Take me to the Steps

123_piezo_jw_03

This project first appeared in MAKE Volume 38, on page 85.

A contact microphone is a small device that can be used to amplify acoustic instruments. Unlike the more common types of microphone, you do not sing or talk into a contact mic. As the name implies, it does its trick by making contact with a solid object and turning mechanical vibrations into electricity.  Because a contact mic does not pick up all the ambient sounds in the room, it allows you to focus in on one instrument without interference or feedback.

If you wanted to amplify your acoustic guitar, for example, you could attach a contact mic to the body of the guitar. Commercial versions use a small suction cup. For this project, we will use gaffer tape. In fact, you may already have everything you need lying around the house. I am, of course assuming that you fancy yourself a bit of a musician and already own a low-powered practice amp. Maybe you have built the Cracker Box Amp from MAKE Volume 09?

The main ingredient in this project is the mic element itself, a piezo disc. Piezos are used as lo-fi bleepy speakers in smoke detectors, old cellphones and computers, and handheld video games, and they emit a high-pitched whine when you give them some current. They also work the opposite way — in a beautiful bit of symmetry: apply current, get vibration; apply vibration, get current. That means the same device can be used as either a speaker or a microphone, depending on which way you wire it up. (This is actually an old trick. Need to record a bit of dialog, but don’t have a microphone? Plug in an extra pair of headphones into the mic input!)

Share yours: #piezomic

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Project Steps

Aquire parts

The fun of this project is to repurpose things you already have lying around. Gather up some old toys from the attic or the thrift store, and pull out the blippity bleepy ones.

Open them up and see if they have a piezo disc or a normal speaker. Hopefully you will find some piezos. You’ll need to break apart some plastic bits to get at the discs —just be careful not to damage the discs.

Wire it up

Cut the audio cable in half. Now you have enough cable and connectors for two contact mics!

Strip off a bit of outer insulation from the cut end of the audio cable. Strip and tin the signal and ground wires.

De-solder the two piezo wires where they connect to the piezo. Now solder and heatshrink the signal and ground wires from your audio cable to those same points on the piezo. If using a headphone jack, make sure not to mix up your positive and negative wires!

Dab some hot glue on the back of the piezo, for strength.

Test out your new mic by plugging the cable into a low-powered amp and tapping on the piezo. You should hear the sound of the amplified tapping.

Finish it up

Use heat-shrink tubing, electrical tape, Plasti Dip, or epoxy to insulate the wires, solder connections, and the piezo itself.

Use gaffer’s tape to attach the mic to different objects like cardboard boxes and paper cups, and acoustic instruments like guitars and kalimbas. You may get a feedback loop if the mic is too close to the amp; not necessarily a bad thing if you are a noise artist. EFX pedals can be used to flavor the sound to your taste.

Going Further — Superpower your project by building a preamplifier circuit for your piezo. Collin Cunningham shows you how in this video tutorial: makezine.com/micpreamp.