The Piezo Transducer is a tiny, cheap and useful piece of electronics.

You can pick one up from JayCar for around $3.00.

With some easy modifications you can turn one into a contact microphone. You can use it as an instrument pick up on the acoustic instrument of your choice!

All you need is some basic soldering skills, a piezo transducer and a female jack input. There are numerous YouTube videos that demonstrate how to do this much better than what i would type here.

I have used one to make a stomp box out of a chocolate box... It has a creamy and caramel timbre

As you can see this is a great technology for the classroom with not a lot of money

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