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.
