Hey, if you’re into electronics and want to try your hand at radio transmission without the hassle of winding coils, this DIY coilless AM transmitter is perfect for you. The schematic you shared, labeled “Coilless AM Transmitter” with “No Tension For Coil,” uses common transistors like the 2SC1815 and BU446, along with resistors and capacitors, to create a signal without inductors. As an experienced electronics engineer, I’ve analyzed it and will guide you through how it works, the parts you need, step-by-step assembly, and practical tips. This is ideal for short-range hobby projects, not high-power broadcasting, but it’s a fantastic way to learn about AM radio. Let’s dive into this fun and educational build.
Why Choose a Coilless AM Transmitter?
Traditional AM transmitters use LC tanks (inductors and capacitors) to generate the carrier frequency, which often means tedious coil winding and tuning. This design skips that by using a transistor-based astable multivibrator to produce a square wave rich in harmonics, eliminating the need for inductors. The “No Tension For Coil” note highlights the ease of avoiding coil-related adjustments or drift issues.
The circuit runs on a 16V supply, regulated to 12V, and modulates an audio input for transmission. It generates usable AM band frequencies through harmonics rather than a pure sine wave, making it a practical experiment for beginners. While the signal isn’t as clean due to broadband radiation, it works well for low-power, short-range use—think 10-50 meters. This makes it a cost-effective (under $20) and educational project to broadcast audio like music or voice to a nearby AM radio. Just keep it legal with low power.
How the Circuit Works: A Simple Breakdown
The schematic splits into three key sections: power supply, RF oscillator, and audio modulation with amplification. Here’s how it comes together:
- Power Supply: A +16V input feeds into IC1, likely a 7812 regulator, to provide a stable +12V. Capacitor C3 (0.1µF) filters noise, ensuring consistent performance despite voltage changes.
- RF Oscillator: This is the coilless magic. Transistors Q1 and Q2 (2SC1815 NPN) form a cross-coupled astable multivibrator. Capacitors C1 and C2 (180pF each) link the collectors to bases, while resistors R2 and R3 (22kΩ each) set the timing, and R1 and R4 (470Ω each) provide bias. This creates a square wave oscillator. Frequency is roughly f ≈ 1 / (1.386 * R * C), with R = 22kΩ and C = 180pF. That’s RC = 3.96e-6 seconds, so f ≈ 182kHz. Below the AM band (530-1700kHz), but odd harmonics (546kHz, 910kHz) fall within range for reception. Q2’s collector output goes through R10 (470Ω) to Q3 (2SC1815) as a buffer.
- Audio Modulation and Amplifier: The AF input enters via C4 (0.001µF) and R5 (10kΩ) to Q5 (likely 2SC1815). R6 (220kΩ) biases it, R8 (1kΩ) stabilizes the emitter, and R7 (100kΩ) loads the collector. Audio couples through C5 (4.7µF) to modulate Q4 (BU446, a high-voltage NPN like BU406). BU446 amplifies the modulated RF, with its collector tied to +12V (possibly via R9 100Ω) and emitter grounded. The output passes through C8 (0.001µF), three 150Ω 0.5W resistors (parallel for 50Ω), and C7 (0.1µF) to a 20-foot 18AWG antenna. This delivers basic AM by varying the carrier amplitude with audio.
Power is likely under 1W, suitable for short-range experiments.
Essential Components for Building
Gather these parts based on the schematic, available from Digi-Key or Amazon:
- Transistors: 4 x 2SC1815 (NPN for audio/RF), 1 x BU446 (or BU406, high-voltage NPN).
- IC: 1 x 7812 (12V regulator, TO-220).
- Resistors: R1/R4/R10: 470Ω, R2/R3: 22kΩ, R5: 10kΩ, R6: 220kΩ, R7: 100kΩ, R8: 1kΩ, R9: 100Ω, 3 x 150Ω 0.5W.
- Capacitors: C1/C2: 180pF, C3/C7: 0.1µF, C4: 0.001µF, C5: 4.7µF, C6: 1µF, C8: 0.001µF.
- Other: 16V DC supply (500mA+), 20-foot 18AWG antenna wire, protoboard or PCB, soldering tools.
Cost: $10-15 with basic tools.
Step-by-Step Assembly Guide
Build it on a breadboard first, then transfer to a perfboard. Here’s the process:
- Set Up Power Supply: Solder the 7812 with +16V to pin 1, ground to pin 2, and +12V out to pin 3. Add C3 across the output and ground.
- Assemble Oscillator: Cross-connect Q1 and Q2 bases to collectors with C1 and C2. Link R2 from Q1 base to +12V, R3 to Q2 base, R1 to Q1 collector, R4 to Q2 collector. Test for oscillation with a scope or radio hum.
- Build Audio Amplifier: Connect AF input via C4 and R5 to Q5 base. Bias with R6 to +12V, R8 to ground, R7 to collector. Output via C5 to the modulator.
- Add Power Stage: Feed RF from Q3 (via R10) to BU446 base. Audio from C5 modulates BU446. Collector to +12V (via R9 if needed), emitter grounded.
- Output Network: Route from BU446 collector through C8, three 150Ω resistors (parallel), C7, to antenna. Ground emitter.
- Test It: Power with 16V, input audio from a device. Tune an AM radio to harmonics (540kHz, 1080kHz). Adjust the antenna for range. Measure output power.
If frequency drifts, adjust C1/C2 (lower values raise the base frequency).
Practical Uses and Applications
This transmitter suits hobbyists: Broadcast audio in your home, explore radio waves, or teach AM basics. Pair with an AM receiver for a complete setup. Range is 20-100 feet indoors, more with a better antenna. Try automating with a Raspberry Pi or adding a filter to reduce harmonics.
In the US, the FCC limits unlicensed AM to 100mW and short antennas—check local laws.
Safety and Legal Notes
The 16V supply is safe, but BU446 may heat up—add a heatsink for continuous use. Avoid live parts and use insulated wire. Legally, it’s for education only. High harmonics can interfere, so keep power low and test in shielded spaces. Add a filter if needed. No public antenna use without a license.
Troubleshooting Tips
No signal? Check oscillator frequency at Q2 collector (should toggle). Dead? Verify 2SC1815 pinouts (E-C-B). Distorted audio? Lower input level to avoid overmodulation. Weak range? Extend antenna or boost supply (watch heat). Interference? Add a filter for harmonics.
If BU446 overheats, swap with BU406 (7A, 200V).
Conclusion
This DIY coilless AM transmitter is a brilliant way to explore radio without coil hassles. Using harmonics from a multivibrator simplifies building while teaching AM fundamentals. Build it, tweak it, and enjoy the hands-on learning. Share your results—electronics thrives on experimentation. Stay safe and legal! And legal out there!
