Several intercom circuits have appeared in http://www.circuit-lab.com/
integrated circuits. The circuit described here uses three easily available
transistors only. Even a beginner can easily assemble it on a piece of
veroboard. 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 15mA 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 toy cellphone, which is easily
available in the market. Suggested cellphone cabinet, with the position of
switches, speakers and mike etc is shown.
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