Discussion

Raspberry Pi Pico as AM Radio Transmitter

juancn: 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?

nom: https://vanhunteradams.com/Pico/AM_Radio/AM.html

pesfandiar: 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.

lormayna: Please use an appropriate filter for the band that you are transmitting, otherwise you will pollute all the near frequencies with spurious.

hulitu: > Raspberry Pi Pico as AM Radio TransmitterThe fact that you are receiving it with an AM radio, doesn't mean that you are transmitting AM.

tejtm: "A rose by any other name would smell as sweet" -- Bill

mk_stjames: 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...

pesfandiar: 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.

mikeyouse: On off keying at 1,000khz is AM transmitting though?

crims0n: AM is short for Amplitude Modulation, and by definition needs a carrier wave. This is more like controlled interference, still impressive though.

anfractuosity: If you PWM a signal, I presume you could add a filter to convert to amplitude changes?

peterbmarks: You think that's fun, rpitx will blow your mind: https://github.com/F5OEO/rpitx

cadr: 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).

fainpul: FM radio with an ATTiny:https://spritesmods.com/?art=avrfmtx

gsliepen: Something similar: https://www.youtube.com/watch?v=7t7_naYJnHo Source code can be found here: https://github.com/spookysys/attiny-synth

bitwize: 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.

westurner: GSMem (2015) 1-5.5m/30m with 3G from the RAM busTEMLEST-LoRa (2025) 87.5m with LoRa over display cablesLoPHY (2024) 700m with LoRAMAGNETO (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)

anthk: >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.

genxy: The RP2350 can write at 175MB/s using the HSTX, https://github.com/steve-m/hsdaoh-rp2350 more than enough for SDR.https://www.digikey.com/en/maker/tutorials/2025/what-is-the-...

ajot: The 101things radio stuff is super cool too!https://101-things.readthedocs.io/en/latest/

STARGA: The Pico is doing AM modulation by toggling a GPIO pin at the carrier frequency and varying the duty cycle for amplitude. It works because at these frequencies the GPIO output plus a wire is essentially a square wave oscillator, and the fundamental frequency component is your carrier. The harmonics are what make this legally sketchy — a square wave at 1MHz radiates energy at 3MHz, 5MHz, 7MHz, etc., which lands in bands you definitely do not want to be transmitting on.The 30cm fade-out distance suggests the effective radiated power is well within Part 15 limits (FCC allows ~200 microvolts/meter at 30 meters for AM broadcast band). But the harmonic content is the real concern. A low-pass filter between the GPIO and the antenna wire would clean up the output significantly — even a simple RC filter with a cutoff around 1.5MHz would attenuate the third harmonic by 10+ dB.Fun project though. The RP2040's PIO state machines are genuinely interesting for signal generation — you get deterministic cycle-accurate timing without fighting an OS scheduler or interrupt latency.

mikeyouse: But on-off keying is amplitude modulation…https://en.wikipedia.org/wiki/On–off_keying

ra: [delayed]

Animats: Emitting a square wave as RF blithers all over the RF spectrum. At least put a bandpass filter on the thing so it stays in-band.

tl2do: 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.

userbinator: A slightly less practical but more fun way is to do it on a ship in international waters. (Bringing a whole new meaning to "pirate radio"...)

tl2do: 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.

guenthert: An oscilloscope is the wrong tool for that. You can tell the difference between kind-of-square and kind-of-sine wave using one, but hardly more. At least DSO often at least have a FFT option and on the new ones with 12bit ADC that might actually be more than just a gimmick.You'd want to use a spectrum analyzer to verify that other frequencies are present only at very low levels. TinySA might be the cheapest option or an used Digilent Analog Discovery.

Kim_Bruning: You think you jest: https://en.wikipedia.org/wiki/Radio_Veronica .A literal pirate ship!

BoxOfRain: There's still one example of a working offshore radio ship, the Ross Revenge in southern England which you can go and visit. She's one of the former Radio Caroline ships, the studios are still fired up every month for a weekend of broadcasting and they run tours. Radio Caroline themselves are still alive and kicking as a legal station broadcasting 24/7 online and on 648 AM; ironically the latter transmission comes from a former BBC World Service site. She wasn't really a 'pirate radio' ship as she was a Panamanian-flagged vessel in international waters so not subject to the Wireless Telegraphy Act in theory, but British citizens specifically would have committed an offence working on her in her free radio days. What really did Radio Caroline in as an offshore broadcaster was the Anglo-Dutch action against the clandestine organisation which supplied the ship, that and the move from a 3-mile to a 12-mile limit which forced her into more exposed waters.Other than the RNI ship she was probably the best-equipped radio ship that ever put to sea, and certainly the strongest. She was a long-range trawler built for Arctic conditions, and the engineering which went into the radio station was really impressive; Peter Chicago her engineer by all rights should be up there with the greats in hacker lore. Most radio ships were clapped-out old vessels at the end of their lives, they were essentially slapped with transmitters and sent to sea to die since you can never take a radio ship back into port once it's broadcast. The Ross Revenge on the other hand was a very strong ship who was left purposeless midway through her life due to the Cod Wars. The generating and transmitting facilities were really sophisticated for radio pirates, there were plenty of redundancies and the ship could radiate multiple medium and short wave services.The broadcast studios and accommodation are still active but most of the machinery spaces and the hull itself aren't in good condition. They've raised half a million pounds for repairs, but that's not actually all that much in the maritime conservation game. Hopefully it will be enough to stabilise the immediate problems with the hull and open a door to lottery funding though. If you're in the area I'd go and see her while you've definitely got the chance!