# Piezo voltage through guitar vibration and Nyquist's theorem

I bought a piezo disk and intend to use it to determine the vibration frequency of a string instrument when a note is played.

https://www.sparkfun.com/products/10293

Can anyone explain how much voltage would be generated by a tuned Low E string (82Hz) compared to the high E (329Hz)?

And if I wanted to detect the frequency of the standard guitar notes I would need to sample the voltage at twice the highest frequency. High E - 329Hz. So I need to sample at 658Hz? Or 658 samples per second to an analog pin of my arduino?

Thank you

• Why piezo? You could use a microphone for that. How the force of the string will be transduced to the piezo element? As for sampling rate: with nyqvist you'll get only a rude triangle, you need much much higher, for example 44khz is CD quality. – Marko Buršič Sep 19 '15 at 14:36
• Potentially a noisy environment so a piezo would be ideal for cleaner reading. Yes, how the force of the string will be transduced, is it possible to calculate it without having to physically take readings? – JamesDonnelly Sep 19 '15 at 14:39
• Why not just get a cheap electric guitar and use the pickup? – Andy aka Sep 19 '15 at 17:14
• Looking to use it on multiple instruments rather than an integrated system! – JamesDonnelly Sep 19 '15 at 17:30

Voltage will be low in the mv range, you should have a high impedance say 1Meg resistor in parallel with the piezo, and then feed that to a little opamp preamp.

As far as sampling, nyquist is the bare minimum, I'd probably sample much higher but give it a try and look at the waveform you get. If low sample rate is enough for you then go for it.

• You might google piezo contact mic to see some tutorials on making one. – Some Hardware Guy Sep 19 '15 at 14:23
• Thank you, I've seen some of the tutorials but they just reproduce the sound but I intend to determine the frequency or the note being played. I have used the resistor in parallel but I don't have an op amp just yet. – JamesDonnelly Sep 19 '15 at 14:27

I watched some pics on the net what actualy is this pickup, and yes its glued on guitar. Now if you want to measure the frequency, you dont need to connect it to ADC, rather you need a schmitt trigger comparator circuit and connect the output to a interrupt input to MCU. Look for frequency measuring with arduino.
Basically you would need to convert input signal from piezo to quadrature pulses, at each rising edge from 0->1 you count the elapsed time and calculate frequency.