interkom mini full duplx

8/13/2019 Interkom Mini Full Duplx

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Circuit diagram:

Parts:

P1_____________22K Log. Potentiometer

R1_____________22K 1/4W ResistorR2,R3_________100K 1/4W ResistorsR4_____________47K 1/4W ResistorR5______________2K2 1/4W Resistor (See Notes)R6______________6K8 1/4W ResistorR7_____________22K 1/2W Carbon or Cermet TrimmerR8______________2K7 1/4W Resistor

C1,C6_________100nF 63V Polyester or Ceramic CapacitorsC2,C3__________10F 63V Electrolytic CapacitorsC4_____________22F 25V Electrolytic CapacitorC5_____________22nF 63V Polyester or Ceramic CapacitorC7____________470F 25V Electrolytic Capacitor

Q1____________BC547 45V 100mA NPN Transistor

IC1_________TDA7052 Audio power amplifier IC


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SW1____________SPST miniature Switch

MIC____________Miniature electret microphone

SPKR___________8 Ohm Loudspeaker

Screened cable (See Text)

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 ofa 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.

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 beloudly 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 clearreproduction 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 twoseparate 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 byusing 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.


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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 almostinaudible when reproduced by the loudspeaker of the same unit.

Do the same as above with the second unit.

interkom mini full duplx

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