Piezo driven tremolo

[dimaugh]

The post title is the closest analogy I could think of.

The raw idea is this:
a piezo used for percussion (preamplified and maybe going through some wave corrector and filters) controlling the gate of a transistor, so that when you hit it, it allows more or less of the guitar signal to pass.
Are there similar designs? What transistor would be recommendable for the task?

Thanks a lot!

[amz-fx]

Here is how you build the actual "stomp box":

http://www.instructables.com/id/Acoustic-Electric-Wooden-StompBox-with-Piezo-PickU/

You can use the voltage output to control any number of projects, including a vca or vcf.

There are lots of diy electronic drum projects that use this type of piezo as a pickup.

regards, Jack

[dimaugh]

That's a very nice looking box, but my question was about the electronics. I have already very specific ideas on how to mount the piezo, but that's another subject.

[amz-fx]

You want to rectify and filter the piezo output before it goes to the gate of the transistor being controlled.

Half-wave rectifier should be okay but the pulses from the piezo will be short and you'll need to experiment with the capacitor to see how large it needs to be, since it impacts the decay time.

regards, Jack

[dimaugh]

Wave rectification and filtering already taken into consideration (see 1st post). The cap value / sustain of the piezo pulses is not that important for this effect, for reasons that it would be a headache to explain here, but I'll experiment with it.

My main trouble is finding the right transistor and its appropriate configuration. I imagine the easiest way might be with a JFET, having the guitar signal shorted to ground when no piezo voltage is applied to the gate. As we hit the piezo, the channel would be partially blocked for a moment, so that less of the guitar signal gets sucked by the ground.
Is this correct, on principle at least?
Any other thing I should take into consideration?

Thanks!

[amz-fx]

Your basic principle is correct. You might need a diode in series with the gate to prevent audio leakage from drain to gate.

J113 will do the job and it is still available in a TO92 package.

regards, Jack