Piezoelectric discs are fun little devices that take advantage of crystalline structures compressing to produce a voltage. You may know them better as speakers! Yes, supplying a voltage will create a distortion in the crystalline structure to produce sound. So whether you produce voltage or supply voltage, piezoelectric materials rely on mechanical strain to induce a reaction in the material.
But what about "hitting" the piezoelectric disc with light? Typically when voltage is created using light, this is known as photoelectric, as seen in solar panels and other doped materials. In this Instructable, I will show you just that - inducing a voltage in a piezoelectric material using light!
Step 1: Items You Will Need
2. Arduino Uno
Whatever method you choose of acquiring these parts, whether it be for cheaper or on different sites, is entirely up to you. Basically all you need is at least one piezoelectric speaker/disc, an arduino that has at least one analog channel (don't forget your USB cable), and a semi-powerful laser pointer (the dollar store laser pointers just won't cut it!)
Step 2: Setting Up Your Arduino
This next step is very simple. Take the red (+) lead of your piezoelectric disc and attach it to the analog 0 (A0) channel of the arduino, and the black (-) lead attaches to ground (GND).
Step 3: Upload Your Code
Next, plug in your Arduino and upload the code provided. This is a simple program that reads the analog channel and outputs the result over the serial line. This makes the arduino a very simple data acquisition device. If you have a National Instruments DAQ device, even better! Anything that can accurately monitor and display voltage at very low currents should work appropriately.
Step 4: Observe the Weirdness
Open up Serial Plotter on the Arduino, and shine your laser pointer on the piezoelectric disc. Do you see a change in voltage? Taking the laser pointer and clicking it on and off a few times, you can see the staggered saw-tooth waveform generated in the picture above.
So what in the world is going on?
My first assumptions was that the laser was just heating up the piezoelectric disc, causing the base to warp slightly and produce a voltage due to the pressure differential; however, this was not the case. The laser used outputs less than 5 mW, which is very, very weak in terms of heating things up from a distance. Second, the piezoelectric disc also produced a voltage with an LED, and infrared light! Woah!
So what's causing the voltage spike? My second guess is that surprisingly, piezoelectric material may also be photoelectric, or rather maybe the opposite and be photoresistive. If the material is photoelectric, the laser light is knocking off some electrons from the ceramic/silicon material that makes up the piezo and allowing them to flow through the circuit. If it is photoresistive, then the decrease in conductivity of the material creates some sort of differential between the brass plate (ground) and the piezoelectric material, possibly resulting in a voltage. This may be the case because I have not observed a saturation point in the voltage - it just keeps going up beyond 5 volts!
Unlikely, but still fun to think about, is that the photons are bombarding the piezoelectric disc like a BB gun and pushing on it. Wouldn't that be something?
What does this mean for you?
Typical photodiodes can be fairly expensive. These piezoelectric discs cost pennies, and are able to detect light at multiple wavelengths - but not really accurate without some processing. You can detect a laser pulsing with them, as shown above. The fact that you can generate a voltage at all from a device that is supposed to only produce voltage with mechanical strain is truly amazing!
I hope you enjoyed this mysterious Instructable and any comments, questions, or discoveries would be greatly appreciated!