Previously, in part 1 of this project, I showed
how I used a cheap USB TV tuner to eavesdrop on and decode my ceiling fan’s wireless
commands. Now that I have that knowledge, part 2 is all about sending those
same commands using a Raspberry Pi. This isn’t just plugging some
expensive radio into the Pi. The Pi itself, believe it or not, can transmit
radio signals on a huge range of frequencies. Obviously it has a radio for Wi-Fi and Bluetooth,
but that’s way off from the frequency I need to transmit. What it also has that most computers
don’t have are GPIO pins that you can easily control. If we can toggle one of these GPIO
pins back-and-forth between zero and one, we generate electromagnetic
radiation in the form of radio waves. Luckily for us, there already is a
tool to do this on the Raspberry Pi. It’s called rpitx, and you can find it on Github. It can generate all sorts of radio signals in
a huge frequency range using GPIO pin number 4. This works without us connecting
anything to the pin at all, because a wire that’s disconnected on one end
can still have current and voltage. This disconnected pin is basically
an antenna - a very small antenna. It’s actually a little too small, because the
radio signal only goes about 10cm. I want to control my ceiling fan, but I’m not going to
tape the Pi to my ceiling to make it happen. However, if I connect a longer wire to
this pin, then I can extend the range. Here I’ve cut a length of
wire about 20cm long. Again, it’s not connected to anything. If I transmit
using this, the signal can go tens of meters. A signal on a specific frequency
is like this - it’s a sine wave. [monotone hum] But with the Pi’s GPIO, we’re working with
binary data, ones and zeros. If we toggle between one and zero at the same frequency,
we get a square wave instead of a sine wave. [rough monotone hum] In reality, it actually looks
more like this due to physics. If we compare the two, here’s the
difference. Isolating this difference, it’s equivalent to more
sine waves stacked together. Basically, anything that’s not a sine wave is
some combination of sine waves at different frequencies. That means transmitting our square
wave transmits on multiple frequencies at once. For a square wave, it’s very simple
to calculate these frequencies. A square wave at, say, 100 MHz contains not only a
signal at 100 MHz, AKA the fundamental frequency, but also at 3x the fundamental, 5x, 7x,
and so on. We call these the “harmonics.” So the bad news is that any time we try to
transmit on a specific frequency with the Pi, we’re also simultaneously transmitting
on many other frequencies. There are a lot of frequencies that are illegal
to transmit on without a license, for good reason. You don’t want your cellphone to drop calls
because your neighbor’s using a remote control. The good news is that this is a
common and well-understood problem. We can send our signal through what’s
called a low-pass filter, which blocks signals above a certain frequency while letting
signals below that frequency pass through. They’re not complicated and it’s very
possible to build your own with the right resistor and capacitor, but I was able
to find suitable filters on eBay for only 7 US dollars. Shipping was pretty slow, but
that’s the price you pay for cheap electronics. For aesthetics I’m adding in an antenna
and a connector for the other end, and all this stuff adds up to about $15. Back to the software side. Rpitx comes
with a bunch of example programs, one of which is exactly what we need. We know
that this remote uses on-off-keying - basically morse code - and there just happens to
be an included program called “sendook”. We just need to tell sendook exactly what we want
to send, including frequency and timing, and … [keyboard typing] holy **** it worked That was proof that I can control my
ceiling fan and light with my Raspberry Pi. I can even throw together a quick web page on my
Pi as proof that I can control it with my phone. The webserver simply runs the `sendook` program with the right arguments whenever you
push one of the buttons on the page. [keyboard typing] At this point, it’s just a proof-of-concept
- in the future I’ll integrate this into Home Assistant so that these controls are centralized
and integrated with all of the other smart-home hardware in my house. If you’re using other
systems like Alexa, I’m sure there’s a way to set up custom actions as well - they just
need to tell your Pi to run the right command. As always, I appreciate all of your comments
and likes! If you want to see more of my videos on tech, automation, or whatever else
I find interesting, please subscribe. Thanks for watching!
