[sdiy] Vactrols and contact mics

[Byron G. Jacquot]

>I'm curious if anyone has experience with driving the LED of a vactrol
>with a contact mic.  The contact mic would be attached to a drum so
>I'm guessing the voltage would be high enough to light the LED but
>would it be very instantaneous or perhaps it needs to be amplified?
>Would it be simple to create some sort of decay?  Ultimately I would
>like to be able to trigger a voltage controlled dimmer connected to an
>incandescent bulb with a drum.

Piezos are not very well behaved...you need to protect any following circuitry from possibly very large spikes, and maybe filer out the self resonance.  Thankfully, it can be broken up into a series of sequential stages, see the following link for some ideas:  

http://leucos.lstilde.org/wp/2009/06/piezo-transducer-signal-conditioning/

Than you might follow that with a comparator to square up the pulse, and feed it into a standard AR envelope, with the A time constant set very fast.

Byron Jacquot