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I have a built a vibration detection circuit based on an op-amp to trigger high speed photography of balloons popping.

(I'm using this circuit)

enter image description here

(From this blog - not my image)

It works well. It includes a 5 volt zener diode to bleed off the potentially high voltages that it can give from strong shocks. (well, a 5.2 zener, since that's what I had)

I have found that the pictures look really good if I put some cornstarch in the balloons before shooting. It gives a spun cotton look to the air inside, but the cornstarch needs to be suspended in the air to get the best effect. Thus I have to shake the balloon, put it in my rig, and quickly turn everything on and pop the balloon, triggering the photo.

It occurred to me that since I already have a piezo element touching the base of the balloon, I could drive a moderate frequency tone through the transducer to create a loud base buzz that would kick the cornstarch into the air inside the balloon.

To do that I would drive the piezo element directly from an output pin on my Arduino (or perhaps run it through a transistor if I need more current, but I doubt if I will. I get pretty loud tones with a current limiting resistor in place that keeps the current well below the 40mA limit on an Arduino output pin...)

Anyway, my question is, will a piezo element being driven as a speaker generate echo voltage that's higher than the input voltage, sort of like the boost effect you get from an induction coil?

And can I be confident that my (5.2 volt, what I have on hand) zener, wired across the terminals of my piezo, will protect my Arduino input line from damaging voltages when I bump the sensor and generate big spikes of voltage from the transducer?

EDIT: Some person deleted the paragraph on why I put cornstarch in the balloon, and why I need it to be suspended in the balloon. I've put it back. Please don't delete it - my question does not make sense without that background info included.

EDIT #1: One of the posters asked for me to post pictures. Here is one:

enter image description here

There's a gallery of images at http://www.pbase.com/duncanc/popping_balloons&page=all

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  • \$\begingroup\$ If I were you, I would use a vibrating motor on the rig instead of a piezo element. It should give you a more aggressive vibration. \$\endgroup\$ – Matt Young Aug 12 '14 at 14:13
  • \$\begingroup\$ @MattYoung, good idea, but I don't have a one of those on hand. The piezo, however, is already in place, wired to my breadboard, and ready to do my bidding. To use a vibration motor I would need to buy the motor, add a power transistor to my breadboard, add a flyback diode, run another set of wires over to the shooting rig, etc, etc. \$\endgroup\$ – Duncan C Aug 12 '14 at 15:36
  • \$\begingroup\$ You will be fine, any voltage amplification would come about due to interactions with your physical media. Model the piezo as a capacitor. It isn't a big deal to drive one from a microcontroller IO pin. To correctly drive one you will want a step-up transformer. I recommend a pager vibrator motor as well. Post some photos! :D \$\endgroup\$ – HL-SDK Aug 12 '14 at 16:14
  • \$\begingroup\$ How about shaking the balloon with a little speaker? \$\endgroup\$ – George Herold Aug 12 '14 at 16:50
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    \$\begingroup\$ @HL-SDK, I edited my original question and posted a picture, plus a link to the gallery. \$\endgroup\$ – Duncan C Aug 15 '14 at 0:24
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Yes, piezo elements can produce higher voltages than they are given when shut off fast. To the driving circuit, there is a inductive component to the piezo's characteristics. You deal with this the same way you deal with driving real inductors, which is with a reverse diode.

Actually in the case of a piezo, it's a good idea to use two diodes, preferably Schottky, one each in reverse to power and ground. That way, no matter what the piezo does as a result of mechanical ringing or getting externally whacked, the resulting voltage won't hurt anything.

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Piezos have no inductance and thus will not generate extra voltage spikes. And in any case, any voltage generated will be still limited by piezo's capacitance, and thus zener will be able to handle it just fine.

Beware that a mechanical echo is a common problem in ultrasonic rangers which also use piezo elements -- immediately after piezo emits a signal, it is still vibrating (mechanically) and thus generating small amounts of power. This may cause false triggers in your detector. Make sure you wait long enough between the end of the buzz and time when you start sampling the sensor.

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