Temperature Sensor and Simple Oscillator Make a Value-Added HF Beacon

Sometimes the best projects are the simple, quick hits. Easily designed, fast to build, and bonus points for working right the first time. Such projects very often lead to bigger and better things, which appears to be where this low-power temperature beacon is heading.

In the world of ham radio, beacon stations are transmitters that generally operate unattended from a known location, usually at limited power (QRP). Intended for use by other hams to determine propagation conditions, most beacons just transmit the operator’s call sign, sometimes at varying power levels. Any ham that can receive the signal will know there’s a propagation path between the beacon and the receiver, which helps in making contacts. The beacon that [Dave Richards (AA7EE)] built is not a ham beacon, at least not yet; operating at 13.56 MHz, it takes advantage of FCC Part 15 regulations regarding low-power transmissions rather than the Part 97 rules for amateur radio. The circuit is very simple — a one-transistor Colpitts oscillator with no power amplifier, and thus very limited range. But as an added twist, the oscillator is keyed by an ATtiny13 hooked to an LM335 temperature sensor, sending out the Celsius and Fahrenheit temperature in Morse every 30 seconds or so. The circuit is executed in Manhattan style, which looks great and leaves plenty of room for expansion. [Dave] mentions adding a power amp and a low-pass filter to get rid of harmonics and make it legal in the ham bands.

Beacons are just one of the ways for hams to get on the air without talking. Another fun way to analyze propagation is WSPR, which is little like an IoT beacon.

[via Dangerous Prototypes]

13 thoughts on “Temperature Sensor and Simple Oscillator Make a Value-Added HF Beacon

  1. I’ve always wanted to make one of these with a solar panel and put it next to a creek near where I live. I’d have it chirp out the depth / flow rate of the water every hour or two. Maybe I’ll build and place it eventually. I just want to make sure it falls under the part 15 rules, which is honestly a little intimidating to me.

  2. Neat idea. I wonder if you could save a few parts by using the internal oscillator on the ATtiny with the 13.56 MHz xtal. You’d need more filtering to knock the harmonics off the resulting square wave, but it might be a net gain.

      1. That circuit is hilarious. It goes into crystal-off-power-down mode when it’s not transmitting, and fires the AVR’s crystal driver circuitry back up to transmit. Absurdly clever!

        But it’s also got to be fairly tweaky. As you change the capacitive loading on the antenna, it’ll pull the crystal frequency with it.

        The ATtiny25/45/85 have an 8x PLL multiplier internally, though. You could run the chip at 13.56 MHz, bump up the PLL, and then drive a GPIO pin with the onboard hardware. This would have a lot more drive capability (whether that’s desirable or not) and still have the same parts count. Anyone tried this approach?

    1. “That’s amazing – what are those little ‘pad’ boards?” The link is in the original post I copy it here:


      Unfortunately for me, the qrpme.com site isn’t loading now. Perhaps the HaD effect?
      Regardless. The “‘pad’ boards” as you call them are just little squares of PCB. You really don’t need to buy them. Just cut some squares from regular stock PCB and glue them down to a carrier PCB which acts as a ground plain (through HF frequencies anyway). Tin-snips or (better) a nibbler tool work OK. In a pinch just carefully use a razor knife to score some Phenolic (not glass-fiber FR4) PCB and snap the squares apart.

      It seems to me that right now the HaD comment editor is mangling line-ends. So apologies for bad format after posting. It is not my fault.

      1. Yes, as Drone says, they are MeSQUARES, with a MePAD for the ATtiny85. Rex also has smaller MeSQUARES, which are handy for work in tight spaces (can’t remember what they’re called, due to his site being down). His MePADs come in sheets with a selection of different types, for both DIP and SOIC devices. As of this morning, Rex’s site is still down, unfortunately.

        Drone is right – you can make your own. If you Google something like ‘make your own Manhattan pads”, you should find some articles. I do like Rex’s pads though. I’m a bit lazy that way. If he ever stops selling them, I may have to take up the style of construction known as ugly construction.

  3. Although he may actually be operating under part 15 rules at his power level, 13.56 MHz is actually an ISM frequency, governed (in the USA anyway) by 47 CFR Part 18, specifically:
    § 18.305 Field strength limits.
    (a) ISM equipment operating on a fre-
    quency specified in § 18.301 is permitted
    unlimited radiated energy in the band
    specified for that frequency.

    (emphasis mine)
    Subject, of course, to limits on harmonics and other spurious out-of-band emissions.

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