Wireless FM Microphone

This is a battery operated Wireless FM Transmitter that can transmit the audio signal up to a range of 100 feet of any spot frequency within the FM band. The transmitter could be assembled on a small Printed Circuit Board approximately at size of 35mm X 45mm. The field strength of the radiated signal will not exceed 50μV / meter at a distance of 15 meters from the unit. The bandwidth of the radiated signal shall be about 200KHz. Therefore, voice audio as well as most of the musical instrumental audio could be transmitted properly within the specified range. This small size of the transmitter can be used as a Wireless Microphone. Therefore this transmitter must be used in a sealed auditorium hall as wireless microphone, reason being the transmitted signal from the wireless microphone may interrupt normal FM receiver within the specified range. Please find the schematic circuit diagram of the wireless microphone at the attached link.

CONSTRUCTION: It shall be better to assemble the components on a PCB. The complete PCB diagram is available on the attached file. Assembly technique of the components is of no difference than building any other circuit. The components also are very easily in the market. Proper care should be taken regarding to avoid any dry or bad soldering.

On completion of assembly of all the components thorough physical checking should be done to ensure there shall not be any sort of wrong component placement and dry soldering.

After the physical checking fit the assembled PCB in a suitable non-metallic case (plastic box) and put one single strand wire or a telescope antenna at the C9 point. Connect the power (one 9V battery).

The alignment or tuning should be done using one standard FM band radio receiver. Place the FM radio in such a position that there should not be any radio station. It shall be better to set the tuning pointer near 87-88 MHz area. Speak in to the electret microphone of the newly assembled transmitter and adjust the C8 (variable trimmer capacitor) using a non-metallic alignment tool so the voice (test signal) could be received properly at the FM receiver.

Place the FM receiver at a distance to 25 to 30 Meters Fine tuning may be required to adjust the above mentioned capacitor (C8) in such a manner so that the test audio signal is heard clearly at the FM receiver. The FM Transmitter is ready to use.

Bill of material and Circuit Schematic

Electronic Circuits – Simplified