We’ve been loving the variety of entries to the 2025 One-Hertz Challenge. Many a clock has been entered, to be sure, but also some projects that step well outside simple timekeeping. Case in point, this AM transmitter from [oldradiofixer.]
The software-only transmitter uses an ATTiny85 processor to output an AM radio signal in the broadcast band. It transmits a simple melody that you can tune in on any old radio you might have lying around the house. Achieving this was simple. [oldradiofixer] set up the cheap microcontroller to toggle pin PB0 at 1 MHz to create an RF carrier. Further code then turns the 1MHz carrier on and off at varying rates to play the four notes—G#, A, G#, and E—of the Twilight Zone theme. This is set up to repeat every second—hence, it’s a perfectly valid entry to the 2025 One-Hertz Challenge!
It’s a simple project, but one that demonstrates the basics of AM radio transmission quite well. The microcontroller may not put out a powerful transmission, but it’s funny to think just how easy it is to generate a broadcast AM signal with a bit of software and a length of wire hanging off one pin. Video after the break.
Angry HAMs screeching about polluting spectrum in 3, 2, 1…
Nope. Not too worried about a few mW at 1MHz when there are AM stations blasting out kilowatts all over that neighborhood of the spectrum.
Part 15 specifically allows license free micropower broadcasting on both AM and FM bands. I think the limits on AM are 100 milliwatts max output and 10 foot radiator (antenna) size. The OP project is well within the limits. Both AM and FM exceptions were created for broadcasting audio content (originally phonographs) to radio receivers for listening. By contrast, commercial AM stations are typically kilowatt range up to 50 kw for very large stations, and hundreds of watts for the smallest, and FM transmitters even higher power since the signals don’t propagate.
There’s been a software-only FM STEREO transmitter for RasPi for some time.
Software only? I see a Microcontroller, battery, wires and an antenna…
Seriously: nice idea!
This is not AM. In AM the RF carrier is modulated according to the information you want to send. Modulation refers to changing something about the RF carrier such as its frequency, phase, or amplitude. In AM the amplitude is varied. What you described is OOK (one-off keying) which is another type of modulation.
Well, turning it on and off at intervals, is modulating between a very low and a very high amplitude isn’t it
It most certainly is AM. Just because OOK is either 100% or 0% modulation, that does not disqualify it as AM. Think of it like how a square is also a rectangle but not all rectangles are squares. A signal can be multiple modulations at once too! Absolutely mind blowing, I know. Ask any AM receiver and it will agree – quite audibly.
But it is never 100% modulation. It’s either 100% carrier or 0% carrier. Modulation is an analog process.
previously seen here?
https://hackaday.com/2012/01/26/sprite_tms-three-component-fm-transmitter/
(At least that is where I got inspiration to try this back in the days)