[vacpress]

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..

Thanks!

[NC Cams]

Look into the circuitry that involves "touch pads", ala those that sense if you touch a lamp to turn it on and off.

As I recall, they use an op amp and the resistance or capacitance change of your finger to turn of/on 120vac circuit.

[vacpress]

http://www.aaroncake.net/circuits/touch.htm

such as this.... i am not sure.. will this circuit trigger off a very small resistance change? the balloons do not seem to effect the resistance measurement...

[NC Cams]

That should be a starting point for you.

I"d think that something with a 1 to 50 mv signal should be enough to amplify so as to trip a switch.

We were able to get a CA3160 cmos high impedance op amp to trip at a slowly changing "peak sensing" voltage circuit fairly easily and predictably - we were looking for a 10 mv differential.

You might want to "debounce" the circuit by feeding the output from the CMOS chip in the link to a 555 based monostable to give you a finite and tuneable output pulse.

[vacpress]

thank you for your quick reply

Yes. Debouncing, but especially the higher driving current and adjustable 'sustain' of a 555 is why I figured 'comparator and 555'. 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 response is very appreciated. I would very much like to help my friend.

I have got an array of op amps handy, but no dual-voltage power supply.. Do I need the + and - supplies to use these sorts of op amp circuits?

Robert

[vacpress]

thank you for your quick reply

Yes. Debouncing, but especially the higher driving current and adjustable 'sustain' of a 555 is why I figured 'comparator and 555'. 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 response is very appreciated. I would very much like to help my friend.

I have got an array of op amps handy, but no dual-voltage power supply.. Do I need the + and - supplies to use these sorts of op amp circuits?

Robert

[lwill]

Since Mylar is not very conductive you may want to try a circuit like one of theese that use capacitance. Just gound one of the ballons and use the other to sence.
http://www.discovercircuits.com/C/capacitance-sw.htm

[sswitaj]

I'm sorry, I'm a confused. DO the baloons touch *each other* or does *she* touch them? Is the surface of each baloon conductive, or is there some sort of film on them that prevents you from using them as a simple contact closure?

[NC Cams]

If you are trying to sense 1 to 10 mv above ground without a bipolare supply, that is going to be tough.

You don't need to get fancy to create a +/- power supply - two 9 volt batteries can be configured to do it - if the proper amp is used, they should last for long enough to have the display function for the desired time.

The CA3160 was sensing a 10mv DIFFERENTIAL in a sample and hold circuit BUT the voltage differential was way above ground.

Linear Technologies has a number of "ground sensing" op amps and comparators that might work for you. I'd still look at crafting up a bipolar supply to simply make life simpler - "finding" that small of a differential is hard enough without having to do it at/near ground without a bipolar supply.

[pminmo]

Mylar is a good insulator and you're probably seeing a capacitive coupling when they hit. Off the top of my head, my sugestion kind of mirrors NCcams, an op amp in the first stage, LM321 or LM324 something that has extremely low input current requirements. Power it with the same supply as your basic stamp and experiment using a gain of 100 to 1000 and see if it would interrupt your basic stamp. Usually a micro has an edge sensitive interrupt. If you can get it to interrupt it reliably, then make your interrupt routine do what you want. i.e. drive a relay or....

[pastera]

Set the balloons up as a "switch" driven with an oscillator running if the high kilohertz - a few volts should be enough (555 timer). Feed that into an OPAMP (741) biased at about mid supply level. Use the output of the amp to trigger a one shot (555).

Aaron

[ViperTX]

...just a short note....look at the circuitry used in Strain gauges....

[vacpress]

Hey everyone

I am still thinking about how to do this best. Thanks for the suggestions.. At this point, I am leaning towards a transistor amp stage, 20-40x amplification feeding to an op-amp which can drive a 555 1-shot.. this seems a bit excessive, but it is based on the various bits of advice ive gotten.

the balloons are, i think, conductive. if you put a meter on the baloon directly, you get a reading depending on how hard you press. the harder you press, the less resistance. they will hopefully hit one another as people walk near them. in testing, this seems to be the case. the contact between the 2 balloons is very delicate and intermitant.. short.. this is why the thing needs so much effort and they cannot be used as simple contact switches.. for obvious safety reasons, a conductive balloon is sort of scary!

dual voltage from 2 9v wallwarts or batteries seems easy enough to rig.. any resource for me to see some op-amp schematics? ive got forrest mimms books, etc... i am a neophyte, at best, when it comes to analog electronics, and especially op amps!

i will post some schematics soon to get advice.

thank you!

-R

[pastera]

if you use a high frequency (80-100KHz) you can capacitively couple the two.
1)555 oscillator connects to one ballon feeding it with a low voltage high fequency signal with a fairly high source impedance (~10K)

2)second balloon is hooked to an opamp based band pass filter (mcp602)
3)depending on the output of the band pass you may need another amp stage to feed the mono stable (555)


aaron

[vacpress]

oh dear... what do you mean capacitively couple the two? how is this better than just looking for voltage? if i send an oscillating signal into the balloon, will it be easier for an opamp to amplify?

thanks

[pastera]

The fact that the two balloons are large conductive areas separated by a dielectric can be exploited by using them as a variable capacitor.
The capacitance is inversely proportional to the distance between the two conductors (balloons). The reactive impedance is then directly related to the distance between the two. The resistance is essentially infinite until the two touch.
By choosing a high enough frequency, the impedance can be lowered to a point where the signal (oscillator) easy passes without actual contact.

Aaron

[vacpress]

AH. that makes perfect sense. it would be an ideal way to sense this. unfortunately, it is beyond my ability to imagine a circuit to fulfill this need easily...

[pastera]

http://ourworld.compuserve.com/homep...wden/opamp.htm

http://www.uoguelph.ca/~antoon/gadgets/555/555.html

It's actually simple - your just using a high frequency oscillator to drive a switch (two balloons) and sensed using a filter to get rid of other signals.

Aaron

[vacpress]

"Set the balloons up as a "switch" driven with an oscillator running if the high kilohertz - a few volts should be enough (555 timer). Feed that into an OPAMP (741) biased at about mid supply level. Use the output of the amp to trigger a one shot (555)."

Pastera,

Thanks alot for the advice. I am right now building a 555 oscilator circuit. High Khz eh....

Thanks`

[vacpress]

Oh!

Ive got a pulse generator. Unfortunately it has 'pulse spacing' and 'pulse width' knobs. It makes sense, I think, that mS is 1000/ths of a second, or Khz, and uS would be megahertz? This thing outputs upto 10 V, can I use it as a experimental source of 'high khz' pulses?

by high, i imagine 100-200khz? is that appropriate?

I am showing how little I actually know! Thank you!