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Fifty Cent Piezo Sensor Can Measure Some Amazing Things

Here is an email I sent to members of our Physics Group at UMaine:

Hi All,

My fifty cent piezo sensors are producing some strange data. Kudos to anyone who can explain this stuff. (I’ll offer an explanation after this email.)

This picture shows the setup: There is a piezo sensor on the bottom and one at the top (with the visible PZT, thats Lead Zirconate Titanate, side facing downward), both attached with thermal glue.

The dark green part that separates the two is an electrically and thermally insulating plastic straw.

Piezo Sensors set up for testing

Ignore the bottom piezo sensor for now. The strange part is the top sensor.
It generates a voltage as measured by either an oscilloscope or measured by an Arduino board.

The strange thing is it generates a voltage when the top brass disk (not the PZT) is exposed to light, and even when I breathe on it! (It also generates a voltage when I put a mass on it, as expected.)

Can anyone explain that?

Here is the data:

I add an extra brass plate disk to the top sensor and you can see the voltage change due to the weight change.

I then short the sensor, and observed a voltage change when the extra top plate was exposed to white light.

(The voltage effect still occurs even though there are two brass plated for the light to go through.)

I removed the extra plate and exposed the top plate to UV 395nm light.
(Note the PZT, lead zirconate titanate, of the sensor is never exposed to light.)

Piezo voltage vs time in seconds

I then exposed the top plate to light, this time with the 395 nm light first and then the white light:

More Piezo Data

Note the time constants for the two different types of light. That's interesting.
I then breathed on the top plate and got a more intense change.
The blue line is the force sensor at the bottom.

Nothing like a good mystery!

Dave

Our Physics Group concluded that the effect is due to the small temperature change that occurs when the light heats the brass plate. The temperature change is enough to change the physical dimensions of the PZT. Our conclusion is that this piezo sensor is an incredibly sensitive temperature sensor! And it only cost 50 cents!

For more info., see Piezoelectric Sensors on Wikipedia.
PhysicsGuy