Triggering with very small voltages (.001 to .050vdc)

RGuyser

Hi gang!

My friend is trying to make an art project in which 2 mylar balloons trigger little audio snips when they hit one another. she has got these little play\record boxes that are triggered with a push switch. I have found that running 9v through the balloons with a multimeter in series set for the sensitive 2v (.000) setting usually responds with from .001 to .050vdc when they hit each other..

i want to take this small input, and trigger a relay..i have a basic stamp available and tons of cmos and ttl logic if that helps.. i can order parts..

i think i need an op amp circuit to trigger a 555 latch of some sort? yeah?

Even suggestions of how to search google for this info would help. I tried 'mesuring small voltages' and it is all about differential voltage measurements for current sensing.. [noparse]:([/noparse]

Thanks!

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Beau Schwabe

"I have found that running 9v through the balloons with a multimeter in series set for the sensitive 2v (.000) setting usually responds with from .001 to .050vdc when they hit each other.."

I don't think this is a reliable approach.... you can lift your big toe and generate a response 5ft away from CMOS input that is left floating. I would be willing to bet that your experiment would
work even without the 9V battery.

Mylar by itself, I believe is an insulator... The metallic balloons you see use Mylar with a very thin coat of aluminum. Unfortunately the aluminum is sandwiched between the Mylar, so making
an electrical connection might be difficult. You might be better off treating each balloon as a "capacitor plate" and try to get a reading that way in a circuit that uses a capacitor as one of it's
oscillating components. an RC oscillator or an LC oscillator I think would work if the R or L values were within a workable range.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

RGuyser

thank you for your quick reply

Yes. Reliability is a factor as this is supposed to spend a month in a public art gallery.. One thing that is encouraging is the fairly reliable response of the multimeter. The balloons are held apart and we fluttered our hands to simulate human traffic in the space. When the balloons knocked against each other fairly hard, there was a .001 to .005 fluctuation in the otherwise flat .000 readout wired in series with the 9v battery.

The first thing I thought of was a really sensitive comparator circuit... But I have only used comparators for IR line tracking sensors.. Nothing this minute... If I understand correctly, I could use a .001 - .025ish reference voltage(somehow) as the comparison voltage, so any time the balloons knock together and transmit more than that, a postive pulse can trigger a monostable circuit of some sort(555?) and in turn trigger a relay or transistor or similar?

The responses are very appreciated. I would very much like to help my friend.

Robert

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Mike Green

You really want to amplify this before you use it for anything else. Try a single transistor amplifier with the negative of the battery connected to ground and the positive connected to one balloon, then the other balloon connected through a 10K resistor to the base of any NPN switching transistor. Connect the emitter of the transistor to ground. Connect the collector of the transistor through a 1K resistor to some positive supply from 5V to 12V (with the supply negative connected to ground). Measure the voltage across the 1K resistor and see what you get. It would be nice to get a substantial fraction of a volt change when the balloons bump.

RGuyser

Yes, I considered an amp, but not before the op-amp.. So this transistor acts as an initial amp, making it easier to provide a relaible comparison voltage for the op amp? Is there a simple way to describe the characteristics of this amplifier? I suppose if i use a.. 9406 is it? 9604N.. WHATEVER. I can look it up... Is there an IC of some sort that makes more sense than a transistor? Easier? Cost within reason is not as important as reliability and ease of construction.

So, Something Like this? I am afraid I do not completely understand your description. Do I need a seperate voltage source, or can it all come from 1 power supply? We will be using a power supply that plugs in probably...

Thanks!

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Bruce Bates

Robert -

You may be able to utilize one of the Qprox proximity sensors found here:
http://www.qprox.com/

Regards,

Bruce Bates

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Mike Green

RGuyser,
Your diagram is correct. You could use a single power supply. I had suggested two in case there was a possibility of a short circuit between balloon 1 and ground (not likely). You could prevent that from being a problem by splitting the 10K resistor (approximately) into 2 x 4.7K resistors and putting one in the lead to each balloon. Then you could run the transistor supply from the same 9V battery (or really any supply up to maybe 12V). A common NPN switching transistor is the 2N2222. It typically has a gain of only 30-50, but you could always add another stage. Anyway, see what you get.

As an alternative, you could use a simple op-amp as an amplifier. Keep in mind that you don't have to use a differential input in that your baseline is zero. Any significant voltage or current above zero is what you're looking for.

metron9

Have you tried simply connecting one balloon to one side of a comparator and the other balloon to the other input of the comparator. When the balloons are in contact the balloon with the higher voltage (capacitance) will give its electrons to the other balloon bringing the DIFFERENCE in the two balloons to 0V. Using 2 comparators and connecting one balloon to the + of comparator1 and - of comparator2 , the second balloon to - of comparator1 and + of comparator2 you will know when either one of them is charged higher than the other one depending on the comparators state, and when the voltage is 0 they must be in contact.

One more thing, this is just a theory I have no idea if it would work but it seems plausable.

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Think outside the BOX!

RGuyser

Hey. It has been awhile. I hope this still gets some attenton. I am running really late now! (bad friend, etc. ugh). Anyways. I have been working on this this evening. I built this transistor circuit with a pn2222 and it wasnt very helpfull. Withe the balloons nowhere near each other the meter read between .001 and .003. When they touched, it read between .002 and .005 - anyways. I examined the audio player modules better and found they are triggered by pulling a pin in an on-board encapsulated IC low. I wired this directly to the balloons - 1 was ground, the other the trigger pin. This actually worked almost. If the balloons were going to be highly aggitated, it would work - but they need to sense when just barely touching.

Impedence? Someone recommended i use the 2 layers of foil seperated by mylar as a capacitor and try to sense the gap via impedence. They mentioned sending a 100+-Khz signal into one balloon, and somehow using an opamp to do the rest? I figure I can trigger the IC pin to ground via an npn switching transistor driven by an opamp.Unfortunately I have no idea how to do this! Thanks!

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