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The
circuit comprises a 3-stage resistor-capacitor coupled amplifier.
When ring button S2 is pressed, the amplifier circuit formed
around transistors T1 and T2 gets converted into an asymmetrical
astable multivib-rator generating ring signals. These ring signals
are amplified by transistor T3 to drive the speaker of earpiece.
Current consumption of this intercom is 10 to 15 mA only. Thus a
9-volt PP3 battery would have a long life, when used in this
circuit.
For making a two-way intercom, two identical units, as shown in
figure, are required to be used. Output of one amplifier unit goes
to speaker of the other unit, and vice versa. For single-battery
operation, join corresponding supply and ground terminals of both
the units together.
The complete circuit, along with microphone and earpiece etc, can
be housed inside the plastic body of a cellphone toy, which is
easily available in the market. Suggested cellphone cabinet is
shown.
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