Speaker Microphone Circuit
Circuit :  Andy Collinson
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This circuit takes an ordinary loudspeaker and allows it to be used in reverse, as a microphone.

This circuits allows you to use a cheap loudspeaker as a microphone. Sound waves reaching the speaker cone cause fluctuations in the voice coil. The voice coil moving in the speakers magnetic field will produce a small electrical signal . The circuit is designed to be used with an operating voltage between 6 and 12 volts dc. The first transistor operates in common base mode. This has the advantage of matching the low input impedance of the speaker to the common base stage, and secondly has a high voltage gain. The second stage is direct coupled and operates in emitter follower. Voltage gain is slightly less than unity, but output impedance is low, and will drive long cables.

As speakers do not give respond well (as a microphone) to high frequency gain is rolled off by C1, the frequency plot against gain (more commonly known as Bode Plot is shown above). This roll-off also helps with RF noise immunity when driving long cables.

As the amplifier is biased to almost half supply allowing for maximum amplitude swings, its distortion is low, a fourier series to the 16th harmonic (fundamental 1kHz) calculated THD at just over 1.5%, see above. Speech quality is not as good compared to an ordinary or ECM microphone, but quite acceptable results can be obtained. Speaker cones with diameters of 1 inch to 3 inches may be used. Speaker impedance may be 4 ohm to 64 ohm. The 8.2 ohm resistor value may be changed to match the actual speakers own impedance.

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