There are 2 main ways to turn the physical vibration of your strings into an electrical current that is then sent to an amp and made into much much louder noise.
One is through electromagnetic induction, and the other is through piezoelectric magic. For induction pickups your strings needs to be conductive, so basically just metal. For the piezo style pickup you can amplify ANYTHING as it works through physical vibrations. I’ll talk more about piezos later, for now i’ll talk more about inductive pickups.
Faraday – the original noise maker
At its simplest it involves a coil, a magnet, and movement.
With a guitar pickup you have a coil of wires wrapped around some strong magnets. When a string is plucked, the movement of that string creates a tiny electrical current in the coil due to the presence of the magnets. movement + magnets/coil = current.
That current is teeny tiny, so is sent to an amplifier, which simply amplifies that current to a level that can be sent to a speaker and heard by our human earholes.
That’s all very well and good, but how does that help with us DIY-ers? Well, you could build your own pickups, that’s not outisde of the realm of possibility. There’s tons of guides on how to do that online.
Another thing you can do, is utilise other electrical components that use Faradays law for different purposes:
Electric motors receive current, into a coil/magnet arrangement and create movement (they spin real fast). They’re essentially the opposite function to a guitar pickup, but using the same law of physics to do so.
Inside a bog-standard electric motor
So, if you were to drill a hole (carefully) in a motor casing, and run a wire through it, all you would need to do is connect the two motor terminals to your amp and you’ve just built a pickup.
You can get fancy and use it in instruments like this one:
But wait, there’s more!
Different types of motor have different coil arrangements inside. So, for example, electric fans have something like this:
This coil was from a pond pump motor:
By attaching the magnets directly onto the springs, the electrical current is vastly stronger/louder. Which is fine with springs, as it won’t dampen the vibration too much. Using magnets directly onto strings won’t sound too great, but I’d love it if you could prove me wrong.
Transformers are parts of electrical circuits that either increase of decrease a voltage for whatever reason it’s needed to do that. So, for example, when you have a power adapter that turns 240v down to 12v, chances are there’s a big fat coil inside that plug that you can utilise for pickup-y purposes.
Speakers use the same law of physics as motors and pickups, the diaphragm of the speaker cone is connected to a very lightweight coil that surrounds or is surrounded by a magnet. When you send a current into that coil it will push/pull the coil and therefore the diaphragm in the same frequency as the current turning it into audible noises.
So, we can utilise that bit of physics and the construction of the speaker cones to induce noise backwards into it. By vibrating the cone of the speaker, we create a current in the coil which can then go off into an amplifier to be made audible.