Abusing RPi GPIO pins as a radio controller | Hacker News Reader

Abusing RPi GPIO pins as a radio controller

riveducha.com

95 points_Microft16 comments

I was thinking "oh wasn't there a project a few years ago that also used the GPIO pins to broadcast FM radio?". Turns out those few years were almost a decade: https://hackaday.com/2012/12/10/transmit-fm-using-raspberry-...

Yeah, I remember transmitting the then-current hit, Rihanna's Umbrella :(

rpitx is probably the #1 reason for getting a raspi these days. If you can find one that's not marked up 200%..

I enjoyed reading about the programming and testing of a kickstarter project that used this technique in their fixed-frequency public radio receiver: https://www.kickstarter.com/projects/centerlinelabs/kickstar...

With a proper low-pass filter, the RF will be fine. I have a "WSPR" beacon going all the time that is heard around the world -- from Antarctica to Europe to Japan to Russia. The RF is generated from an I/O pin, low-pass filtered, and amplified to 200mw which is a common power level for WSPR.

> Some people wanted to know if this is doable with an Arduino. The Arduino simply can't toggle its digital out fast enough to transmit at this frequency

There is no need to be able to toggle a pin fast enough to be able to transmit at a given frequency. You can rely on the harmonics to get you to a much higher frequency than your code is toggling the pins.

Which means the output will be lousy with harmonics and intermodulation. Even a tight band-pass filter won't clean up the mess.

It will be anyway. To create clean RF isn't all that simple.

It isn't simple if you use the wrong tools for the job. It can be simple if you don't generate square waves.

http://ok1ike.c-a-v.com/gnat_40.pdf

There are 100's of similar simple circuits that generate RF that is clean enough.

Right up until you hook up that spectrum analyzer...

Have a read of any of the Amateur Radio books and magazines over the past hundred years or so.

There are no shortage of simple Analog transmitters which meet FCC specs.

This kind of things will never get FCC approval ;-) I would be curious to see the spectrum around the transmitter on a spectrum analyzer :-)

It will look like a somehow downfiltered square signal, because that's what it is.

Well, he puts a low-pass filter before the antenna.

specific bandpass filter even better

That was a fun video, easy intro to analyzing wireless remote control signals with simple, almost free software.

Reply on news.ycombinator.com