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# how to make something like the UFO energy ball that use in teaching electric conductivity? Answered

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There are a bunch of different ways to do this.  For example, if you feed Google(r) this image search,
then you'll see a bunch of different pictures of circuit diagrams.

I claim that what most of these circuits have in common is a pair of bare metal "probes", wires, metal studs, or something like this.  Electrically the probes look like an open circuit (ideally infinite resistance) when nothing is touching the probes.  When skin is touching the probes, a resistance, in the range of roughly 100 K ohm to 10 M ohm,  is sensed between the probes.

So that's most of the model right there:  the circuit sees (senses, detects) skin touching the probes, as a resistor. The exact value of this resistance depends mostly on how moist the skin is.

Supposedly it is possible to use a skin resistance measurement like this to detect the presence of small amounts of sweating, caused by psychological stress, e.g. that associated with lying. So sometimes circuits like this are sold, or presented as, a "lie detector" circuit.

So "lie detector" is another phrase you can use for searching for searching for these kinds of circuits.  Some other phrases used to describe the same thing: "skin resistance", "skin conductance", "galvanic skin response"
http://en.wikipedia.org/wiki/Skin_conductance

Another important feature these circuits all share, is that they are all powered by batteries, so that the actual voltage seen at the probes is LOW, usually less 10 volts or so, DC.

A skin-resistance detecting circuit connected to the mains, 110 or 220 volts AC, would be a very bad idea, because you don't want people touching wires that are are touching wires that connected to mains voltages.   So whatever you build, design it to run on batteries for safety reasons.

I am not sure which circuit to recommend, because I am not sure what you like building with.  For example, I have had good luck with op-amps in the past, so I would probably choose a circuit that uses op-amps.  Also not sure how simple or complicated you want it, or what kind of output, sound or light, or what.

Probably the most simple thing to try is to just get a digital multimeter,
http://en.wikipedia.org/wiki/Multimeter
and set it to measure resistance, probably on its highest scale (on mine this is 2000 K ohm = 2 M ohm full scale).  Then try to measure the resistance through your hands holding the probes.  When I tried this, the resistance was initially out of range (higher than 2 M ohm), but I was able to get a reading after putting some water on my hands, and that brought the resistance down to around 500 K ohm.

aizuddinrazak (author)2012-07-07

wow thanks thats what ive been looking for, im trying to build something similar to this and still don`t have any idea to do it.

Jack A Lopez (author)2012-07-07

What does the UFO ball actually do when you touch its probes? Is it a response that is proportional the resistance (or conductance) of your touch, like a tone that increases in frequency as you get a better, more conductive, grip on it? Or does it merely detect that someone is holding it, and then sort of go through some sort of pre-programmed response, that is the same every time?

More importantly, what do you want your circuit to do?  Just sort of turn on, indicating someone, or something, is touching it, or do you want a response that is sort of proportional to the conductance it senses?   There are trade-offs to each of these approaches.

With the first one, the one that just turns on at certain sensed resistance threshold, you can make it really sensitive, so you can have it turn on in response to one, or two, or three, or ten, people resistances in series, like you know people holding hands, or otherwise touching each other somehow and making a conductive (resistive) circuit.

The second one, for example the ones that make a tone whose frequency is proportional to the sensed conductance ( by making this resistance set the frequency of an oscillator, usually an RC oscillator), I think that is more the traditional "lie detector" type toy.

aizuddinrazak (author)2012-07-07

im about to do the one that will response to certain sensed resistance threshold with sound coming out of it when people holding their hand. just to teach my student about the electricity. is there any way it can be easier?

Jack A Lopez (author)2012-07-08

I have attached some pictures of the most simple circuit I can think of, and this is just two NPN transistors (2N3904) wired as a Darlington pair, with a LED and 1 K ohm resistor in series, connected to to the collector.

As shown in the picture, the base current to this transistor pair flows from the positive (+) side of the battery through the "resistor" to be tested, in this case, my hand.

I put a 1 K ohm resistor in series with the base of the first transistor to protect the transistors, in case someone decides to place a very small, too small, resistance in between the test probes. E.g. if a short (a piece of wire, or a paper clip, etc.)  were placed between the (+) terminal of the battery and the base of the first transistor, too much base current would flow and burn out at least one  of those transistors.

aizuddinrazak (author)2012-07-08

Thank you so much jack.it really help a lot. :)