Watch piezo drive method...

[alank2]

Hi Everyone,

I've got this idea that I want to hollow out a casio watch and put an old style HP quad LED display in there instead so I am looking at the piezo mounted to the back metal cover of the watch.  I wanted to see how the casio watch module drove it so I measured it and I expected to see a cyclic wave but instead saw a single pulse that was almost 100V for 3uS.  Does that make sense?  Was using a 10x scope probe on it with the spring ground lead.

What would be the easiest drive circuit for a piezo from a 3V uC pin?  Can different frequencies be done (even if the device is only optimal for one frequency?)

Thanks,

Alan

[Zero999]

100V sounds a bit high.

Was this measured with the transducer connected?

An inductor connected in parallel with the transducer is often used to increase the power output.



Even more power can be delivered by using a small autotransformer.

[alank2]

I measured it with the piezo disconnected, maybe that was why it was so high...  Is your driving circuit as simple as it gets?  An inductor and a driving transistor?  Could a mosfet be used too?  How big an inductor?

[Mr Smiley]

You pick L so that, at your required frequency you have a resonant circuit. So you need to know roughly what the capacitance of the piezo is at your chosen resonant frequency. If you want to know the maximum audio frequency of your piezo, connect it in series with a resistor and connect them across a signal generator, sweep up the frequency from low to high and you'll hear the peek sound. You'll also sweep through a few other maximum peeks as well. choose the one you like and use that frequency to work out your value of L

Also remember that you can increase the sound levels with a resonant chamber.

The principle is, the short pulse sets the tuned LC(piezo) resonating; passing energy back and forth between the inductor and the capacitive piezo, that energy transfer is decaying but taking longer than the initial pulse. So for a very short pulse you have an extended audible sound. Keep pulsing it at the right time and it will continue to resonate.

If you have a scope, a quick and dirty experiment is to put a coil ( any value for now ) in parallel with your piezo, connect the scope across the two and then just quickly momentarily  tap a supply across the circuit, you'll see on the scope the resonance and decay. Single shot is good for this and measure the period of the sine wave you have on the screen to work out the resonant frequency for that LC combination