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Low-cost
Transistorised Intercom
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| Several
intercom circuits have ap-peared in EFY 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. |
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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.
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| 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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Click on the Image for its larger version
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