The relay switch has a fine wire coil inside which is perfect for a pickup. Combined with a strong magnet it can produce a really good sound.
This pickup only works for one string. However, if you want to use it for multiple strings, you can combine multiple pickups to create one larger unit.
Step 1: Parts
PCB relay switch: I found the relay I used on an old circuit board. Look around first! You can find them on plenty of circuit boards that are being thrown away.
1/4" mono audio jack: You can pick these up pretty cheaply at most electronic stores (Maplin/RadioShack). Also check out eBay. I bought 3 or 4 for about a fiver.
Wire: Any thin wire will do really.
A small strong magnet: As strong as you can find. The stronger it is the louder the output!!
In the first picture above you can see inside the relay. The left side of the relay is the part we're interested in, the coil. This will be the main part of our pickup.
The second picture shows what most relays will look like on the board.
Step 2: Cutting Out the Coil
First of all we need to identify the pins we'll be using as outputs for our pickup. This will be the two, as shown above, which will be on their own away from the main cluster of pins. You need to be careful with these two pins.
Depending on the way you're relay has been produced will determine how to proceed.
If the relay, as above, had a cover with clips simply remove the cover.
If it's glued, using a knife remove the top of the relay. This will make it easier to cut away the sides without damaging the components within.
I then carefully used a hack saw to cut the relay in half.
Once you have done this you can remove the thin metal cover over the top of the coil. This is the part that moves up and down when the electromagnet is switched on/off. Do not try to remove the centre core of the coil.
You're coil should now look a little like picture 2 above. Do not remove the pins attached to the coil. These pins are attached to either end of the coil and make it much easier to wire it to the audio jack.
Step 3: Magnifying the Coil
Step 4: Wiring
Naturally you can add other components to the circuit so that volume, tone etc. can be varied. However this instructable just covers the basic pickup.
This is the pickup I used in my One Stringed Music Stick instructable (https://www.instructables.com/id/One-Stringed-Music-Stick/.)