Step 1: Materials
Gather these materials.
1-Old smoke detector/co2 detector.( Be sure to only use one that is not being used.Fire and co2 detectors have their vital purposes,you're safety)
2-Old computer mic cable.
3-Pill bottle cap
4-(optional) 1/4 adapter jack,these come with most store bought headphones.
Take apart the smoke/co2 detector.In the one i had after i removed the housing the Piezo beeper fell right out.Try not to drop it or damage it,this will be you're pickup.The piezo element is a small silver disc with another disc on the inside.This is what makes that annoyingly loud beeping sound when you over cook you're meal.We will be soldering the wire to this disc reversing the function so that instead of making sound,it will pick up sound when attached to you're guitar.
While you're at it now's a good time to do some prep work for the Pill Bottle cap.The cap will be you're housing for the pickup.
Drill a hole in the middle of the side big enough for the wire to be threaded through.You will need to have the wire already through for the next step.Thread the cap a ways down the wire so it won't be weighing/dragging down the wires when you are soldering as explained in the next step.
Now you will need to take the working computer mic cable and strip the wires.You might have to play around with it a bit to find the ground wire,on mine the ground wire was the bare unshielded one.What you want to do is solder the ground wire to the outside of the piezo element.There should be an inner disc,you want to solder the other wire to that part.
The picture below shows the completed soldering job with the wire already threaded and knotted through the pill bottle cap.
Tip:A fair knowledge of soldering is a must.There are several soldering instructables.If you're not sure of you're soldering abilities do a search and learn how to do it right.
Now you have a working pickup.What you want to do is make sure that you aren't getting any feedback from it.If the ground wire is touching the inner disc due to a mistake with the soldering job now would be the best time to re do the joint.Make absolute sure that the bare wires are only touching what they are supposed to.Plug you're wire into a guitar amp with the 1/4 plug or the mic input of you're computer.If you are getting a buzz that's no goofd,it's time to go back and find out what you did wrong.
Now that you've tested it and it sounds good to you it's time to glue on the housing.
Tip:Buzzing can occur for many reasons,If you have a faulty cable that isn't shielded properly or has something wrong with it an excellent solder joint will do nothing.Make sure you have a good enough cable.
Now you will need to glue on the housing and you're done.
Calculate a good length to knot the cable.The knot will ensure that when the pickup is mounted and you step on the cable it won't snap off at the solder joint.With the distance judged so as that the cables aren't moving freely inside the cap you want to knot the cable at the correct length.If the wires move too much within the housing they can fray and snap off
You will now need to glue on the cap to the back of the pickup.I used in this case Krazy Glue.I beaded the glue around the edge of the cap and carefully placed the pickup on.Depending on the type of glue you may want to set it down and place something on top of the cap to ensure proper adhesion and clamping pressure.
When the glue dries you will have yourself a pickup.
To mount the pickup you will now need some sticky tack.Roll it in you're palms to make a small strand.Take that strand and make it into a ring then tack it to the bottom of you're pickup then mount it onto you're acoustic.Depending on you're tastes different parts of the guitar will produce the "sweet spot" you might be looking for.
Final Thoughts:I made this pickup for some simple home recording.I would not recommend playing on stage with this fragile pickup.Although if you are very mindful of where you are at all time or spend most of you're time sitting this may be a good option.I have not yet tested this in a loud environment so i cannot see how it will be effected by loud stage volumes.However it is designed to work sort of like a mic,therefore i am guessing it will be very feedback prone when amplified as these types of pickups are prone to.
Thank you for you're time.
Enjoy and post you're own creations below :D