This wireless microphone circuit uses only a single transistor, with few additional passive components. This FM transmitter is very compact and need only a single cell 1.5Volt battery, even works on 1.2V rechargeable battery. Here is the schematic diagram of the circuit:
The transmission range could be between 30-50m radius in open air, and practically depends on the surrounding transmission path and obstacles. The transistor is not critical, any transistor with high hFE, high frequency bandwidth (more than 150MHz), and low base-emitter capacitance will work: 2N2222A, PN2222A, BC109B, C, BFxxx series, and even well known BC238 is suitable for this transmitter.
The inductance L is the critical part and should be handmade. Use an AWG24 (0.5mm) enameled copper wire and make 2-4 loose turns with about 4-5mm diameter. Try to loosen or tighten the coil, or trim the variable capacitor, and try to receive the signal on around 90MHz. Don’t forget to tighten the coil using a glue to fix the coil, avoiding mechanical deformation that change its inductance value.
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