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I have two polystyrene balls wrapped in copper tape. When the balls touch, the tape completes a circuit from a digital pin on an Arduino inside one of the balls to an Arduino inside the other (both powered by 9v batteries). This allows them to send information to each other via the copper tape path.

I was testing the balls out, and they were working too well. Even when the balls weren't quite touching, the signal was getting through! Turns out that's because the signals were travelling via my body. Even if I held one ball in one hand and the other ball in the other hand, digital reads on one Arduino were correctly reading the digital writes from the other.

This is actually great for my project!... unless it's not safe. So I'm looking for two things:

  1. A reliable reference on what is and isn't safe in terms of voltage/wattage/amperage travelling from hand to hand.

  2. A second opinion on the voltage/wattage/amperage that two Arduinos will actually pump through me (from digital pin to digital pin).

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  • You say the tape completes a circuit when the balls touch ... do the balls have some wires connecting them, besides the tape? Or are you just doing capacitive signalling?
    – James Waldby - jwpat7
    Commented Jan 15, 2017 at 5:44
  • @jwpat7 I guess "completes the circuit" is the wrong wording. One Arduino sets a pin to 0V or 5V in a recognizable pattern, that pin leads to a pin on the other Arduino (via the copper tape), where the voltage is read. When the balls aren't touching, the pins aren't connected.
    – Craig Gidney
    Commented Jan 15, 2017 at 5:58
  • So it is like you have two metal globes, with no wires coming out, and when they are connected you are able to detect signals between them? If so the mechanism is capacitive and current probably is on the order of nanoamps if that. What's your signalling protocol? Do you have to have some min or max data rate, Manchester coding, and so forth?
    – James Waldby - jwpat7
    Commented Jan 15, 2017 at 6:07
  • @jwpat7 I haven't decided on the protocol yet. I just confirmed that the distinction between high and low was getting through. The data rate I need is super low (<10bps) so basically anything I use will work. The main complication is cross-talk, or negotiating who sends and who receives.
    – Craig Gidney
    Commented Jan 15, 2017 at 7:05
  • If you ever switch to a wall-adapter, make sure it's not one of those ultra-cheap chinese ones.
    – Gerben
    Commented Jan 15, 2017 at 13:05

2 Answers 2

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If you are talking about running at 5V, then I doubt there is any safety issue. After all, 9V batteries (the sort you put in your smoke detector) just have bare terminals and no-one tells you not to touch them.

There is quite a bit of related information at How much voltage is “dangerous”?. That link seems to have enough information that there isn't a lot of point of repeating it all here.

There is a reference on that page to someone holding a metal pole connected to a 12V battery wading through water which completed the circuit and he was unable to let go of the pole. But that is 12V (more than twice 5V) and in circumstances where there was good conduction (his body was wet).

In the interests of science I set up a small test. I connected myself to a 5V lab supply via an ammeter. I grasped the terminals firmly (with dry hands) and measured about 3 µA of current. From the page I referenced above there seems to be a consensus that 1 mA would be just noticeable, so this is about 300 times less.

I found if I wet my fingers I got up to about 20 µA of current, still a lot less than 1 mA.

That page mentions that 60V is getting into the range where you might get a shock.

Hopefully it goes without saying that if you are going to touch metal balls then you need to be certain that 5V is the maximum, and that some sort of failure in a power supply might not deliver a lot more. But if the balls are battery powered you should be pretty safe.

You've added that they are powered by 9V batteries. That doesn't change my answer much. The most that could "escape" due to some malfunction is then 9V. Under normal operation you would have 5V from a digital output pin.

I measured my body resistance as being around 1.5 MΩ, which is consistent with getting 3 µA of current through it.

I = V / R
I = 5 / 1500000 = 0.000003 A
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  • 1
    Have you tried licking the terminal on a 9V battery, they don't say you shouldn't do that either :)
    – Code Gorilla
    Commented Jan 17, 2017 at 13:06
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    Yes I did that when I was a lot younger. The pain caused me to stop doing it. :)
    – Nick Gammon ♦
    Commented Jan 21, 2017 at 4:29
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In fact the voltage is almost irrelevant. The current is the issue. 30mA is normally considered the limit - this is the value RCD are set to trigger at.

Even low voltages applied to intimate places can be fatal. (I have a couple of wires in my heart, these can certainly cause a significant impact).

There is little risk with the low energy available, but if you want to be safe you can use a series resistor. I have (a very early) touch sensor installed on lights in my house. The touch pad is effectively connected to 240V, but is quite safe as the pad is connected through 2 10MΩ resistors. (2 are needed as they are rated at 500V.)

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  • 2 are needed - plus that is a precaution if one fails and becomes closed circuit.
    – Nick Gammon ♦
    Commented Jan 15, 2017 at 4:17
  • In fact the voltage is almost irrelevant. The current is the issue. - Check out "It's not the volts that kill you, it's the amps"
    – Nick Gammon ♦
    Commented Jan 15, 2017 at 6:01
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    @NickGammon I didn't "read" the link, because it is to a video. Like most such issues this is not simple, because people are not a simple linear resistance. I bow to the recommendations of the of the Standards authorities, which admittedly only apply to "Low Voltage" (contrary to most expectations this is 1000V - with its own caveats).
    – Milliways
    Commented Jan 15, 2017 at 7:06
  • If you had "read" the link you would see that he agrees with you. It isn't simple. Higher voltages, however, break down the surface resistance of things like bodies.
    – Nick Gammon ♦
    Commented Jan 15, 2017 at 8:31
  • @NickGammon, apparently there are no wires coming out of the spheres (see comments to question) so typically any volts or amps occuring will be from capacitive effects as a few nanocoulombs of charge move around.
    – James Waldby - jwpat7
    Commented Jan 15, 2017 at 9:18

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