The reason why you can find these modules is because 434MHz is an ISM band in some parts of ITU Region 1. You won't find an garage door opener there in the US. In the US you should use the 315MHz version which allows for license free use.

While 434MHz is in the 70cm amateur band you really should have an ID coming out of this thing when it transmits. All you need to do is shove your call sign out at 20wpm Morse code at the end of your transmission.

Otherwise it's a cool little hack.

There are some comments on the SparkFun page suggesting these transmitters are "Part 15" compliant (or at least can be, if only transmitting intermittently): https://www.sparkfun.com/products/retired/8946

Thanks for letting me know. I made sure the ATtiny84s could be re-flashed in circuit in case something like this came up. (I also have 315MHz modules, and will prefer those in the future. Swapping out the transmitters and receivers will only take a bit of resoldering.)

Actually, according to the ARRL band map, 434MHz is for repeater links, so even with a license you really shouldn't use it for this kind of stuff, unless it is below the "intermittent" limits on that FCC link.

This is really great for both the Amateur Radio community (proper) as well as amateurs of radio concepts. Like a lot of things, computers are becoming more and more things in more and more places. Your grandfather's crystal set is today's DSP with an Arduino. That's just fantastic.

Software Defined Radios are also fascinating! That's a very different post, though. :)

Nice post. For a slightly more comfortable setup the Arduino based JeeNode's (http://jeelabs.org) are worth mentioning. Super low power and quite a nice library.

For frame syncing you can also have a look at Hoffman-Neuman sequences, applying convolution between the expected frame sync sequence and the incomming bit stream.

I thought this would be yet another reference to a sci-fi story where someone is marooned on a planet with nothing, and they use their chemistry knowledge to re-implement enough metallurgy to build a spark-gap radio and get rescuced.

I'd read that

You have read Andy Weir's "The Martian"? (http://www.amazon.com/The-Martian-Novel-Andy-Weir-ebook/dp/B...)

Those stories are already out there. They're just hard to Google.

Is it necessary to periodically transmit? What are downsides to only transmitting on state change?

There's no way to know that the payload has been received - they're transmitters, not transceivers. The messages are idempotent anyway, so just retransmitting makes the whole system simpler.

Losing the state change in the noise. The receiver is the more interesting end in a setup like this.

It's almost like you didn't even read the article...