This circuit is commonly credited to Japanese multimedia artist Tetsuo Kogawa. It takes audio input through a 1/4″ phono jack and, constructed as shown, without the optional antenna connections, will broadcast an FM radio signal about 30 feet.
This is the standard model of Mr. Kogawa’s simplest FM transmitter, which is slightly more complex than his most basic design in that it includes a trim capacitor to adjust the transmitting frequency. It can be powered by a 9V battery and uses a hand-turned copper coil.
I’m using the PCB and parts from Sonodrome’s old kit, but the circuit is extraordinarily simple and could be built on perfboard or on a panel almost as easily. Sonodrome provides free PCB art if you want to etch your own board. Kogawa himself provides instructions for building the transmitter on an unetched copper-clad panel.
Take a piece of 19 AWG enameled copper wire, about 4" long, and wind at least four turns about the threads of a 1/4-20 bolt or machine screw.
Rotate the bolt counterclockwise to unscrew the coil from the thread.
You want a total of four turns in the coil. Use small pliers to bend two legs down, as shown, and side-cutting pliers to clip them to about 1" long.
The mounting holes for the coil legs should be 12mm apart on the surface of the PCB. The act of installing the coil on the board should stretch it to the correct length, but you may have to tweak it a bit with pliers or a screwdriver to make sure the rate of coiling is even between the two legs.
Remove the washer and nut from the phono jack and thread it, from inside the case, through the hole you drilled in Step 1.
Put the washer over the threads and tighten the nut down from outside the case, to secure the jack. Finger-tight is fine.
Attach a 9V battery to the clip, pad it with a scrap of bubble wrap, and stuff it in the case before sealing up.
I'm going to to modify my transmitter with a jack for an external regulated power supply. You may want to do the same, or at least add a power switch between the battery and the board. But for testing purposes, this set-up will suffice.
Turn your radio on again, pick it up, and walk away from the bench 'til the signal fails. Mine was loud and clear to about 30 feet.
Depending on where you live, operating even a very short range FM transmitter like this, without a license, may conflict with applicable laws and/or regulations. Be sure to investigate carefully before turning it on, and err on the side of caution if in doubt.
I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and makezine.com. My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.