Build a Low Noise Contact Microphone (inducer/transducer)

This instructable will show you how I built a great low noise contact microphone for my friend's double bass!

Thanks to the transformer, the microphone has a very good signal to noise ratio. That means that the inherent hum/noise of contact microphones is almost gone! The microphone can also be used in reverse to play back audio.

This instructable is inspired by Nicolas Collins Hack of the Month Club -- Project #6: Drivers & Transducers . Please check out his other videos, they're pretty awesome!

• Piezo disc
• Audio output transformer 1200/8
• Glass jar metal lid
• Cardboard
• Pill bottle
• 1/4 in (6.5mm) female phone jack
• Shrink tube
• Wires
• Guitar cable 1 ft (30cm)

• Soldering iron
• Hot glue gun
• Wire cutter/stripper
• Double sided foam tape

1. The idea was to use the lid as a cover for the sensitive piezo disc. So the first thing I did was to hot glue cardboard inside the lid to make the piezo disc able to lay flat against any surface I attach it to.
2. Next I reinforced the solder joints with hot glue and striped the microphone cable and tined the wires.
3. Use heat shrink tubing to make the connection more rigid.
4. Glue it in place.
5. I made a hole in the bottom of the pill bottle and reinforced it with a washer. I fed the cable through the hole and tied a knot so it couldn't slip back out.
6. The transformer is connected with the high ohm primary winding on the piezo side and the low ohm secondary winding on the amp side. Solder to the outer connections and insulate with heat shrink.
7. I made another hole in the pill bottle lid and attached the phone jack. Connect the wires from the phone jack to the outer pins on the transformer's secondary winding.
8. This is the circuit we are building.

Ok, first off, I'm not an electronics expert. This is the first time i've ever used transformers in my builds. All I know is that when we're using a transformer with a piezo disc like this, we're doing something called impedance matching. The main difference I noticed, compared to contact microphones without transformers, is that the signal to noise ratio is much better. I think it has to do with having a non-galvanic connection between the piezo and the amp. According to the video link in the intro, this build can also be used in reverse to make the piezo behave like a driver. Attach it to different materials to make them vibrate and act like a speaker membrane.

Almost finished! Just needs a paint job. I use double sided foam tape to attach it to stuff. This also helps a lot with sound quality.

Try placing the mic in different places to find a good sound. The pill bottle sits under the bridge or tailpiece.