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EDITED:

Two arduinos, each one powered by a battery. Each arduino is connected to a piece of metal. When pieces of metal from both arduinos touch each other, the arduinos have to realise the event.

The problem: by having a different ground, they cannot detect each other electrical signals. Is it there any workaround for this?

I've take a second guess: each arduino uses the piece of metal it is connected to as a capacitive sensor. This way, both arduinos are able to "detect" that the pieces of metal have come together. But, this has the problem that if a human grabs both pieces of metal at the same time, the arduinos will believe that both pieces of metal have make contact.

The problem is: how to make the arduinos able to detect their respective pieces of metal touch each other, without giving a false positive when a human grabs both metals at the same time.

Any ideas about how to resolve this problem?

Edited: About the strength of the signals each arduino takes when the metals as capacitive sensors touch each other, it is of the same intensity and as such non-differentiable from the signal the arduino gets when the human touches the metal.

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    \$\begingroup\$ Are you trying to design something that detects if two unconnected modules "come together"? If so please specify orientation and operational distance requirements. Please also state if barriers come between the two modules, what should happen. \$\endgroup\$ – Andy aka Apr 24 '18 at 9:55
  • \$\begingroup\$ I don't understand what you mean with "detect each other". Everything would be much easier if you show us a schematic. \$\endgroup\$ – pipe Apr 24 '18 at 13:54
  • \$\begingroup\$ Ive edited. Hope it is clearer now \$\endgroup\$ – pfernandez Apr 24 '18 at 16:51
  • \$\begingroup\$ Please do provide as much of the use case details as possible. What you are asking for may not be practical but with more background a suitable solution may be possible to suggest. \$\endgroup\$ – KalleMP Apr 24 '18 at 21:41
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For reliable communications, you must include a second wire, connecting the grounds. If you wish to maintain isolation, I’d suggest something like an optoisolator between them.

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