brainwagon "There is much pleasure in useless knowledge." — Bertrand Russell

20Dec/110

Back to QRSS…

For some reason, I'm getting back into the universe of QRSS, or very slow Morse code. I goofed around with this for a while, writing some software to record audio and produce the necessary FFTs so that you can read Morse from these incredibly long Morse messages, but lately haven't done much lately.

Hopefully, that will change during my time off for the holidays.

Somewhere in a junkbox, I built this circuit (from Hans Summers, G0UPL):

It's really just a Colpitt's oscillator, with a reverse biased LED acting as additional capacitance to "tune" the frequency around the nominal crystal frequency. When I built this circuit, I played around a bit with using an Arduino with an digital to analog converter to drive the LED back capacitance, and that worked fairly well. Here's a timewarp back to that project, in the form of two of my earlier YouTube vids:





It worked, but I never put it on the air. But I got to rethinking it a bit more, and thought that maybe I'd assemble a new version and see how much I learned. I've got some ideas for improvements:

  • Make it "real". Think about the layout a bit, and built it to be completed inside an (Altoids?) case.
  • The 4.7pf cap is too much to be driven by 0-5v (it induces too much frequency swing). Use a "gimmick" cap in its place.
  • Build in a small "dummy load" (just a 50ohm resistor), and add some test points so you can monitor the supply voltage and current from (say) an Arduino. It would be interesting to monitor temperature as well.
  • Oh, and use an Arduino to key it. Or maybe just an ATtiny.
  • Spend some time thinking about solar powering it.

I also was considering making a better dedicated antenna for it. I suppose I could just use a 30m dipole, but I think it might be fun to try something a bit smaller. Alan did some experimentation with a center loaded short dipole which might be fun, but I'm still pondering that.

The micro-power transmitter is fun, and unlikely to cause anybody any anguish, but might also be a bit hard to get any real DX with. So, I started thinking about what I needed to change. Higher voltage, to generate more power. A buffer and power amplifier to generate maybe 100-200mw. And a harmonic filter, so I could be a good citizen. But then I realized something: Hans was offering a kit that had all of those features already, and for about $15 US. So, I've got one of those on order too.

I hope to be on the air come the New Year. Stay tuned.