Piezo sensor

[prprog]

This circuit works as I want. When the piezo disc is press the LED lights according to how hard the piezo is press. Good..but... the piezo needs to be
press really hard to make it work. Could you please suggest how to make it more sensitive? (I will prefer not to add any IC or any additional transistor. Diodes, capacitor and resistor are OK to be added.)

Thanks,

 

[duffy]

It's odd that circuit works at all - that's a JFET, the led should be on all the time. Did you draw that gate diode the right way? The band is the cathode, with the line.

The cutoff voltage on that JFET is -8V. Hard for a disc type piezo to put out that much. Why don't you want to use a different transistor? An ordinary bipolar only needs +0.7V.

 

[audioguru]

The FET has a very wide range of sensitivity. Buy many of them and test for the most sensitive one.

 

[crutschow]

The cutoff voltage on that JFET is -8V. Hard for a disc type piezo to put out that much. Why don't you want to use a different transistor? An ordinary bipolar only needs +0.7V.

You can't use a bipolor since it requires base current to turn on and a piezo disc can not generate any appreciable current, only voltage. Thus it needs to see a very high impedance such as a FET gate.

 

[duffy]

Yes you can, works fine, I've built this circuit before. It doesn't generate much current, but it's more than enough to drive the base of a 2N2222 and light an LED.

 

[duffy]

Here's the circuit.

 

[crutschow]

Yes you can, works fine, I've built this circuit before. It doesn't generate much current, but it's more than enough to drive the base of a 2N2222 and light an LED.

How long does the LED stay light when you press the button?

 

[duffy]

I think it was... aww nuts, now I'm going to have to build the stupid thing. Wait right there, brb -

...

Here you go. Answer is: depends on how you press it. Press it slowly at the right rate and it stays on for almost a second. Tap it and you get a blink. Dropping a coin maybe an inch onto it also gets a good flash out of it. The "button" is a piezo disc, bigger ones work better.

See how I got that diode hooked up, prprog? The piezo is like a piston in a pump, sorta. The diode and the base-emitter junction act like two valves in the pump.

 

[crutschow]

OK, now I understand. I was assuming that the light should stay on as long as the button was pressed, and for that you would need a FET.