Wiring Schematic Diagram

This FM wireless microphone transmitter can transmit speech over a short range. It can be used as a simple cordless microphone. This circuit has good frequency stability and has range over 1 Km (under good conditions). This project features RF amplifier buffer (10dB gain), an AF preamplifier to boost the modulation and good microphone sensitivity. You can use it for guitars and remote control system.

This circuit quite simple to build. The two BC547 transistors can be replaced with any small-signal NPN transistor, such as the 2N2222. L1 is 3.25 turns in spiral form and is an integral part of the PCB foil pattern. The final stage is a BC557 PNP general purpose device. If you use different devices then you should select the 1M0 resistor for 5-volts DC at the collector of the first transistor. Select the 47K resistor for 3 – 4 volts on the collector of the third transistor.

 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.