A piezoelectric device also works in reverse: applying an electrical charge will cause the piezo to make a sound. Because the discs are inexpensive to build and fairly robust, they’re used in everyday products that need to buzz or beep, such as toys, appliances, and mobile devices. That means they’re everywhere, and the savvy DIYer can source piezo discs easily from broken and discarded electronics. I have students who treat their DIY piezo project as a treasure hunt, trying to see if they can build one completely from scrap.
If you’ve decided to source your piezos this way, it’s very likely that the discs will already have a pair of wires attached. That makes things a lot easier, because all you have to do is attach the ends of those wires to the wires of a shielded cable and you’re ready for action. For the sake of this article, however, I will step through the process of wiring up a disc from scratch so you can see what’s involved.
In this project, we will connect the piezo disc to the wires in a standard, shielded guitar cable: we will attach the strands of the outer shield to the metal disc itself, while the cable’s center conductor wire gets attached to the disc’s ceramic center. Although you can hear how a piezo contact mike sounds by using alligator clips to make the connections, such a flimsy setup won’t survive any practical uses. Therefore, we need to solder the wires to the disc in order to use it as a drum trigger.
Fig. 3 Preparing a shielded cable for soldering. I’ve twisted the outer wire shielding together and exposed the conductor wire in the center.
Soldering a cable to a piezo is fairly straightforward when you have the right tools, though it can be a bit tricky if you don’t have soldering experience. The delicate part has to do with heating the disc. If the iron touches it for too long you will irreparably damage the disc. Thankfully, piezos are relatively cheap (depending on where you get them), so don’t be afraid to destroy a couple as you learn this skill.
You will get the best results using a modern soldering iron that has a fine tip. For example, the ubiquitous Weller WES51 soldering station includes the proper type of soldering iron, a holder, and a sponge (which you will dampen and use to clean the iron’s tip when it’s hot). If you don’t want to buy a soldering iron, it’s likely you know somebody who has this model or something similar.
For this project, you will also need rosin-core solder, a pair of wire strippers, shielded cable, and a 1/4" plug (like the kind used for electric guitars), and electrical tape. If you can sacrifice a cable that already has a 1/4" plug at one end, that’ll save you the hassle of soldering the connector later.
To prepare the cable, strip away 0.75" of the protective layer to reveal the first layer of wire (known as the shield). As you pull the shield to one side and twist it together, you will be left with the coated wire in the middle. Strip off the insulation to access the internal wire, and twist its strands together. (This center conductor wire needs to be longer than the shield wire in order to mitigate stress on the solder connection, because it reaches further into the disc.) Now you should have two separate wire strands. You will attach the strand from the center of the cable to the center of the piezo, and the cable’s shield to the metal part of the disc.
Before you plug in your soldering iron, it’s a good idea to wrap a bit of solder around the tip. As the iron heats up, the solder will melt around the tip and protect it from oxidizing when not in use. Then, just before you begin soldering, wipe the tip on the damp sponge to clean off any excess solder.
Now it’s time to tin each of the wire strands. This is done by heating each one up with the iron and then pressing the solder against the heated strand. The goal is the let the wire strand soak up a fair amount of solder, which will help it make a solid connection when you attach the wire to the disc.
Here’s where it gets a bit tricky — touching the soldering iron to the disc itself. First, tin the areas of the disc where the cables will attach: Touch the metal part of the disc momentarily with the iron to heat it, then melt a dot of solder there. Then, touch the iron to the center portion, near its edge, and melt a dot of solder in that spot.
Finally, attach the cable to the disc by soldering the cable’s shield strand to the tinned edge of the disc, and the inner strand to the tinned spot on the center of the disc. Each time you complete a solder joint, give the parts a few moments to cool and harden before you move them so you don’t weaken the connection.
At the other end, you will attach the connector of your choice. Because you’ll be using these as drum triggers, it makes sense to use a standard 1/4" guitar plug. However, you can just as easily use a two-conductor 3.5mm (sometimes called an 1/8") plug. The smaller one allows you to connect your contact mike to portable recorders and miniature amplifiers (such as the battery-powered RadioShack Mini Audio Amplifier, a staple for hackers and circuit benders).