Previously, I have read about a wireless Morse thermometer designed by Steve Weber, KD1JV. I played around with the basic idea in a YouTube video I made. It basically powers a little Colpitts oscillator from an IO pin on an Arduino. Weber’s circuit did much the same, just using a small 8 pin AVR and a temperature sensor to send the current temperature via Morse code.
Over on the Open Emitter blog (sorry, don’t know whose blog this is) there is a twist on this basic idea: instead of using a separate oscillator, use the oscillator that drives the microcontroller as the emitter (probably of just a few microwatts into a short wire antenna). To on-off key the oscillator, the Atmel chip is put into power-down mode, and woken up via the watch dog timer. I think it’s a cool idea, and worthy of experimentation. I’ve got some ATTINY85 chips on sale (just $1.10 each) which will probably be breadboarded into a test circuit, using either some of my 10.140Mhz crystals or maybe some of the 80m or 20m crystals that I have lying around.
Check out the article: