Intercom - How to explain the circuit

[unknown_figure]

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.


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?

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

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

 

[crutschow]

The IC is a power amplifier to amplify the small voice signal from the MIC and power the speaker at the opposite intercom.

R3 and R4 bias the MIC. C7 filters the MIC bias voltage.

C1 couples the AC MIC signal and blocks the DC bias.

C8 rolls off high frequencies from the MIC.

VR1 is a variable resistor of some sort to control gain.

C3 apparently decouples the amp or compensates it. R1 and C2 apparently roll off the amps frequency response or compensate it. Without knowing the amp part number, I can't know exactly what the parts do.

C5 and R2 provide high frequency roll off at the output for high frequency stability.

C8 couples the AC signal to the speaker and blocks any DC out of the amp.

 

[unknown_figure]

the amp is LM386.

R1 and C2 give high gain

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

Apologies for not adding. It was on separate paper which I didn't scan so had to manually write each now.

Thanks a lot for your help. I believe am now getting how exactly it's working. I know it's basic electronics and have being reading for the past few days to improve that area.

C8 couples the AC signal to the speaker and blocks any DC out of the amp.

you meant C6

 

[crutschow]

C6 it is.

 

[ivigeorge]

the function? the objective?

 

[Mike odom]

R3 and R4 bias the mic, but more importantly, R4 turns the current flowing into the mic into a differential voltage for the audio amp input (amp does not read mic voltage directly).

 

[shokjok]

Why are both speaker tabs grounded? A DPDT PTT switch is needed in this design.

 

[audioguru]

R3 and R4 bias the mic, but more importantly, R4 turns the current flowing into the mic into a differential voltage for the audio amp input (amp does not read mic voltage directly).

No, look again.
Pin 2 of the LM386 power amp IC is connected to GND and the filter capacitor C7 also connects to ground.
Therefore the mic signal is not differential.

The volume controls are wrongly sketched as rheostats instead of as volume controls.

 

[audioguru]

Why are both speaker tabs grounded?

You are sharp. I didn't see that error.

 

[unclejed613]

the intercom that was always fun to explain was the one where there was only one amplifier, and the speaker at one end or the other was used as the microphone while the speaker at the opposite end was the speaker...... that one used to be the most common home intercom.

 

[audioguru]

A speaker makes a horrible microphone due to its strong resonance making a boomy sound.
Gas stations use them and the cashiers usually barely speeky zee Engrish so nobody knows what they are saying.

 

[4pyros]

You are sharp. I didn't see that error.

Extra dots, LOL