A simple demonstration of piezoelectricity
by Dr Jonathan Hare - The Creative Science Centre, Sussex University

(taken from: Simple demonstration of Piezoelectricity. J. P. Hare, IOP press, Journal of Physics Education, May 2006, vol. 41, p.212-213)




Photo of the simple apparatus to demonstrate piezoelectricity. An LED is wired to a piezoelectric transducer.
The LED briefly lights when the device is flicked - neatly demonstrating that electricity has been generated by stress and strain.

Everyone is familiar with the high pitched “beep-beep” of a car alarm re-setting itself, a wrist watch alarm or the sound of a smoke detector. The sound is made by a piezoelectric transducer. An alternating electric voltage is applied to a thin slab of piezoelectric material which flexes at the same rate. The device is only efficient around a narrow band of frequencies but at this resonance it works so effectively that a very loud noise is produced for very little power and size – hence their use as a sounder in many electrical devices.

Piezoelectric crystals are used in cigarette and gas stove lighters, where tension produced by a lever produces 100's of volts - enough to create a spark to light the gas. We can also use this effect to show piezoelectricity in a safe and more straightforward way.

How to make the demonstration
The piezo-transducer consists of a thin slab of piezoelectric material sandwiched between two flexible metallic connections (usually a thin piece of flexible brass sheet metal and on top a thin gold layer). Obtain a piezo-transducer, as large as you can (perhaps salvaged from an old toy or purchased from an electronic supplier [3]). Connect an LED between the two transducer connections (see picture above).

Hold the transducer (with attached LED) in one hand between thumb and forefinger to one side. Use a finger on the other hand to flick the opposite side of the disc. The flexing creates stress in the piezoelectric material which produce a corresponding voltage, enough to briefly light the LED – neatly demonstrating piezoelectricity!

Note: a neon indicator can sometimes work in this simple demonstration as the voltages produced by the piezo crystal can be 100V [2]. The current is very low so it does not represent a health hazard.

References and Notes
[1] The Penguin Dictionary of Electronics, E. C. Young, any edition
[2] The Forrest Mims Circuit Scrapbook, Vol. II, LLH Tech. Pub, 2000
[3] Maplin Electronics codes: piezo-transducer: YU82D, super bright LED: UF72P the two should be less than a pound.
[4] Mike Bullivant, the Chemist in the Zanzibar (5th) series of the Rough Science TV series made piezoelectric Rochelle salts. This was used as the microphone element for a hydrophone to listen to marine life underwater. For those interested in the details of the chemistry of making your own piezo-crystals these are to be presented in a forthcoming article by M Bullivant and J Hare in the RSC journal, Education in Chemistry.