The circuit provides a simple illustration of a door phone intercom system having a two-way communication using the features of LM386 amplifier.The door phone circuit consists of speakers rated at 8 ohms that is used as an output speaker and as a microphone.
Intercoms have been utilized in many places like in residential and commercial area, in local area paging systems, in area of rescue assistance systems, in apartment entry systems using hardwire, in access control of telephone entry systems, emergency call systems and nurse call systems. In the world of intercom systems, generally intercoms are being used in video systems, integrated security, baby intercom, commercial systems, security systems, telephone systems, wireless systems, home intercom and other services including installation, repair and maintenance
LM386 may be used in several applications such as power converters, small servo drivers, ultrasonic drivers, line drivers, TV sound systems, portable tape player amplifiers, AM-FM radio amplifiers and intercoms
In this doorphone circuit, an 8 ohm speaker is used both as a microphone and also an output device. The BC109C stage amplifies in common base mode, giving good voltage gain , whilst providing a low impedance input to match the speaker. Self DC bias is used allowing for variations in transistor current gain. 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 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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