A low power 2N3904 one transistor FM transmitter is shown below. Such FM transmitter is also called wireless microphone FM bug because it works with a simple electret microphone in the FM range(88MHz to 108MHz). This FM transmitter using single 2N3904 transistor and microphone can be build easily at home.
The 2N3904 BJT transistor is used as an RF modulator. A capacitor of 0.1uF is used between the +ve and -ve power rail to reduce power source noise. An electret microphone is used here. The R1 of 10kohm is used to power up the electret microphone because electret microphone needs power. The audio input from the microphone enters the transistor 2N3904 base via the R2 of 100ohm and electrolytic capacitor C3 of 1uF. The function of the C3 capacitor is to block dc component of the input signal from the microphone circuit. The R3 resistor of 39kOhm is used to bias the transistor. The inductor L1 and variable capacitor VC1 forms the LC tank that is tuned to FM frequency range. The inductor L1 is just a 6 turn coil of laminated copper wire(#20 gauge wire). The antenna is around 25cm of ordinary wire. The circuit is powered using 1.5V battery and no amplifier so it is a low power transmitter.
See next Two Transistor FM Transmitter which is improved version of this transmitter.
