It's shown in the 8th image in the rotating set of images at the bottom. Some text in one of the images references Tetsuo Kogawa, so I guess the circuit is from here: https://anarchy.translocal.jp/radio/micro/howtosimplestTX.ht...
It would be really interesting to set up a little weak transmitter as a kind of geocache like find somewhere. You could have a super small microprocessor (ATTINY85) playing an audio loop, and also managing power to only transmit at certain times.