Guesstimate - 5mm thickness marble touch sensor feasability

[daqq]

Hi guys,

Do you think that a touch sensor under a 5mm thick plate of marble would be sensitive enough to reliably work? Or is the whole idea wrong? Not position sensitive or anything - just if the button has been pressed.

If yes, could you recommend a specific device? I know Atmel, Cypress and Microchip makes them.

Thanks,

David

[georges80]

The cypress capsense stuff can be set to be quite sensitive in the 'proximity' mode. I would think 5mm of marble should work with a decent sized 'sensor pad' on the bottom side. Get a capsense kit (often free at the various cypress sponsored training sessions) and try it out.

My wag is that marble and glass are likely similar and glass works very well with a ito etc pad on the 'other' side. Marble is non-conductive and has a similar permittivity as glass so seems quite feasible.

cheers,
george.

[tautech]

If it can be done through 10mm toughened glass, 5mm marble should be no problem one would think.

[ajb]

There are commercial solutions that are speced for up to 6mm of glass: http://www.apem.com/Capacitive-switches-for-underlay-applications-v9-d-992.html

[miguelvp]

I have a "marble" laminated countertop and magnets attract to spots, so I don't think that comparing glass to marble even when thinner applies to sensors, I'm not sure what capacitance aberrations the marble (probably faux marble) will have.

The magnet jumps between about 1/5th of an inch to one inch spots within the surface when pushed around the surface.

[gman4925]

Allen Bradley give data http://literature.rockwellautomation.com/idc/groups/literature/documents/ca/c116-ca504_-en-p.pdf on using their sensors with various materials and Marble has a relatively high dielectric constant so the marble might be a bit hard to detect a person through, so you would need a sensor with decent range.

This http://www.digikey.com/en/articles/techzone/2011/may/capacitive-commutation-catches-on mentions touch sensing through marble so it should be possible.

I'd suggest just getting a cheap industrial capacitive proximity sensor with 20-30mm range and trying it out.

[SeanB]

I have a "marble" laminated countertop and magnets attract to spots, so I don't think that comparing glass to marble even when thinner applies to sensors, I'm not sure what capacitance aberrations the marble (probably faux marble) will have.

The magnet jumps between about 1/5th of an inch to one inch spots within the surface when pushed around the surface.

That is a melamine sheet with a marble look alike print on it, underneath it is plain chipboard or MDF board. Likely the magnetic parts are the screws or nails used to attach it to the subframe it is mounted on. You will have regular lines of nails, and then odd spots where the screws and mounting frames are.

As to the real marble, it will work until it gets wet, as it is basically a compressed limestone, and it is somewhat hygroscopic. Your sensor area will have to have drift compensation to avoid the moisture build up affecting it, or you will have to provide a local heater to drive moisture out actively. It will still stop working at some moisture level, even if it does not look damp.

[daqq]

Thanks guys!