Impedence sensing between 2 Mylar\Foil Balloons

RGuyser

Hey!

I know this is in another topic also, but i am in a hurry, and also it will be more clear rexplained. Thanks for the help guys!

I have a friend who I told I could get her art project working. It is getting closer to the due date and she has told this art gallery in NYC that it would be ready (poor callie, relying on a sod like me)

Anyways. I have 2 shiny mylar balloons and want them to trigger a little sound recorder module when they hit one another, even if it is a fairly soft hit. I have considered using the fact that the balloons are a thin layer of shiny aluminum foil covered with a thin layer of mylar - essentially like a capacitor. Given this, there should be a way to sense impedence between them.

I was told to try running a 100+- KHz signal through one balloon and somehow using an opamp to further trigger my audio device, which is triggered by pulling a pin to ground. Incicently, hooking the balloons to ground and the trigger pin almost works out, but it requires too agressive a encounter to trigger.

Anyways. Please help! I will love you forever! Do it in the name of contemporary art... These things are supposed to be part of my friends first show outside of chicago.. Thanks!

Robert(and callie, by proxie)

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Tracy Allen

Maybe something like the attached diagram. Do you have a Litz wire attached up the string to make contact with the metalized surface? One balloon would attach to the +5 volts or +9 volts, and the other to the gate of a sensitive mosfet transistor. The gate would have a high ohms (10 meg or ??) resistor to ground, so the transistor is normally off. But the capacitance of two balloons touching would couple in a 5 or 9 volt pulse into the gate and turn on the transistor briefly to trigger the audio device. The mosfet could be something like the VN10, but it should be a small signal device with not too much gate capacitance of its own. It would be possible to use an op amp or a CMOS inverter circuit in place of the transistor.



I see you got quite a few responses in the other thread, and I think those were all on track. A mosfet or the input of a CMOS gate is probably better than a 2N2222 for this, because of the high input impedance of a mosfet, as it is much better able to respond to a small dollup of charge. But I am assuming that the coupling between the balloons is mainly a capacitance, that the aluminization is on an inner layer, not on the outside. The reliability of this scheme will depend on good electrical connection to the aluminization layer. Can you describe how (if) you have wires attaached to the balloons? There are electrically conductive copper tapes that can be used for this purpose.

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Tracy Allen
www.emesystems.com

Post Edited (Tracy Allen) : 11/1/2006 5:44:49 AM GMT

Jonathan

Just a thought...

What about using the balloons as capacitors in a RCTIME type circuit? I'm not sure if the difference in capacitance as the balloons moved together would be enough of a difference to reliably detect. If it did work though, low part count and very simple.

Jonathan

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