- This circuit is a simple approach for converting a loud speaker into a microphone.
- When the sound waves fall on the diaphragm of a speaker, there will be fluctuations in the coil and there will be a small proportional induced voltage.
- Usually this induced voltage is very low in magnitude and useless. Here in the circuit the low voltage is amplified using transistors to produce a reasonable output.
- The transistor Q1 is wired in common base mode and produces the required voltage gain. The transistor Q2 is wired as an emitter follower to produce enough current gain.
- The voice quality of this circuit will not be as much as a conventional microphone but quite reasonable quality can be obtained.
- To set up the circuit, keep the preset R2 at around 10 Ohms and connect the battery. Now adjust R2 to obtain the optimum sound quality.
NOTES:
- Assemble the circuit on a general purpose PCB.
- Power the circuit from a 9 V PP3 battery.
- A 3 inch speaker can be used as K1.
- All capacitors must be rated at least 15V.
- An 8 Ohm speaker or head phone can be connected at the output to hear the picked sound.
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