The circuit provides a simple illustration of a door phone intercom system having a two-way communication using the features of LM386 amplifier.

1. Intercom System with
Telephone-Based Door
Entry

2. •Intercom – short for intercommunication, is an electronic communication
system made of microphones and loudspeakers; used for the purpose of
announcements, cooperation, association, direction, or private conversations
•LM386 – a low voltage audio power amplifier designed for use in low voltage
consumer applications; its features include low distortion, self-centering output
quiescent voltage, ground referenced input, voltage gains from 20 to 200, low
quiescent current drain, wide range of supply voltage, minimum external parts
and battery operated
•Dual Pole Dual Throw (DPDT) Switch – provides a normally open and
normally closed contact for each pole which make and break two separate
circuits

3. The door phone circuit consists of speakers rated at 8 ohms
that is used as an output speaker and as a microphone. This
conversion of microphone to speaker and vice versa is made
possible when the variation on the voice coil, made by the
audio waves arriving at the speaker, creates a few electrical
signals.
This signals that are
passing to the amplifier are
being converted to audio
signals coming out of the
speaker. Since the speaker
has low impedance, the
BC109C transistor
functions to provide the
same amount of low
impedance input while its
amplification operates at
the common base mode to
provide a reliable voltage
gain, while the current gain
modification is permitted by
biasing its own direct
current.

4. The role of LM386 in the circuit is to
provide power to the speakers operating
in non-inverting mode. As a power
amplifier, it increases the voltage gain of
the speakers. The overall gain of the
circuit is set by the 5K preset while the
volume of the speakers is being
controlled by the 10K ohm
potentiometer. A potentiometer is a
manually adjustable resistor used to
control the potential difference or
voltage across a circuit.
To allow a 2-way communication
where one talks while the other
listens, the switch is manually
operated. The dual pole dual throw
switch is responsible for reversing
the operations of the loudspeakers
between end points. The normal
setup of an intercom requires long
cables and it is possible because of
the low impedance and less voltage
gain of the circuit.

5. 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.
Source:
http://www.eeweb.com/blog/circuit_projects/intercom-system-with-telephone-based-door-entry
Image from:
http://qga.com.au/product-category/intercom-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.