I’ve been meaning to build one of these for ages.

Can anyone guess what it is?

It’s not a Taser, or one of those shocking buzzer things – the clue is that the coils are well “potted” and have very different gauges of wire.

Answers on a postcard please, or in the comments…

UPDATE : Correctly identified as a “guitar sustainerizer” otherwise known as an Ebow.

The circuit is a simple audio amplifier based on the LM386 (or in my case the NTE823) IC.

The two coils are taken from old telephone pickups – one (input) has a small rare earth magnet on the back, the other (output) was unwound and rewound with about 300 turns of #32 wire – this coil now has a resistance of just under 8 ohms. I tried a 4 ohm output coil, but the IC got too hot so I doubled the turns.

Note – I recently found a better candidate to make the coils with – tiny PC speakers.

The input coil has a resistance of 250 ohms – this is probably too high – I read somewhere that 50 ohms is probably better – so I may remove some of the turns on the coil.

Tomorrow I’ll publish the circuit and the etch mask, along with a video of the device in action.

…and here’s the patent 🙂

UPDATE2 : Here’s the schematic, placement and etch mask – click to enlarge