Contact Microphone

This device is designed to allow you to hear the vibrations moving through a solid object. The piezo disk in the cap converts the vibrations it feels into electrical signals in the form of positive and negative voltages. This signal are then made bigger as it goes through the amplifier and smoothed out a little as it pass through the resistors and capacitors. The amplified signal then goes through the volume knob, which determines the amplitude of the voltages, and to the headphone jack. Your headphones then convert the voltage signal sent to the jack back into acoustic energy with its speakers. Simple as that!

Once you have the PCB boards ready (I cut mine out with an Othermill desktop CNC machine), the first step is to solder in the volume knob and power switch. Both of these are placed on the non-copper side of the PCB and their pins are soldered on top. The amplifier is then placed in the middle of the board so it's little notch is facing away from the volume knob. Before soldering, I bend out the amplifier's pins on the underside so it is secured to the board.

The 220 uF and the 0.05 uF capacitors are then soldered on the board near the volume knob. Make sure that the white stripe on the 220 uF capacitor is closer to the center of the board (the voltage signal can only flow one direction through this type of capacitor and the minus sign on the white signifies the "downstream" voltage side). The 10 (brown, black, black) and 10K (brown, black, orange) ohm resistors are then placed on the other side of the board - the 10k connecting the mic-/amplifier signal to ground and the 10 connecting the 0.05uF capacitor to ground.

The resistors and capacitors work to condition the signal and make it nicer to listen to. The values are fairly arbitrary and any replacements with similar values should work just as well.

The final step before assembling the board with the case is to solder in the battery connector and piezo. The leads for the battery connector should be put through the holes from the non-copper side of the board as shown, then soldered from the top. To connect the piezo leads to the copper side of the board, I put solder over their holes and melted it as I placed the leads in. Notice that the 9V+ and the red lead of the piezo are connected, the 9V- goes through the switch to ground, and the black lead of the piezo goes into the amplifier as well as through the 10k resistor to ground.

I added a 4-stripe 3.5mm line out to the contact mic to make it easier to record the sounds on a phone or computer - this is not needed if you just want to use headphones with the contact mic. I soldered the ground to the outside clip of the jack (the bottom strip), then soldered the signal line to the remaining three leads (the left channel, the right channel, and the mic channel). This allows you to listen to sound produced by your computer or phone through the contact mic if you have headphones plugged in while the Aux out is plugged in. I used a drill to twist the two wires together into a nicer line.

The .stl files for the 3D printed housing are attached, but the designs can all be found in this OnShape project. The three parts of the case are the tube to hold the circuit, the piezo cap, and the back cap. The tube could be printed any without support material, but the piezo cap will need support material that can be sanded away after. Depending on your printer's resolution, you may have to add some tape on the rim of the tube for a snug fit with the caps (or you can modify the design in OnShape to get the right tolerance).

Once the tube is printed, the first step is to put the headphone jack in place on one ledge and the aux chord (or not) through the hole next to it. The soldered pcb should be placed in copper side up, with the battery clip threaded through the tube to the bottom side. If using the aux out, you should solder that first.

To insert the pcb, the board should be tilted sideways so the volume knob and power switch are inserted into their openings, then the board can be pressed down so the headphone jack fits into it's through holes (you may have to pull the aux chord through the hole as you set it down). The headphone jack should then be soldered into the board from the top.

After sanding down the support material for the 3D printed piezo cap, you can glue in the piezo disc. I place it face up and use copious amounts of hot glue to hold it in place.

Here you can see the final product (fancy back cap with broken off top shown). You can also see the masking tape I used to make a nice press fit for the caps around the tube. Happy making!!