For my first post to the forums I have a design of mine to present. The basic premise is that the piezo element is situated under the skin of a drum (bass drum for example) and that when struck, the resulting voltage switches the FET and thus the LED array (also located inside the drum) on for as long as the piezo element is recieving enough of a pressure wave (drum still making sound).

The circuit diagram I have attached has been put together from my limited high school electronics knowledge, an online LED calculator and a piezo switch circuit I found at http://www.discovercircuits.com/DJ-Circuits/piezoswitch2.htm

The value of the cap has been changed to reduce the LED 'on' time from about 0.5s to 0.05s

A few questions however:

Due to the relative speed at which drums can be played, I want a fast decay. is the capacitor the only thing constraining this?

I read that the sensitivity of the piezo can be adjusted with a variable resistor in parallel with the element, hence the 1M variable in the circuit, is this correct or is there a better way? also, wont the variable resistor also effect the timing of the LED 'on' period?

Is the output of the Piezo likely to be big enough to exceed the 20V rating of the FET?

I am planning to use a 6V wall wart transformer, is this likely to give me shockingly bad DC that will adversley affect the circuit?

Anything glaringly obvious I have missed?


Cheers.
Marcus Sargent
Invercargill, New Zealand.

 

Hello,

The pot you have placed over the piezo element will only damp the voltage.
The timing can also be affected by changing the 4M7.
When you lower this value the discharge speed will go up.

Greetings,
Bertus

 

See the attached schematic; a slight re-work of what you've drawn.

D2 is a 15v Zener diode. It clamps the output of the piezo to prevent damaging the MOSFET. If you can't find a 15v Zener, you could use a couple of 9.1v Zeners in series for 18.2v, or whatever other combination you can easily get. Radio Shack used to sell 9.1v Zeners.

R1 prevents the resistance from being set all the way to zero. R2 is a 1megohm pot. Changing the pot's setting will change the LED ON time. The maximum time on will be around 15mS, or 1/66 second. Increasing R1 and/or R2 will increase the ON time. Increasing C1 may increase the ON time as well, but the piezo is only going to put out a certain amount of current when struck; if the cap is too large, the piezo won't supply enough current to charge it up enough to turn on the MOSFET.

With N-ch MOSFETS, I like to show them on a schematic with the source down; P-ch shown with the source up. You can place them any way you like, but being consistent helps in more rapidly understanding the functioning of the circuit.

"Wall wart" power supplies are not typically regulated. YMMV, Caveat Emptor.
Calculate the value of your LED's current limiting resistors as:
Rlimit = (SupplyVoltage - Vf(LED)) / LEDcurrent
LEDs usually have a MAX forward voltage, Typical forward voltage, and current specification. Use the typical Vf in your calculation.

 

Have you got this circuit working yet? I am planning on using it to drive a pc keyboard... so i want short 'on' time to trigger a keypress