You can use a normal piezo. One project I worked on used 3 simple
piezo sensors arranged in X, Y, Z planes on a large machine. These
low cost sensors were identical to a normal piezo sounder, but
didn't have the hole in the top to let the sound out!

Used variable low pass filters to look at the band of interest,
rectified and fed into A/D. Machine alarm sounds when vibration in
any axis exceeds preset level 1, shuts down when exceeds preset
level 2.

Check out the MuRata PKS1-4A10, Farnell P/N 731-973. Output
approx 40mV/G.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  brent.brown@clear.net.nz

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