Using piezo transducer to power an LED

[JohnJohn20]

Using a piezo transducer to power an LED

Hi.

I saw a page where a mechanism tapped a piezo transducer diaphragm with an LED across it causing the LED to flash. (See https://rimstar.org/materials/piezo/piezoelectric_crystal_gift_card.htm).

I tried doing this and found that the LED does flash the first few taps but then stops working for further taps.

I really need an oscilliscope to look at the output of ther piezo.

My question is:
- As the piezo produces current in both directions and I only have a single LED across it, will the piezo" charge up (like a capacitor I guess) with a polarity opposite to the LED?

Also, how fragile are these transducers? I need to tap it quite hard to to get the flash.

Thanks.

 

[kam1787]

no they do not charge up like a capacitor

yes, they are fragile

 

[Audioguru]

The light from the LED powered by the piezo is so dim and for such a short duration that you can barely see it. Why bother with it?

A piezo wafer is stiff and resonates (like a bell) when hit. The resonance causes it to produce AC.

 

[jpsganesh]

I remember few years back I checked it. Piezo element I took it from a beeping buzzer like bike indicator one. I connected piezo to DSO without any load. Then I tapped it from one side, I noted the reading at +ve side 32 to 38 volt and negative side some 6 to 8 volt. Then I continued the test with different loads like resistor, capacitor and different values I noticed for a small load also voltage reduces drastically. Voltage came down to 5 to 7 volt at +ve side around 1 volt at -ve side. This happens because it generates enough voltage but current is very less.
When u connect a LED at this environment one possibility LED will get damage in a short period. So u need a Zener to protect it and if u connect a small value capacitor it can store small amount of current which will extent LED glow time. Preferably use a diode in series with piezo at right direction to avoid -ve voltage. If u want more power u need more piezo elements.

 

[FvM]

Some suggestions are ignoring elementary laws of electric circuits. A piezo element is a capacitive source and will in fact charge up to a DC voltage if the load current can't flow for both polarities.

Uisng an antiparallel (parallel with reversed polarity) diode to the LED or two antiparallel LEDs would be a simple way to overcome the problem.

 

[JohnJohn20]

The idea was to have a wind mill hooking a hammer which dropped onto the piezo to cause the LED to flash. Simple yet rewarding. Why not bother?

 

[Audioguru]

I doubt the LED will flash when the piezo breaks.