hi there, I want to make a contact mike using a piezo transducer, however I'm not sure what gain will be required. Any ideas?
(alternatively, do you know what voltage a piezo outputs when it is tapped by a fingernail?)

 

No way to tell or guess unless you try it.
You will need a gain of about 100 to 300.

 

a gain of at least 100? That's very helpful, thanks! So does a piezo produce an output of a few millivolts?

 

Can't say for sure. The devil is in the details. You just have to try it out.

 

Place I used to work did some good things with piezos such as level sensing. They bought crystals with a resonance of either 1 or 3 MHz and we'd amplify the signal by at least 1,000 so we would get a digital square wave that would directly drive digital chips.

 

Piezo transducers make a poor microphone with many peaks and nulls in its frequency response.

Microphones are never "tapped". If you tap it then its output level depends on how hard you tap it.

Its output level also depends on if you whisper far away or if you scream up close.

The output level also depends on its load impedance.

 

...I want to make a contact mike...

You mean, like a percussion pad? A mic, IMHO, is for detecting air pressure variations - sound waves. No contact with any solid. Spit is frowned on, but happens.

...using a piezo transducer...

Why did you choose to use a piezo for your project? There may be better options, but we can't comment without understanding what you hope to accomplish.

 

A contact microphone is designed to be a contact microphone. If you try to make one from a piezo transducer then it might sound awful.

String musical instruments sometimes use a contact microphone:
Guitar, banjo, violin, piano and many more.

 

A piezo lighter creates a 2 mm spark with a hard tap; no amplification needed- just feel it.

 

oh I just want to experiment with piezo transducers to pick up sounds from random objects.. such as pans, trays, etc. I made a semi-crappy spring reverb using a piezo, so I can see how it would be useful.

 

I made a semi-crappy spring reverb using a piezo, so I can see how it would be useful.

Was the spring-reverb used for guitar amp? I have a sample reverb circuit taken from an audio amp as attached....May be you can use it for your reference.

Allen

 

that's great, thanks! I like getting new schematics than I can put to use or at least learn from. Have you tried building this circuit?