In this door-phone circuit, an 8 ohm speaker is used both as a microphone and also an output device. The BC109C stage amplifies in common base mode, providing low input impedance to match the speaker and good voltage gain.
The 270k resistor provides simple DC bias and the load resistor is 27k. The speaker produces a weak varying DC output when used as a microphone typically several 100uV. This varying DC signal must be separated from the fixed DC bias voltages and this action is performed by the 100u and 0.47u decoupling capacitors.
An LM386 is used in non-inverting mode as a power amplifier to boost voltage gain and drive the 8 ohm speaker. The 10k potentiometer acts as the volume control, and overall gain may be adjusted using the 5k preset. The gain of the LM386 can be as high as 200, this is required because of the small audio signal from the speaker. The double pole double throw switch, reverses the loudspeaker positions, so that one is used to talk and the other to listen. Manually operating the switch (from inside the house) allows two way communication.
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