Raspberry Pi Pico as AM Radio Transmitter
pesfandiar.com53 points by pesfandiar 4 days ago
53 points by pesfandiar 4 days ago
I want to point out that what keeps this 'OK' is that the little wire is so 'electrically short' compared to the actual wavelength at 1000khz (a real quarter wave antenna at that freq is like 75 meters)... and thus this limits the power of this 'transmitter' to probably nanowatts.
If the PIO pin could drive a fair amount of current at 3.3v into a long enough wire at that frequency you'd start to get into milliwatts, and AM radio is NOT a band that even amateur license operators can broadcast over a a certain power on. FCC part 15 dictates no more than a 3 meter antenna for personal devices at AM frequencies which is what does the power limiting essentially.
The harmonics fall off quick enough on such a setup that it wouldn't really be a problem - but the only way to really KNOW that is to have a real solid understanding of how this 'radio' you've just made is working, meaning how that square carrier wave is really being driven off the PIO pin, and thus you need the requisite EE knowledge and/or ham radio test equipment and experience.
I've seen more and more of these 'ChatGPT coded up a radio transmitter' posts and it kinda rubs me the wrong way. I'd like to see more calculations and disclaimers for people showing some responsibility with radio, and if it drives people to studying and taking an amateur radio license test that would be for the better...
At least you are supposed to be allowed to transmit on AM (and FM) bands at very low power, at least under FCC rules. That's how they can sell those adapters for car radios. Some other ones you definitely are not supposed to use as far as I know, like the old analog TV signals - I've seen a couple of those sorts of projects recently. On the other hand, you can go and buy an illegal TV transmitter on Amazon right now.
Also a reasonably even bet that you already own a low-quality wall power supply that will produce more interference than anything you're going to be doing with a Pico and a 4" jumper wire (I've found a couple of offending devices in my house), but I'm certainly in no position to tell you if you should or shouldn't do something.
Mea culpa.
Without the proper knowledge or measurement equipment, I observed that the audio would fade out after a 30 cm distance. Combined with running it for mere seconds to test and record a demo, I assumed to be in the clear with the spirit of the regulations. Appreciate the reminder to be responsible with RF.
I don’t know what the regulations are in your country (looks like you are maybe in Canada?), but in many countries it is straightforward to get an amateur radio license, and then you can have all sorts of fun (under the rules).
Transmitting on AM broadcast frequencies is generally prohibited unless it meets an extremely low-power exemption , even if you have amateur license(I have a Japanese amateur radio license). A practical way to reduce risk is to put a large resistor before the antenna so the radiated power stays within that exemption. You could start with 100 MΩ; if the receiver cannot pick it up, try 10 MΩ, and so on.
You think that's fun, rpitx will blow your mind: https://github.com/F5OEO/rpitx
FM radio with an ATTiny:
Something similar: https://www.youtube.com/watch?v=7t7_naYJnHo Source code can be found here: https://github.com/spookysys/attiny-synth
Please use an appropriate filter for the band that you are transmitting, otherwise you will pollute all the near frequencies with spurious.
Given GPIO frequency limits, reproducing a beautiful sine wave for a 1000 kHz carrier is a real challenge. He should borrow an oscilloscope and measure the output waveform.
If you use GNU+Linux/BSD or anything with an X server, by tweaking the modelines you can broadcast a song over AM by using harmonics from your screen.
Search for Tempest for Eliza.
I don't get why PWM wouldn't work? Would the harmonics make the tuner ignore the signal?
Because the speaker is still slow, so if it got to it, there should be audio, but maybe the circuit filters out the PWM signal outright?
Highly recommend his Pico-based microcontroller course: https://www.youtube.com/playlist?list=PLDqMkB5cbBA4GisLzRSqw...
The PWM-based modulation is interesting, but as an amateur, I couldn't fully understand it or trust that the radio receiver reliably picks up the duty cycle as amplitude.
If you PWM a signal, I presume you could add a filter to convert to amplitude changes?
This is the first use people cooked up for the MITS Altair computer, which at the time could only output to its blinkenlights without expansion. Before a tiny company called Micro-Soft released BASIC for the thing, some madlad at the Homebrew Computer Club found a way to spin the CPU in loops tight enough that the interference could be picked up as tones on an AM radio, allowing for music to be created. Good to see the old traditions are still alive.
GSMem (2015) 1-5.5m/30m with 3G from the RAM bus
TEMLEST-LoRa (2025) 87.5m with LoRa over display cables
LoPHY (2024) 700m with LoRA
MAGNETO (2021) CPU-generated magnetic fields
"Rowhammer for qubits" describes hypothetically using electron tunneling and magnetically biased bit flips in standard RAM to simulate quantum operators.
I've heard stories of ham radio clubs teaching how to make a coaxial antenna out of coaxial cable (cable TV copper cable)
"Can you hotwire this computer to transmit a tone through the radio?" — Transformers (2007)
>I've heard stories of ham radio clubs teaching how to make a coaxial antenna out of coaxial cable (cable TV copper cable)
That's Teleco 101, basically the first lessons from a Teleco trade/vocational degree. And OFC known at any electrician degree.
Based on a repo I found that twiddled the memory bus, I transmitted audio from a wav file via Pulse Density Modulation - https://github.com/anfractuosity/musicplayer
> GSMem (2015) 1-5.5m/30m with 3G from the RAM bus
What the fuck, that's crazy. For those similarly bewildered, look here [1].
[1] https://www.usenix.org/conference/usenixsecurity15/technical...
> Raspberry Pi Pico as AM Radio Transmitter
The fact that you are receiving it with an AM radio, doesn't mean that you are transmitting AM.
On off keying at 1,000khz is AM transmitting though?
AM is short for Amplitude Modulation, and by definition needs a carrier wave. This is more like controlled interference, still impressive though.