Medium

Simple Intercom (Two-Speaker Wired)

A speaker can also work as a crude microphone — wired together with a second speaker through a simple amplifier, the two form a wired intercom, the oldest trick in the audio electronics book.

Circuit Diagram

+9V Speaker 1 (as mic) C coupling Q BC547 C → speaker 2 E → GND bias R Speaker 2 Speaker 1 doubles as a rough microphone — the amplifier drives speaker 2

How to Build It

  1. Connect the first speaker (used as a microphone) to a BC547 transistor amplifier's input, through a capacitor.
  2. Connect the bias resistor from the positive rail to the transistor's base.
  3. Connect the transistor's collector to the second speaker, with the speaker's other terminal to the positive rail.
  4. Connect the emitter to GND.
  5. Speak near the first speaker — the second should play back (faintly) your voice.

Bill of Materials

ProductCodeQty
Speaker cable KAB0352 2
Capacitor (electrolytic/ceramic/SMD/film) — 1
Resistor DR47K-8/9 1
Transistor (incl. IGBT) — 1
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