Full-duplex Intercom

Posted on Feb 4, 2014

This design allows to operate two intercom stations leaving the operator free of using his/her hands in some other occupation, thus avoiding the usual `push-to-talk` operation mode. No complex changeover switching is required: the two units are connected together by means of a thin screened cable. As both microphones and loudspeakers are always in

Full-duplex Intercom
Click here to download the full size of the above Circuit.

operation, a special circuit is used to avoid that the loudspeaker output can be picked-up by the microphone enclosed in the same box, causing a very undesirable and loud "howl", i. e. the well known "Larsen effect". A "Private" switch allows microphone muting, if required. The circuit uses the TDA7052 audio power amplifier IC, capable of delivering about 1 Watt of output power at a supply voltage comprised in the 6 - 12V range. The unusual feature of this design is the microphone amplifier Q1: its 180 ° phase-shifted audio output taken at the Collector and its in-phase output taken at the Emitter are mixed by the C3, C4, R7 and R8 network and R7 is trimmed until the two incoming signals almost cancel out. In this way, the loudspeaker will reproduce a very faint copy of the signals picked-up by the microphone. At the same time, as both Collectors of the two intercom units are tied together, the 180 ° phase-shifted signal will pass to the audio amplifier of the second unit without attenuation, so it will be loudly reproduced by its loudspeaker. The same operation will occur when speaking into the microphone of the second unit: if R7 will be correctly set, almost no output will be heard from its loudspeaker but a loud and clear reproduction will be heard at the first unit output. The circuit is shown already doubled in the diagram. The two units can be built into two separate boxes and connected by a thin screened cable...

Leave Comment

characters left:

Related Circuits

  • New Circuits



    Popular Circuits

    20 Khz ring counter
    Low-Battery Indicator
    Acoustic Field Generator Circuit
    Arduino BarGraph
    UHF Antenna Amplifier Circuit
    monostable multivibrator 555 timer
    simple 12db/octave filter
    A variable speed fan motor thyristor circuit
    Dedicated headphone amplifier integrated amplifier LM4880-02