Software-Driven AM Radio Transmitter Contest Set for 2025
In a fascinating demonstration, oldradiofixer has shown that it's possible to create a software-only AM transmitter using an ATTiny85 microcontroller. This project, which is a valid entry for the 2025 One-Hertz Challenge, challenges traditional perceptions of AM radio transmission complexity.
The transmitter operates by generating a carrier frequency on an output pin of the ATTiny85, toggling pin PB0 at about 1 MHz to create an RF carrier in the AM broadcast band. Amplitude modulation (AM) is then implemented by turning the 1 MHz carrier signal on and off at varying rates, effectively modulating the signal with audio tones.
To create the audio content, oldradiofixer mapped four notes of the Twilight Zone theme by controlling how long the carrier was active during each note, repeating once per second. A simple antenna, a length of wire attached to PB0, acts as the transmitting antenna, requiring no additional RF hardware or modulators.
To build this transmitter, follow these essential steps:
- Use the ATTiny85 to generate a carrier frequency on an output pin: Load the modulation code onto the ATTiny85 using an appropriate programmer or Arduino IDE setup.
- Modulate amplitude by switching the carrier ON and OFF: The project toggles pin PB0 of the ATTiny85 at about 1 MHz to create an RF carrier in the AM broadcast band.
- Encode audio or melody as on/off patterns: For example, oldradiofixer mapped four notes (G#, A, G#, E - the Twilight Zone theme) by controlling how long the carrier was active during each note.
- Connect a simple antenna directly to the microcontroller pin: A length of wire attached to PB0 acts as the transmitting antenna.
This project demonstrates the principles of AM transmission by digitally generating the carrier frequency and implementing amplitude modulation purely in code, with minimal hardware beyond the microcontroller itself and a wire antenna. The output power is very low but enough to be received by nearby AM radios.
You can find a video of the project after a break, which is likely to provide more detailed information about its workings. If you want to build this yourself, look for example code from oldradiofixer or related Hackaday posts to see how to toggle the output pin at 1 MHz and implement amplitude modulation timing sequences for the melody.
Key technical details:
| Parameter | Description | |-------------------------------|------------------------------------------------| | Microcontroller | ATTiny85 | | Carrier frequency | ~1 MHz (generated by toggling PB0 pin) | | Modulation method | Amplitude modulation by toggling carrier ON/OFF| | Audio content | Simple melody encoded by timing carrier toggling| | Antenna | Length of wire connected to output pin PB0 | | Output power | Very low, suitable for close-range reception |
The transmitter's simplicity makes it an interesting demonstration of AM radio transmission, and its ease in generating a broadcast AM signal is notable. Tune in on any old radio, and you'll hear the transmitter playing a simple melody, specifically the Twilight Zone theme. The project's setup involves the use of cheap microcontroller technology, making it an accessible project for many enthusiasts.
The software-only AM transmitter uses technology to digitally generate a carrier frequency and implement amplitude modulation purely in code, showcasing the potential of modern radio technology. This technological feat allows even enthusiasts to generate a broadcast AM signal with minimal hardware, such as an ATTiny85 microcontroller and a simple wire antenna.
