Low cost intercom
color="lime">Click here for the circuit diagram
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
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.
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.