I made this contact microphone and thought that it would be a very approachable project to do, so here it is. It is a simple design that will allow you to record using a contact microphone and allow you to do some simple filtering.
Step 1: Things You'll Need
1 Piezoelectric Sensor
1 500kOhm or 1MOhm Potentiometer
1 0.1uF capacitor
1 1/4" or 6.35mm input jack
3D printed case parts provided (feel free to make your own)
Step 2: Schematic
This circuit uses a low pass filter to output sounds from the piezoelectric sensor (or contact microphone). The potentiometer is used as a variable resistor so that when you turn the dial you change what the filter is filtering.
The potentiometer only has the middle pin and one of the outer pins connected to the device. You only need these two to create the variable resistance. The capacitor is wired to ground with the black wire of the piezoelectric sensor, and the ground wire of the audio jack is also wired to ground.
Note* I know that my potentiometer is 100kOhm in the schematic, I recommend 500kOhm instead because it will give you a larger filter range.
Step 3: Putting It Together
Here is an image of the device wire up. I place the capacitor across the power and ground lead of the input jack, and then I solder the black wire of the piezoelectric sensor to the ground lead of the input jack. Then I solder wires onto the middle pin and one of the outer pins of the potentiometer. I take the nuts and washers off of the potentiometer and input jack, slide them through the holes of the 3D printed case, and then I tighten them in place with the nuts and washers.
I solder the red wire of the piezoelectric sensor to the middle wire of the potentiometer, and then I solder the other wire on the potentiometer to the less wired lead of the input jack.
Note* the frame of the input jack is usually meant for ground and the tab sticking out of it is usually meant for the positive input. Please keep this in mind and make sure you know which one you are supposed to solder to for the input signal and ground.
Step 4: You're Done!
Now you can place the bottom of the casing on by using glue or tape (I used hot glue because I was too lazy to make the bottom of the case a friction fit.
You can plug the contact microphone into a guitar amplifier and it will output the sound of whatever it contacts.