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.
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.