This was a fun little project – and it gave me an idea for a future one. Sometimes, I find that the more complex undertakings, which require more planning, can get to the point that they “take me over” somewhat. At that point, for me, some of the fun starts getting squeezed out and that, of course, absolutely cannot be allowed to happen. This is the time when simple and fun projects save the day.

QRPp beacons have been a “thing” of mine for a while now, ever since I put the Sproutie SPT HiFER Beacon on the air, sending it’s 12 wpm SPT ID out continuously on 13558KHz, with a mighty 4.6mW to a much shortened loaded dipole. That signal was spotted by a few people, including one spot 900 miles away. SPT is not on the air now, but it’s sitting here in the shack, “just in case”. It was a legal, though unlicensed, beacon, courtesy of the FCC Part 15 regulations. The idea that a signal which is so miniscule that it doesn’t even require a license to be radiated, can be spotted so far away, is quite wonderful to me. Others have had their HiFER beacons spotted from one coast to another – and that’s with regular Morse code speeds. Imagine what you could do with QRSS on that band – which some experimenters do.

After assembling a QRP Labs U3S for my ham friend, my mind turned back to some of my personal interests, and mental stash of “things to be built”. In the world of experimental HF beacons, as well as the legal HiFERs, there are also a good number of signals dotted throughout the HF spectrum that are not licensed (i.e. pirate beacons). I remember, shortly after moving from Los Angeles to San Francisco, I was living in a ground-floor apartment just 3 blocks from the Pacific Ocean. It was wonderful falling asleep at night to the sound of fog horns. Just as evocative, and exciting, was hearing, for the first time, one of these low-powered pirate beacons on my FT817 hooked up to a Buddipole inside my apartment. It was one of a cluster of beacons around 4096KHz, and situated in one of the southern deserts – Mojave, I believe. None of the beacons in that cluster were putting out more than around a watt, and they were out in the desert, sending their brave little signals across hundreds of miles to my receiver in the SF Bay Area. Magical! These beacons have all sorts of different sounds, varying from chirping, bleeping and whooping, to actual Morse ID’s. A few have temperature sensors, and send the current temperature. I think one even has a windspeed indicator. It may not necessarily be particularly useful information, but it adds a bit of interest to the predictability of a simple ID sent over and over again.

That’s what got me to thinking of building a little temperature beacon and, after some browsing and searching, I came across a simple and effective design by Sholto K7TMG. You can read his very informative write-up on it here –

The K7TMG HF Thermometer

It’s a simple one-transistor Colpitts oscillator with the positive supply line keyed by an ATTiny13 micro-controller. The genius (as with so many things these days) is in the code that Sholto wrote. It converts the analog voltage from an LM335 temperature sensor to Morse code and announces it, about once a minute, in both Centigrade and Fahrenheit. When Sholto’s article was first published, he was offering pre-programmed ATTiny13’s for $5. He has since discontinued that offer but instead, has made both the source code and a hex file available on his site. The links are in the article above. I had trouble compiling his source code which, I’m fairly sure, was due to user error at my end, so I simply burned the hex file to an ATTiny 13V-10PU using my Etherkit Etheprog ISP and KC9ON Programming Adpater from Third Planet Solar. Any similar AVR in-system programmer would work fine, such as the Tiny AVR Programmer from Sparkfun, or you could use your Arduino to program it. I’ll leave it to you to figure that out. There’s plenty of good info out there in internet-land.

Before I start raving about this little circuit, I want to show you my version, which is identical to Sholto’s, except with the addition of two resistors at each end of the trimpot to make calibration of the thermometer a little easier, and a different value of the molded choke in the oscillator collector circuit (his was operating at 4MHz). I used a different value of trimpot too. I did try to contact Sholto, but didn’t hear back, so I hope he won’t mind me showing my slightly altered version of the schematic here. However, there’s nothing particularly original about it. As I said, the original work, and the clever part, is in the code, which Sholto has already made publicly available.”