FM microphone kit from Aitendo

In one of my random visits to Aitendo I got the AKIT-334 FM microphone kit on sale (200 yen).

Unfortunately, once I compared the kit's circuit diagram with the components I actually got, I finally understood why the kit was selling so cheaply. In particular, I got a 20pF capacitor that should have been a 10pF; and 2 resistors with the wrong values that I replaced from my own stock. Also, thanks to this site I figured out that I had to cut the track that was shorting 2 of the C3 trimmer capacitor's leads. As if that wasn't enough, I also had to modify the LC resonant tank so that the transmitter would align to the FM frequency range in Japan (76~90 MHz). In particular, I made a solder bridge between the pads where capacitor C2 was supposed to be, and then put C2 (in my case 20pF) in parallel with the C3 trimmer.

The board comes without a coil so I had to make one myself. For me that was the fun part of building this kit. First, I calculated the theoretical value for the coil on this LC resonance calculator. You can also do it by yourself on any calculator application (I use qalculate) using the following formula:

Resonant frequency (Hz) = 1/(2*pi*sqrt(L*C)).

I tried to cover a bit more range so for 76.2MHz (C==40) ~ 108MHz (C==20), I got that my coil's inductance (L) should be 109nH. Next, you need to figure out the physical properties of your 109nH coil. For that use the following formula:

L = perm * (N^2 * Pi * R^2) / length

The permeability of air (perm) is 1 so that is a fixed value unless you use a core. By trial and error I ended up with these values:

109nF = 1 * (7^2 * pi * 2.4e−3^2) / 8e−3

That means a coil made of 7 turns, 2.4mm radius (including the wire), and 8mm length. For the magnet wire, I bought 0.6mm diameter (⌀) polyurethane copper round wire from Akihabara's Ikenoya radio, located in the famous electronics mall radio center (Tokyo hacker space has a nice list of electronic shops in Akihabara here). When I made the coil however, theory and practice differed and I had to tune my coil a little bit using an LC200A LC meter that I ordered from Aliexpress at a very reasonable price (~4000 yen).

This is how the FM microphone kit looks like when assembled. Notice how I applied a bit of hot glue to components that seemed to have more mechanical stress (the power switch, the battery case and the antenna), and I used a short cable for the antenna.

Finally, I used this FM radio receiver that I assembled from a kit sold at Sengoku (Akihabara). Don't worry about interfering with commercial radio stations since this kit can radiate just enough power to listen to yourself from a few centimetres, or perhaps a bit more depending on your antenna and receiver quality.