Low-Cost Transistorised Intercom

Several intercom circuits have appeared in http://www.extremecircuit.net
using 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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