There is abundant information available on the Internet regarding the construction and use of a transducer (Piezo). But, the purpose of this 'How To' missal is skip all the technical jargon and to share with you how I utilize the piezo transducer in the creation of my cigar box guitars, to transmit sound to an amplifier. I believe in the 'KISS' theory for most of what I do. (Keep It Simple, Silly)
First, it is important to understand a transducer. The transducer is a microphone, which picks up vibrations around it. For more refined details, refer to the Internet jargon.
Generally, I use a disc piezo placed near the bridge on the inside of the box, but on occasion I use a rod piezo placed directly under the bridge on top of the box. Placement of the rod is a little more involved, but it may produce better sound. However, if you want the best sound transmission, use a magnetic pickup.
The positive lead (usually red) will be attached to the positive post on the output jack, and the negative lead (usually black) to the ground post on the jack. That's it.
However, I like things a little more sanitary than loose wires floating around, so I do a couple things to refine the assembly. Leads on the piezo are short (about three inches long), but you will need a finished assembly to be 9 or 10 inches long to fit properly in the box you're using for the body of the guitar. So, I stagger the length of the the wires coming off the piezo so the exposed ends are not next to each other. Then, I add (solder) 7 or 8 inches of 24ga. wire to the existing leads to reach the required overall length. When I've completed this, I slip a piece of 1/8" shrink tube over the wires (leaving only about an inch of each wire exposed to attach to the jack, and heat it with a hot air gun to make a tidy little assembly. Then, I solder the leads to the jack, and I'm in business. If you don't want to mess with the shrink tube, I suggest that you cover the exposed joints and twist the wires to make a better looking package.
Parts for this project will cost you about five bucks, or less. But, if you don't mind throwing money around, you can buy the ready made harness from various sources on the eBay. I've seen them offered from $12 to $24, plus shipping. I think I'll continue to make mine, 'cause it's simple, and I don't like giving money away!
Placement -- The versatility of the piezo presents several options for placement in your cigar box guitar (stomp box, drum head, etc). And, the piezo can be cut to a desired shape for ease of location, without harming it's effect.
It really doesn't matter where you place the piezo, because it will pick up sound. But, what will matter is the quality of the sound it transmits. Some builders locate it under the bridge; beside the bridge; on top of the box; on the outside of the bridge; under the fretboard; and even under the nut. The options are many, but each will produce different results, and some may not be very pleasant.
Under the bridge -- A very good option, because the piezo reacts to vibration, is to place it under the bridge, sandwiched between the bridge and the top of the cigar box (soundboard). In either of these options, a small hole must be drilled in the box for wire leads to pass through to be attached to the output jack.
Inside the box -- This is my favored solution for mounting a piezo. Finding the 'right' location may be a bit of trial and error, and it may vary with each guitar you build. But, I have found that if I position the piezo as close to the bridge as possible, and directly under the bass string, it generates the best sound.
However, in the process of trial and error, I've also learned that 'how' the piezo is mounted makes considerable difference in sound and amp feedback. Some builders glue the piezo directly to the box surface. Others encase the piezo in wood pieces that are glued to the surface. Some even use a screw to attach it. Many cover the glued piezo in various substances (silicon sealer, foam insulation, etc.).
What works best for me is to encase the piezo between two layers of double-sided 3-M picture mounting tape. You know, the dense foam material about 1/8" thick and 1" wide, that comes in a roll, and has a green/white protective cover that can be peeled away leaving a very sticky surface.
I cut two squares from the roll, leaving the protective cover in place. Then, I carefully center the back of the piezo on the sticky surface of one of the pieces. Then, I pop the other sticky square surface on top of the piezo (the top is the part with the ceramic ring) -- voila, it's enclosed and ready to mount. So, I now peel off the protective cover from the surface covering the 'top' of the piezo (make sure you remember which surface is the top, 'cause sound is transmitted better if the piezo is mounted with the ceramic ring surface next to the box). With this done, there is still a surface with the protective cover in place, right? Well, peel off the cover and apply a square piece of tinfoil (slightly larger than the sticky surface). Rub hell out of the entire piece to ensure that the sticky surfaces are well adhered. The tinfoil works to eliminate feedback.
The sound produced by this method is more mellow and less 'tinney' and 'scratchy', with no feedback at my amp.
Multiple Piezo Mounting -- By adding more than one piezo to your build, you will be offering some interesting sound options for your creation. Wiring multiple piezos can be done in 'parallel' or 'series', but the recommended method is to wire in parallel (each piezo has it's own set of wires leading to the output jack), which produces more consistent sound results.
Adding Pots and Switches -- Volume potentiometers and switches can be added to your piezo harness (a 250K audio taper pot or 500K pot can be used, but the 500K seems to work better), if you wish to control sound at the guitar, rather than at the amp.
Options -- The interesting thing about creating cigar box guitars is that there are no rules. Experiment. If something does not work like you want it to, try something different. It is your individual ideas that will make your guitar unique and different, so don't be afraid to step outside the box.
You're in business! Play hell out of your guitar, while driving your wife, kids, and pets nuts. q:-)