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Piezo touch sensor problem: biasing a piezo element

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undercurrent

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Hi everybody, what I am trying to accomplish is a compact piezo touch switch for LED lamps, 12 volts in, 12 volts out, switching occurs through a touch sensitive piezo buzzer that controls a BC547, which in turn acts as a non-mechanical switch for a typical NE555 bistable circuit.

The circuit I am basing all this of can be found on Google Images: 555flipflop.gif. I've just replaced the mechanical switch basically. The piezo is connected from ground up to a diode to eliminate negative voltages, and a zener to cut out extreme voltage peaks, then going straight into BC547's base.

Obviously I try to keep the component count down, it's supposed to fit into a quite flat cylindrical button 30mm in diameter!

So the problem: I'd like the switch to act on a light tap or touch, and need to bias the piezo element, which will feed out peaks of 0.3-0.5 Volts when unbiased. Because BC547 is expecting a Vbe of at least 0.7 Volts. But the biasing doesn't work. I am pretty new to electronics, I made a simple voltage divider (100kOhm and 10kOhm giving 0.44 Volts) and tried placing it both in parallel and in serial with the Piezo. Tinkered around in all sorts of ways. But that voltage just doesn't seem to add up with the output of the piezo. That bias seems to basically smother out any output from the piezo.

What component is the piezo closest to anyway in it's neutral form, can it be considered a capacitor maybe?

So please advice: how on earth does one bias a piezo buzzer? The problem seems to be really at the piezo bias level, I have checked the signals at different points with my compact Helleman oscilloscope.

By the way, the circuit works just fine without biasing, it's just that the tapping necessary is a bit towards the violent way, and I don't want the button to break too soon!
 
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SABorn

New Member
I used a piezo for the target of a game using water pistols, and it works with a light tap.
What i done was to stick the piezo to a paper label and then fed it through the laminator to encase the piezo in a plastic sheet, that i could glue down to form a skin of a small drum.
The drum allows greater sensitivity as the membranes of the piezo need to be able to flex to produce a voltage.

Here is the input circuit for the piezo that i used.

Pete.
 

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alec_t

Well-Known Member
Most Helpful Member
What component is the piezo closest to anyway in it's neutral form, can it be considered a capacitor maybe?
Yes, it's essentially a capacitor. Stressing it varies its stored charge distribution and hence results in a voltage change across the element.
 

undercurrent

New Member
The drum allows greater sensitivity as the membranes of the piezo need to be able to flex to produce a voltage.

Here is the input circuit for the piezo that i used.

Yes, I consider if my current testing buzzer isn't too small. It has a round piezo sandwich area of about 8mm diameter on top of a larger metal square, an odd thing tht I've ripped out of some equipment years ago. The buzzer I plan to buy has about 4x times the area. Maybe I don't need to squeeze in an opamp stage before the BC547 to amplify the signal. There's hardly any space anyway.

Another question: I've done a test, and this circuit leaks 9mA in standby mode, consistent with the datasheet leakage of 8mA of a NE555 chip. Is that considered an "okay" leakage or is it too much? This is not a battery-fed application, the power will come from a VAC outlet.

Does there exist a standard allowed leakage of standby circuits?
 
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