About The Circuit
This is the simplest way of converting a loud speaker to a michrophone. Though the output voice is not so conventional, an average quality output will be obtained for sure. The circuit has to be assembled neatly in a PCB.
The basic working is that when the sound waves hit the diaphragm of the speaker, the coils in the diaphragm will fluctuate, causing a small considerable amount of induced voltage. Voltage gain is obtained by connecting the transistor Q1 in the common base mode. Current gain is obtained by connecting the transistor Q2 as an emitter follower. The circuit requires an input voltage from a 9 V PP3 battery. The capacitors must have a minimum rating of 15 Volts. The speaker K1 should have 3 inch thickness. Inorder to collect the produced sound in the output an 8 Ohm speaker or headphone is more than necessary.
It must be noted that while setting the circuit, keep the preset R2 at around 10 Ohms and connect the battery. Later adjust R2 until you get good sound quality.