Hey, if you’ve ever wanted to broadcast your voice or music to a nearby radio without fancy gear, this stable FM transmitter circuit is a great starting point. Pulled from a clear diagram, it uses basic parts like the LM2936 regulator and a 2N2219 transistor to create a reliable signal around the FM band. Let’s break it down step by step so you can build it yourself.
Circuit Overview
The diagram shows a compact setup powered by 12V DC. An electret microphone captures audio, which gets regulated cleanly by the LM2936 low-dropout voltage regulator IC. That stable supply then powers an oscillator built around the 2N2219 NPN transistor, with a hand-wound coil and capacitors setting the frequency. Output couples to a simple wire antenna for transmission up to 50-100 meters in open space.
Key parts stand out right away: a 5.6k resistor for biasing, 0.47uF and 10uF caps for filtering, and that distinctive 25-turn 0.1uH inductor on a 5mm former. The LM2936 ensures voltage stability to prevent drift, while the transistor does the RF work.
This design claims stability, thanks to the LM2936’s low quiescent current and reverse protection, keeping the tank circuit locked. No crystal here, so it’s a basic Colpitts-style oscillator, but the regulated power makes it reliable.
Components List
Gather these off-the-shelf parts from any electronics shop. Most are cheap and common.
- LM2936 voltage regulator IC (TO-92 or similar package, fixed 5V output like LM2936-5.0)
- 2N2219 NPN transistor (or 2N2222 equivalent, metal can TO-39 package for better heat sinking)
- Electret microphone (small condenser type)
- Resistors: 5.6kΩ, 1kΩ (x2), 10kΩ
- Capacitors: 0.1uF (x2), 0.47uF, 10uF electrolytic, 4.7uF (labeled 047)
- Inductor: 25 turns of 25SWG wire on 5mm former (air-core coil, about 0.1uH)
- Variable capacitor: 10pF trimmer (for tuning)
- Antenna: 10cm wire or 25SWG loop
- Power: 12V DC adapter or battery
- Misc: PCB or perfboard, hookup wire
The LM2936 is perfect for battery or noisy supplies. It drops from 12V to a stable 5V with just 15uA quiescent draw, handles reverse transients, and outputs up to 50mA.
How It Works
Start with the power path. 12V feeds the LM2936 input, which outputs steady 5V to the circuit. This clean rail powers the electret mic and oscillator, minimizing noise.
Mic outputs a weak AC signal, biased via 1k and 5.6k to the 2N2219 base. That transistor’s collector tank—inductor L (25 turns), 10pF trimmer, 27pF cap—oscillates at FM frequencies (88-108MHz). Adjust the trimmer to set your station.
Audio modulates by varying base voltage, creating FM shift. 10uF cap couples RF to antenna. The 0.1uF and Gr stabilize the bias point.
Why stable? LM2936 prevents voltage sag or ripple that causes drift in unregulated designs. Coil Q stays high for a clean signal.
Step-by-Step Build Guide
Build on a perfboard, keep leads short for RF.
- Mount LM2936: Input pin to +12V via cap, GND pin to ground, output (5V) to circuit rail. Add 22uF output cap for stability.
- Wire 2N2219: Emitter GND, base via 5.6k from 5V and 1k from mic/audio. Collector to coil, trimmer to 27pF-GND. Feedback 0.1uF collector-base.
- Wind coil: 25 turns 25SWG on 5mm tube. Connect properly.
- Antenna: 10cm from 10uF on the collector.
- Power via LM2936. Check the 5V output first.
Total time: 1-2 hours. Box it up.
Tuning and Testing
Power on. Use FM radio 5-10m away, tweak 10pF for carrier hiss.
Add voice—clear modulation. Range 30-50m. LM2936 keeps it steady over time.
Safety: Low power, but respect FM rules.
Troubleshooting Common Issues
- No TX? Verify LM2936 output 5V, polarity, and no shorts.
- Drift? Check regulator caps, coil secure.
- Weak signal? Antenna length, supply clean.
- Hums? Better grounding.
Swap 2N2219 with 2N2222 if needed.
Why This Design Rocks for Beginners
Ultra-stable power from LM2936 makes it foolproof—no drift headaches. Low parts count, battery-friendly.
Great for experiments. I’ve tweaked it for a longer range.
Variations and Upgrades
Add an audio preamp before the modulator. RF amp for distance. Arduino tuning.
Final Tips for Success
Quality LM2936—fakes drop out early. Spectrum app for freq check. Battery power reduces noise. Enjoy!
