Low-Power FM Transmitter Circuit
Build a simple circuit to broadcast audio to any nearby FM radio.
Required Components
- 2N3904 NPN transistorx1
- Electret microphonex1
- Trimmer Capacitor (C1, 5-30pF)x1
- Capacitor (C2, 0.1μF)x1
- Capacitor (C3, 4.7pF)x1
- Resistor (R1, 10kΩ)x1
- Resistor (R2, 1kΩ)x1
- Inductor L1 (see note)x1
- 9V battery and clipx1
- Antenna wire (~20cm)x1
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Circuit Description
This circuit is a single-transistor FM transmitter. It's a type of Colpitts oscillator, where the frequency is determined by an LC (inductor-capacitor) tank circuit. An electret microphone picks up sound, which modulates the transistor's base current. This modulation slightly changes the transistor's capacitance, which in turn varies the oscillator's frequency (Frequency Modulation). The signal is then radiated through a small antenna.
How It Works
The core of the circuit is the LC tank circuit made from inductor L1 and the trimmer capacitor C1. This sets the carrier frequency. The 2N3904 transistor provides the amplification needed to sustain oscillation. Audio from the microphone creates small voltage changes at the transistor's base, which modulates the carrier frequency. The output is taken from the collector and fed to the antenna.
Circuit Schematic
Components List
- 1 × 2N3904 NPN transistor
- 1 × Electret microphone
- 1 × 5-30pF Trimmer Capacitor (C1)
- 1 × 0.1μF Capacitor (C2)
- 1 × 4.7pF Capacitor (C3)
- 1 × 10kΩ Resistor (R1)
- 1 × 1kΩ Resistor (R2)
- Inductor L1: 6-8 turns of 22 AWG wire, 1/4" diameter
- 1 × 9V battery and clip
- ~20cm wire for antenna
Installation Instructions
- Carefully build the circuit on a breadboard or perfboard. Keep all leads as short as possible to minimize stray capacitance.
- Create the inductor L1 by wrapping wire around a pen or drill bit. Scrape the enamel off the ends.
- Connect the antenna to the transistor's collector.
- Power the circuit with a 9V battery.
- Use an FM radio and tune it to an empty frequency (static) between 88-108 MHz.
- Carefully adjust the trimmer capacitor C1 with a non-metallic screwdriver until you hear the static on the radio disappear. You should hear silence or a slight hiss.
- Speak into the microphone. You should hear your voice on the radio! You may need to fine-tune the capacitor for the clearest signal.