The intercom circuit
described here uses two transistors, an audio transformer, and a few
passive components in addition to condenser microphone and low-wattage
speaker (refer Fig. 1). The complete unit can be made on a
general-purpose veroboard. The microphone signals are amplified by a
two-stage transistor amplifier, while the speaker is driven through an
audio output transformer (similar to the one used in transistor radios).
When ring button (push-to-on switch S1) is pressed, capacitor C3
gets connected between the base of transistor T2 and the top end of
primary winding of audio output transformer. As a result, the amplifier
circuit wired around transistor T2 gets converted into a Hartley
oscillator and produces an audible tone for call-bell. To build a
two-way intercom set, make two identical units with the speaker of each
circuit installed near the other unit as shown in Fig. 2.