Alright, I got this idea, I'm redoing the pickguard on one of my guitars and am using a semi transparent material.
I could figure out how to wire an LED with a stereo jack or something that would light up beneath it, but i want it more interesting.
Im sure some of you have seen videos where the lights flash depending on what's being played, it kind of goes along with whatever string movement the pickups sense i'm guessing.
How would I wire/make such a thing?
Cusp of Magic
im pretty sure its more than just putting a LED across a few wires.. LEDs have voltage drops from around 2-4 V usually if im not mistaken. Your pickups have a voltage output of something in the milivolts. try google?
To do this, youd need a preamp in your guitar to boost the vltage of the pickups, or as said bfore the output of the pickups can't feed the LED.
I feared that'd happen.
So how would I go about just wiring an led by itself, usable by a simple on/off switch?
Whether the guitar is plugged in or not, an entirely different circuit seperated from the guitar's circuit?
Cusp of Magic
LEDs usually have a drop of like 1.5-3.5 volts, differs for different colors and brightnesses, so you have to take that into account.
you would also need to add a resistor to limit the current through the LED so that you don't blow it out.

I would get a battery(might be able to use preamp) and just add a new switch or wire it into a killswitch (if you have one already). figure out the voltage you need to run it, and run a wire to a resistor then to the LED, then from the LED to ground.

I study computer/electrical engineering at college, so I'm going solely off of my knowledge of LEDs and not the wiring of your guitar, so take that into account when you read this.
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Vs=Voltage source
Vd=voltage drop of the LED
I =Current
R=Approx resister value needed

(Vs - Vd)/ I = R
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couldn't it cause interference if you don't find some way of shielding it?
I have 5 in my guitar and the interference is really minimal, just a very small click/pop when you turn them on
Best way to do it is to run a 9V PP3 battery into a switch, into a resistor, then into the required number of LED's. This way of doing it would be much easier then trying to incorporate it into the guitars wiring, which I can see being a complete nightmare to do.

If you sand the tops of the LED's it will help to distribute the light emitted through the pick guard if they are mounted flush.

TBH though, there is a company in the UK that sells them pre-made, and it would be a lot easier to get one of their kits. However, if money is an issue then build it yourself.