Full-duplex Intercom

Comments:
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 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.

P1_____________22K  Log. Potentiometer

R1_____________22K  1/4W Resistor
R2,R3_________100K  1/4W Resistors
R4_____________47K  1/4W Resistor
R5______________2K2 1/4W Resistor (See Notes)
R6______________6K8 1/4W Resistor
R7_____________22K  1/2W Carbon or Cermet Trimmer
R8______________2K7 1/4W Resistor

C1,C6_________100nF  63V Polyester or Ceramic Capacitors
C2,C3__________10µF  63V Electrolytic Capacitors
C4_____________22µF  25V Electrolytic Capacitor
C5_____________22nF  63V Polyester or Ceramic Capacitor
C7____________470µF  25V Electrolytic Capacitor
Q1____________BC547  45V 100mA NPN Transistor
IC1_________TDA7052  Audio power amplifier IC
SW1____________SPST  miniature Switch
MIC____________Miniature electret microphone
SPKR___________8 Ohm Loudspeaker
Screened cable (See Text)

Circuit operation:

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.

Notes:

• 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 having the length desired.
• The cable screen is the negative ground path and the central wire is the signal path.
• The power supply can be a common wall-plug adapter having a voltage output in the 6 - 12V dc range @ about 200mA.
• Enclosing the power supply in the box of one unit, the other unit can be easily fed by using a two-wire screened cable, its second wire becoming the positive dc path.
• To avoid a two-wire screened cable, each unit may have its own separate power supply.
• Please note that R5 is the only part of the circuit that must not be doubled.
• Closing SW1 prevents signal transmission only, not reception.
• To setup the circuit, rotate the volume control (P1) of the first unit near its maximum and speak into the microphone. Adjust Trimmer R7 until your voice becomes almost inaudible when reproduced by the loudspeaker of the same unit.
• Do the same as above with the second unit.