LM 386 is just right for a door phone application. It is very useful application in the present urban households. One condenser microphone and a speaker are kept conveniently outside and another set with amplifier are kept inside. With a door phone installed at the main door, it will be easy to recognize the caller outside the door. Nevertheless, it is still a better method for shooing away those indomitable salesmen with bodybuilder or body toner, or some other plastic gadget.
Circuit is shown in Schematic. The amplifier along with the power supply is kept inside fully accessible by the insider. One microphone and speaker are kept outside the main door such that the caller can speak into the mike and listen to the speaker. When the caller knocks the door, insider can switch on the door phone and ask him to identify. Then the insider can switch on the outside mike to allow him to speak. This activates the inside speaker but switches off the inside mike. When the insider releases this switch, inside mike and outside speaker will be connected. This is simplex system.
There are two condenser microphones (MIC1, MIC2) biased by R3 (10k resistor). Bias is switched by SI such that any one mike will operate. Mikes are coupled to the amplifier by C2, C7. VR1 is the volume control. Similarly there are two speakers (SPK1, SPK2) which are switched by S2 from the amplifier. Both these switches are operated by the insider and are ganged. The switches are ganged so that when outside mike is on, inside speaker will be connected and vice versa. Rl preset is a gain control, which you can adjust and leave it at optimum level. You still have volume control on hand. Please note that all these controls should be inside in the hands of the householder.This amplifier is used with a power supply of 12V full wave. Regulated supply is not required. Use good quality capacitors as bad filtering would result in annoying hum
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