I'm at a very early stage of thinking about this project, and would love any guidance on potential pitfalls, issues I may not have seen or even completely alternative ways of achieving similar effects.
The idea is to drive some RGB LED strips from an arduino uno, based on voltages being received from variable resistors
The twist is that I would like the variable resistor to be operated by people touching both their hands onto foil contacts wrapped around wooden tubes in order to complete the circuit, with the values being read depending on how close their hands are together.
I think it should be safe to send the 5v from the Uno to the "wands"?
However bearing in mind this will be being used by members of the public and I cannot 100% guarantee they will have dry hands perhaps this is not true?
I am a little unsure if the range of voltages produced will be great enough or varied enough to provide enough data to give full control of the lights, Or if the variation in people's skin resistance will be an obstacles. Is it likely I will need to amplify the output after it's been through a person in order to get a meaningful reading?
I am expecting to have to do a bit of experimentation with measurements and foil to get this working at all of course, but I just wanted to see if anyone has experience of a similar project, or can point out something I am missing that will make this impossible to achieve?
In addition, ideally I want to do this for 8 separate LED strips, controlled by 8 separate "variable resistors". I think this means I will need two Unos in order to have enough analog in, plus the 24 digital Outs required to control the RGB LED strips. Does that make sense?
If this proves to be a difficult/unsafe way to achieve what I'm after I think capacitive touch sensors may be another option, perhaps with foil lining the inside of hollow tubes.
Skin resistance will vary greatly between people; sweat on the skin; how well they grab your handles; and probably a few more variables. You may be able to detect "contact" vs "no contact" but that's about it.
with the values being read depending on how close their hands are together.
No... Moving your hands closer together won't make a difference. It will depend on moisture, contact area, and skin-distance between the wands. i.e. There is more resistance between your right and left hands than between two fingers on the same hand.
You'll also probably need to filter the readings because human bodies act as antennas and they pick-up the 50/60Hz power line frequency, and it's "worse" at high impedances. You can use a capacitor or look at the smoothing example.
I think it should be safe to send the 5v from the Uno to the "wands"?
Yes. Anything under 50V is considered safe by most government regulatory agencies. (Over 50V has to be insulated & isolated in a way that the user can't touch it.)
wvmarle:
Skin resistance will vary greatly between people; sweat on the skin; how well they grab your handles; and probably a few more variables. You may be able to detect "contact" vs "no contact" but that's about it.
DVDdoug: Try it with your ohmmeter first. No... Moving your hands closer together won't make a difference. It will depend on moisture, contact area, and skin-distance between the wands. i.e. There is more resistance between your right and left hands than between two fingers on the same hand.
Sorry I wasnt clear here, I meant to say "closer to/further from the contacts, but I guess that the difference in resistance between a cm of foil and 50cm foil is not huge in any case?
You're right of course, I can just try it and measure! But doesnt sound hopeful. I wonder if there is a better material. I was thinking of a similar experiment with graphite on paper, touching crocodile clips in different places to get different voltages...
Perhaps the capacitive touch method makes more sense.
the_duckchild:
Sorry I wasnt clear here, I meant to say "closer to/further from the contacts, but I guess that the difference in resistance between a cm of foil and 50cm foil is not huge in any case?
The difference in resistance is minute, and easily 15 orders of magnitude less than the difference in skin resistance between different people.
To measure where it's held, a capacitive sensor will do a bit better already but still there's a great difference between people here, too.
The best you can do is to have a large number of contacts (e.g. by using thin concentric metal rings in your grips), and then check which contacts are touched. That gives you the best way of telling how far apart those hands are.
With the capacitance option, I'm just intending to use how close people's hands are to the sensor to change the LED colour, rather than have them close the circuit and change the colour by moving their hands.... I think that's probably just as much fun really and sounds like it might be a lot easier to execute from this;
Most capacitive sensors register no more than a few cm, and the actual reading will be affected by the environment (humidity of the air), the size of the person approaching it (the bigger the person, the larger their capacitance), etc.
it is apparently dependant on resistor and sensor size, and could be up to a two feet (which is way more than we need I think)
Interesting about the humidity. Fortunately, exactly what reading triggers what LED colour, or how close someone has to be to light up the LEDs are not too important, so I hope we can get it fucntioning with a little experimentation.
Rather than using skin the pressure with which a person held a batton (wrapped in conductors separated by conductive foam) would change the resistance. It would be crude but the the range would be more predictable and reproducable.
Don't forget that it is not just voltage that is dangerous. A car battery is only 12V but drop a spanner over the terminals and you will have some fun because of the large current.
