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[SOLVED] Intercom - How to explain the circuit

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Newbie level 3
Jul 3, 2011
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Hi, all

I am currently doing intercom duplex project. I understand how to do the PCB layout using DIP trace and doing the labor work after that but I can't probably explain how the circuit functions if someone asks me.

intercom circuit.jpg

Caption: The above screenshot contains two similar circuits joined together by wires for Intercom.

I understand what resistors, capacitors, IC, etc in general do but in relation to this circuit how could I be able to put the little descriptions I know of these components into better explanation. For example, what's the function and why is IC used for this circuit?

Please help.

PS: I might come off as a noob and it might be true but so be it if that's what it takes to understand it :-D :cool:

Looks like a LM386 audio amplifier, but component values are missing.

Looks like a LM386 audio amplifier, but component values are missing.

component values are on different page which I haven't scanned

Name - Description - Units
IC - LM386 (8 pin) - 1
Resistor - 1Kohms - 1
Resistor - 10R - 1
Resistor - 2.2K -1
Resistor - 8.2K - 1
Capacitor - 1uF - 1
Capacitor - 10uF - 4
Capacitor - 0.01 (103) - 1
Capacitor - 47uF - 1
Capacitor - 0.001 (102) - 1
MIC - Microphone - 1
VR1 - 10-59K PRESET - 1
PTT - Push Switch - 1

for each circuit

R1 + C2 = to increase amplifier gain

The LM386 has incorporated a feedback internally, though the gain is set at 20 internally, but it can be raised almost 10 times higher - that is up to 200, just by introducing a resistor and a capacitor across its pin 1 and 8.
Initial gain =15k/(1.35k +0.15k)
Adding R1 the actual Gain= 15k/(R1||1.35k +0.15k)

C3= additional power filter for the input stage of the LM386
C4= additional power filter
C5 +R2 = 'Zobel network' correcting loudspeaker impedance.
This RC group is used to make the impedance of a loudspeaker presents to its amplifier output appear as a steady resistance.

C6= a coupling capacitor. The output is capacitively coupled. The DC output voltage is Vcc/2 at pin 5 so we need to capacitively coupled in order not to disturb the internal biasing.
C1= a coupling capacitor, the microphone is also capacitively coupled in order not to disturb the internal biasing.

R3 + C7= power filter to keep noise from the power supply inducing noise in the audio signal.
R4 = external load resistor for the electret microphone.
MIC = electret microphone
The electret is really a capacitor microphone.The electret holds a static charge for a very long time and eliminates the
need for a HV bias supply. Inside the cartridge is the capacitor. Sound causes one of the platesto change its distance from the other plate so changes bias voltage. But the impedance is very high so there's also a junction FET inside the microphone with its gate connected to the capacitor plate, source at ground and drain to the output pin where is the external drain load resistor .

C8= high frequency filtering.
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