Miniature FM Transmitter is a compact and effective project ideal for experimenting with radio communication. Whether you’re a student or an amateur radio enthusiast, this circuit offers a great introduction to the basics of FM transmission and audio electronics.
This Miniature FM Transmitter circuit was featured in ETI Electronics Projects and is simple enough for beginners while still delivering decent performance. Operating in the FM broadcast band (~88–108 MHz), it can transmit audio from a built-in electret microphone to any FM radio within a short range—perfect for demonstrations or experimentation.
The circuit captures audio via the electret microphone, amplifies it, and uses two transistors in a basic oscillator-modulator configuration to transmit on the FM band. The LED provides a visual power indicator. A hand-wound coil (8 turns on a 5mm former) and associated capacitors form the frequency-determining tank circuit. The range and frequency can be adjusted slightly via the tuning slug and antenna length.
The Miniature FM Transmitter uses a two-transistor oscillator and modulator circuit to pick up sound through the electret mic and broadcast it as an FM signal.
- Q1 amplifies the microphone signal.
- Q2 works as a high-frequency oscillator, modulating the RF carrier with the amplified audio.
- The LC tank circuit (coil + capacitors) sets the operating frequency.
- The LED provides a simple power indicator.
- A wire antenna extends range to 50–100 meters, depending on surroundings.
L1 – Oscillator Coil
- 8 turns of 0.8 mm enamelled copper wire
- Wound on a 5 mm plastic coil former
- Ferrite slug inside the former (for tuning frequency)
- Referenced part: DSE Cat L1010 (coil former) + DSE Cat L1302 (ferrite slug)
- This is the main tank coil in the RF oscillator section
L2 – Output Coupling Coil
- 2 turns of 0.3 mm enamelled wire
- Wound directly over L1 (same former) – tightly coupled (like a transformer)
- Acts as a coupling coil for next stage
- L2 inductively couples the RF output from the oscillator (L1) to next stage. This ensures efficient power transfer without direct electrical connection, improving isolation and impedance matching.
Miniature FM Transmitter – Construction Notes:
- Wind L1 first (8 turns), spaced evenly.
- Then wind L2 (2 turns) over L1, usually at the cold (grounded) end of the coil for better performance.
- Make sure to scrape the enamel coating off the wire ends before soldering.
Note: Transmission on FM frequencies may require a license in your country. Use this transmitter strictly for educational or experimental purposes in a shielded environment. Always follow local regulations.
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