you guessed it right!) two sets . I'll call it SetA and SetB to make things clearer. Now whatever I speak in the mic in SetA should not get amplified in the speaker of the same set and similarly the speaker's output(voice from SetB) shouldn't be fed into the mic again.This audio feedback is what we call as the Larsen Effect and it becomes a very vital role to minimise this whenever you deal with audio frequencies .You must have felt the Audio feedback many times -a high-pitched squealing noise during musical performances; that's why we use monitors to avoid such effect. Now in this circuit the audio input signal is first amplified by a BC547 transistor and an intermediate output(before feeding into the power amplifier) is taken from collector as well as emitter of the transistor.As we know the output is in-phase and out-of-phase to the i/p signal in emitter and collector respectively, therefore by using a 22k Timpot we can minimise the input from the mic.The cable (o/p) from SetB is fed at the collector of the transistor at SetA which we can hear in the speaker.

Can you see the switch (red colour) up there ? Well if you close this ,there won't be any signal transmission but signal reception is independent of the switch.As we can't get any log potentiometer in UDUPI, we managed with a simple 22k pot and it worked fine (of course not amplified as the way it should).We worked using 9 Volts and the Duplex Intercom is running absolutely great!

For any queries and suggestions about the circuit, comments are always welcomed or mail us at
dydflyer[at]gmail[dot]com
or
kaushanihp18[at]gmail[dot]com