Software-Based Transmission of AM Radio Signals in 2025: Achieving One Hertz without Hardware
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In a fascinating demonstration of simplicity and innovation, [oldradiofixer] has developed a software-only AM radio transmitter using an ATTiny85 processor and a single wire. This project, which is a valid entry to the 2025 One-Hertz Challenge, showcases the accessibility of AM radio transmission technology.
The design of this AM transmitter is characterised by its minimalism. It relies solely on the ATTiny85's digital output and precise timing in the code to generate an AM radio signal. The RF carrier is created by toggling pin PB0 at 1 MHz, and the amplitude is modulated by varying the toggle rate or duty cycle in software, representing AM signals.
To implement this project, a GPIO pin on the ATTiny85 is connected to a single-wire antenna. Firmware is written that toggles this pin at the desired carrier frequency, typically within the 540–1600 kHz AM broadcast band. The modulated output is then fed directly to the antenna wire, which radiates the AM signal.
Practical considerations for implementation include using ATTiny85 timer interrupts or tight software loops to generate a stable carrier frequency. Modulating the amplitude can be achieved by varying pulse widths or on/off cycles corresponding to the audio waveform. A simple antenna, such as a wire about 1m long, can be connected to the output pin through a coupling capacitor or resistor to protect the pin.
The AM transmitter developed by [oldradiofixer] transmits the Twilight Zone theme every second. The video of this project is available after the break for those interested in seeing it in action. Example code and detailed explanations can be found in [oldradiofixer]'s demonstration, as well as in other projects from July and August 2025.
This method is ideal for low-power, educational, or experimental AM radio transmissions using minimal hardware. Despite its simplicity, the transmitter generates an AM radio signal that, while not powerful, is receivable up to about 80 meters.
References:
[1] [oldradiofixer]'s project post (July 2025) [2] Another demonstration of a software-only AM transmitter (August 2025) [5] Detailed explanation of the software-only AM transmitter (August 2025)
This project, showcased by [oldradiofixer], utilizes contemporary technology to create a software-only AM radio transmitter, showcasing the accessibility of AM radio transmission technology. The transmitting device, fashioned using an ATTiny85 processor, generates an AM radio signal via digital output and precise timing in the code.
