DH-FM-V4.0 FM Transmitter Module With Microphone – 0.7–9V, 88–108MHz

The DH-FM-V4.0 FM transmitter is a compact FM wireless microphone transmitter module that converts ambient sound into a standard FM radio signal for short-range wireless audio transmission. Built around a miniature electret microphone and an FM modulation circuit, the module captures sound, processes it, and broadcasts it across the 88MHz to 108MHz commercial FM band, allowing any standard FM radio receiver—including a smartphone with a wired headset acting as an antenna—to pick up the audio. Unlike simple RF transmitter boards that require external audio input, this module integrates the microphone, modulation stage, and antenna connection on a single coin-sized PCB, making it a self-contained wireless audio pickup solution.

The module's wide 0.7V to 9V operating voltage range gives the user direct control over transmission power and range. At the maximum 9V, it draws approximately 24mA and delivers 10.2dBm (about 11mW) of RF output, sufficient for 50 to 100 meters of open-space transmission when a 70cm wire antenna is connected to the ANT pad. Lowering the voltage proportionally reduces current draw and output power—at 5V, for example, the module produces about 6.1dBm (5mW) and draws 14mA, which is adequate for many indoor and short-range applications. The transmission frequency is adjusted via a trimmer capacitor on the board, allowing fine-tuning across the FM band.

The board measures approximately 35 × 15mm (coin-sized) and features clearly marked +, −, and ANT pads on the back for power and antenna connection. It is a passive audio transmitter—it does not include a display, digital tuning, stereo encoding, or RDS. The built-in microphone is highly sensitive and is intended for medium-volume environments; speaking directly into it from a very short distance or placing it near a loudspeaker can cause saturation and distortion. The DH-FM-V4.0 is widely used in voice pickup, short-range audio links, sleep monitoring for elderly or children, educational electronics projects, and simple wireless microphone builds.