For the first time, this circuit is the design of the Canadian / English language and was designed by Mr Laurier Gendron of Burnaby in British Columbia, Canada, and myself. Make sure you visit this web site Laurier, Dandy Little Circuits Useful. This page is also available in French by clicking on the flag.
In this doorphone circuit, 8 ohm speaker is used both as a microphone and also an output device. BC109C stage amplifies in common base mode, giving a low input impedance to match the speaker and the voltage gain is good.
The 270k resistor provides a simple DC bias and load resistor is 27k. The speaker produces weak DC output varies when used as a microphone is usually 100uV few. This DC signal is varied to be separated from the DC bias voltage is fixed and this is done with a 100u capacitor and 0.47u.
An LM386 is used in a non-inverting mode as a power amplifier to increase the voltage gain and drive 8 ohm speakers. 10k potentiometer acts as a volume control, and overall profitability can be adjusted using the 5k preset. The advantage of the LM386 can be as high as 200, is necessary because a small audio signal from the speaker. Double pole double throw switch, reverses the loudspeaker positions, so that one used to talk and others listen. Manual operation of the switch (from the house) allows two-way communication.