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Piezoelectric info

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kinarfi

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I tore an old pocket watch apart the other day and trashed most of the guts, but kept the back which has a piezoelectric disk on it. How do get sound, beep, or what ever out of it?
Kinarfi
 
It is probably a piezo transducer which has only 2 wires. Feed it a signal at its resonant frequency of a few volts peak-to-peak and 3kHz to 6kHz.
 
It probably doesn't have any wires - the face of the piezo contacts a spring on the circuit board, and the other side is electrically connected to the metal of the watch's back, which is grounded when it is screwed on.
 
Thanks, Duffy, you're correct, spring contact, so I need to excite the crystal with a 3 volt p-p signal. does it matter if it is a triangular or square wave and I assume it will react best at the resonant frequency.
Kinarfi
 
The watch has a small inductor to increase the voltage to 20v. You need a proper circuit to get anything out of the piezo diaphragm.
 
does it matter if it is a triangular or square wave

Either waveform will work. You should get some sound at 3V - but like Colin says, a higher voltage is better. Piezos use higher voltage and lower current than a speaker with a coil. Crank your signal generator up to 12V or even higher. The only time I shattered one was when I drove it well over 100V.
 
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Normally the diaphragm gets a sinewave as any square-wave gets converted to a sinewave due to the 22n capacitive value of the piezo. There are high quality piezos and low quality. The high quality types give an output at 3v but a watch is only 1v5. At 3v the sound is very similar to a music card but the diaphragm must be housed to get a good output.
 
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