There are lots of fox hunt guides so I won’t repeat all the details. But A few that weren’t obvious or I did differently:

  • I used the earpiece from my Radioddity GMRS radio. By popping open the mic/button on the cord, I was able to solder onto conveniently labeled pads rather than having to separate out the wires. M+ = mic input, S+/- for the speaker includes S- as ground, PT needs to be grounded for PTT.
  • I used an NPN transistor (1kohm resistor on the base) to connect PTT (connected to the collector) to S- aka ground (connected to the emitter). Most guides say to use a relay, but even though this still showed 26 ohms or so resistance when open, it works fine to trigger transmission.
  • I used an old Trinket 5V I had lying around, since it has a wide battery voltage input (5-16v I think, accepts a 9v battery fine). However it uses an ATtiny85, which the Arduino tone() function doesn’t support. I ended up using http://www.technoblogy.com/show?20MO to generate the tone, and then wrote some very simple / non-general Morse generation code (glad to share the full sketch if anyone’s interested).
  • A 560ohm resistor from the 5v DO pin on the Arduino worked fine to drive the line in. And I didn’t have to remove the existing mic element. I was unsure what voltage range I was going for on line in.

Next step is antennas, and setting up in some enclosure with a HAM HT instead of the GMRS radio I was testing it with. I found a variety of antenna guides, but glad for other/better resources: