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I want to make a wearable metal trinket with the ATtiny85 chip acting as a capacitive sensor (with the same technique that this Arduino library uses), so it can detect when it's being worn.

Are there any health risks to touching what is effectively a low-voltage live wire for an arbitrarily long amount of time?

Here's the schematic from that link:

Capacitive sensor schematic

Be patient please, I'm a beginner :)

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    \$\begingroup\$ Nah. Probably not. Try it. On someone else maybe first. \$\endgroup\$ – Olin Lathrop Aug 8 '17 at 17:49
  • \$\begingroup\$ There really is no answer to that question. If it's battery operated the risk is probably zero if you're not creating high voltages in there... but someone somewhere will tell you it can disrupt some bodily system anyway.... \$\endgroup\$ – Trevor_G Aug 8 '17 at 17:49
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    \$\begingroup\$ U can bet someone somewhere will claim it gives them a migraine.... \$\endgroup\$ – Trevor_G Aug 8 '17 at 17:53
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    \$\begingroup\$ I'd be more concerned about the metal than the current. Some people are allergic to nickel and other base metals. Copper (and alloys) can leave green stains on the skin. Best is something guaranteed hypoallergic and intended for use in jewelry. Failing that, solid stainless steel (may have allergy issues) or gold plating on a base metal (like stainless steel.) Current flow can increase the rate of corrosion. \$\endgroup\$ – JRE Aug 8 '17 at 18:08
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    \$\begingroup\$ a reflective or plain CDS sensor would be safer \$\endgroup\$ – dandavis Aug 9 '17 at 1:34
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Yes, there may be a risk for long term wear. If your electrodes have an ohmic contact with the patient, and they have a DC bias, and they are made of a reactive metal like copper, then you may be generating metal ions on the patient's skin. Metal ions may cause a chemical burn.

I have an 8 year old scar to illustrate this. For a few days, I was experimenting with an EKG circuit and I used makeshift copper electrodes. The burns (two more: on another arm and on a foot) took a month to heal.

enter image description here

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  • \$\begingroup\$ Thanks for your answer, Nick. I had no idea that chemical burns take so long to heal, I'll definitely research them more in depth. \$\endgroup\$ – Edoardo Aug 8 '17 at 18:52
  • \$\begingroup\$ I'm planning on connecting the wires to a stainless steel casing, rather than connecting them to the skin directly, does that have the potential to generate metal ions on the skin too? \$\endgroup\$ – Edoardo Aug 8 '17 at 18:54
  • \$\begingroup\$ Obviously the plate will wear but gold or platinum plating will reduce this issue substantially. Using stainless as a base is better but will make it harder to plate. Many plating shops can do this. If placement is not completely crucial, moving them periodically can also reduce effect. I managed to get one of the reactions as above from a Microsoft Band 1, and the issue will be more pronounced in areas the user sweats. Using an electrode gel (as used in medicine) should also help avoid this (as well as improve connection) as it ensures connection is not confined to a single tiny spot \$\endgroup\$ – Rendeverance Aug 9 '17 at 11:14
  • \$\begingroup\$ ^^^ If your not in contact with the casing all the time its unlikely to be an issue and stainless should be fine, it depends on what you call an arbitrarily long amount of time? \$\endgroup\$ – Rendeverance Aug 9 '17 at 11:16
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    \$\begingroup\$ One way to ensure no DC bias is to put a capacitor in series with the contacts. For example 100 nF or larger ceramic cap shouldn't interfere with the measurement at all but will block DC current. Though if the metal itself is reactive this will not be enough. \$\endgroup\$ – jpa Aug 9 '17 at 14:38
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Capacitive sensors don't require direct contact, they work via capacitive coupling:

enter image description here

Check out the picture you've posted: the guy doesn't touch anything, he just keeps his hand close to the foil, creating a capacitor.

Just keep your electrodes insulated (e.g. covered by a thin plastic film or coating), and you'll avoid chemical burns and corrosion issues completely.

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Stainless Steel watch bands give the best skin results or leather if you prefer. but there will be some galvanic DC voltage with any chemical reaction.

But my finger tip is somewhat calibrated in xxx pF depending on surface area and contact force so for data transfer , this is all you need. Waving your hand nearby is about 1pF so f must be high to lower coupling impedance.

Adding a small metal contact does not improve much since it is in series with the air capacitance; but obviously depends on size relative to fingertip.

Noted I took this as an XY problem with a Z answer but other XY answer by Nick is valid, yet ignores the purpose Z.

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