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 ). 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 . The current is very low so it does not represent a health hazard.
References and Notes
 The Penguin Dictionary of Electronics, E. C. Young, any edition
 The Forrest Mims Circuit Scrapbook, Vol. II, LLH Tech. Pub, 2000
 Maplin Electronics codes: piezo-transducer: YU82D, super bright LED: UF72P the two should be less than a pound.
 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.
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