I've been winding coils! Instead of buying a sewing machine, I put together a winding station from parts I had lying around. This weekend project got me to play with a couple of microprocessors I had recently been ignoring in favor of the cortex m4.
The typical guitar pickup consists of a very thin wire (generally >40 AWG) that has been wound in a coil, which has on the order of 8,000 rotations in it. The exact number of rotations in the coil affects the frequency response of the pickup. A magnetic core is placed inside the coil.
A guitar pickup can also be operated in reverse as a driver. This is much less typical in musical applications, but two well-known examples of it are the E-bow and the Moog guitar. In this case, the signal present on the coiled wire can be induced onto the object.
An inductor is very similar to a pickup, but usually has fewer rotations, and therefore it can use a slightly thicker wire. These are useful for making passive filters and sine-wave generators.
Transformers are a bit trickier, as they require two or more coils to be wound around the same core. These can be used to passively decouple signals, trade voltage for current, etc.
My setup can wind all of these (although things like transformers require a little more supervision), and consists of an OSEPP board (an Arduino-like 8-bit microprocessor), an h-bridge circuit designed by my friend Cooper Baker, and a pair of stepper motors.
A bit of tension is good while winding to keep the coils tight. Getting the 47-AWG wire not to break as the motors applied tension was the trickiest part. I'm still not totally satisfied by my solution, but it works fairly well: I made little leaf springs out of thicker gauge wire, and put loops on the ends. The wire to be wound on the coil feeds through the loop. As tension is applied, a component of the stress is absorbed by the leaf springs. There are doubtless other, more suave ways of doing this, but I just wanted to make some coils and be done with it.
A video of an early prototype of the setup is here:
I am programming the OSEPP board from a Raspberry Pi, which runs headless on the LAN on my apartment, so I can issue the winder commands from any of my computers. It's well-suited for this line of work, and I wish it could cross-compile code for my 32-bit microprocessors, but the process takes long enough on a multi-core processor so I have not tried it (yet). Although, it would be lovely to have a battery-operated, field-programmable 32-bit microprocessor...
The pickup / driver coils will be used for more standard string instruments, as well as an arrayed application which is going to be totally amazing. More later...