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I'm designign a device capable to sense proximity with other identintical devices and communicate that to a central node via bluetooth. The problem I'm trying to solve is showed in figure.

enter image description here

So far I identified two strategies:

  • IrDA: cheap but there must be free line of sight between devices
  • NFC: expensive, need 4 NFC chips and design antennas

Instead I'm wondering if I could use capacitive sensing, as implemented by ICs like the MPR121, to implement the above scenario. Could metallic pads on each side of the device be configured to provide a unique capacitance value when in proximity of another device pads?

Thanks!

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  • \$\begingroup\$ "IrDA: cheap but there must be free line of sight between devices." Is there not line of sight? IrDA with an ID code transmitted from each side is the best bet. "At" = A-top, "Ar" = A-right, etc., but it would need to be just above the level required for reception otherwise reflections will be a problem. (Try pointing your TV remote at windows, walls, etc., and see the amount of bounce you can get.) \$\endgroup\$ – Transistor Mar 17 '16 at 16:36
  • \$\begingroup\$ You may be able to use inductive sensing: Some ideas at ti.com/lsds/ti/sensors/inductive-sensing-applications.page \$\endgroup\$ – Peter Smith Mar 17 '16 at 16:58
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For the reason given in JRE's answer, a straight up capacitance measurement won't work. The chip you linked makes several simultaneous capacitance measurements, so you could have several measurement points on the cube, and a unique pattern of present and absent plates on the other cube to sense. This isn't going to work well though, for several reasons:

  • The sensors can only spot the plates when they are up close. How close? Roughly the separation between sensors.
  • The unique pattern of plates would look wrong to the sensors if the blocks are slightly off centre, or rotated. You could fix this by giving the pattern of plates translational and rotational symmetry, but then you're going to need a lot of sensors to distinguish a small number of cubes.
  • The sensors will probably not handle interference well. What's the capacitance of the floor they are sitting on, for example? It could be hard to predict.

An alternative approach using capacitors would be to have one large plate on each face of each cube. When close together, high frequency signals can be transmitted between plates, but if there is a big gap, or the plates don't face each other, the signal would be attenuated. Each face of each cube would transmit it's identity as a digital signal encoded on the high frequency carrier. You'd probably have to alternately 'speak' and 'listen' on each face.

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Capacitance between two plates depends on the distance between them. So, no this won't work.

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  • \$\begingroup\$ thanks! any other simple strategies you could suggest? \$\endgroup\$ – Simone Mora Mar 17 '16 at 16:27

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