how does a knock sensor work?

[Phil Hunter]

Jared wrote:
> It's a "piezeoelectric" device.  It has a crystal
> that, when it vibrates,
> makes a small voltage.
> 
> Piezeoelectric crystals can also work the other way
> around: apply a voltage
> to them and they vibrate.
[snip]

This is a little misleading, apply a constant voltage
and the crystal will bend, apply an oscillating
voltage and you can get the crystal to vibrate at
it's mechanical resonance. Depending on the how the
crystal is manufactured, you can get a very narrow
range of resonance frequencies. This is how crystal
oscillators work.

Conversely, bend a piezo crystal and it will generate
a voltage, apply a vibration and it will make an
oscillating voltage. The range of frequencies that
the crystal will respond to is controlled in the
manufacture.

And of course, this is still an oversimplification...

-phil

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