04 April 2016

Nice-looking waterproof matrix keypad: how?

Say that, for some aesthetic reason, you need to embed a keypad inside the case of a project. Or because you want to build something splash/dust/dirt-resistant. But you need some human interaction, in the form of buttons to press.

Such an application could be a kitchen timer.

How to protect keys against splash/dust/dirt? Keep them inside the box, but then mechanical actions will not work. I came up with these possible alternatives to a membrane matrix keyboard:
  • capacitive sensors
  • hall effect (need a "magnetic" pen/tool)
  • photoresistors (only if the case is transparent)
Either I used wrong keywords, or nobody has documented online a matrix keypad not made out of mechanical switches.

Something must be wrong in my ideas. So far I came up with these cons:
  • capacitive sensors are unreliable, sensitive to static/noise, ...
  • hall is power hungry in large deployments, at 5 mA standby for each sensor (but a single row/column could be scanned)
  • LDRs need transparent case, a ligthed environment and are somewhat slow to react, in the order of 10 milliseconds
Any further thoughts?

I am about to buy a bunch of LDRs and Hall sensors to test it out...


Stephen said...

Cypress Semiconductor sells some nice capacitive touch sensor chips, including some that can handle a keypad matrix. Using one will require reading app more and doing a PCB layout, but you could make it look as nice as you want.

Electronic surplus dealers in the US often have membrane 3x4 and 4x4 keypads available on the web. If I recall, I've seen them on eBay from Chinese sellers as well. These have a solid, waterproof sheet of plastic on their face, and they are just a few USD. I don't know if there are Italian sources, but the Chinese sellers say they will ship anywhere.

Paolo said...

Thank you Stephen for your hint. The "problem" is actually more aesthetic than electronic. See pictures at http://q61.org/en/nixietmr/ . I think that keypad does not look good there; YMMV. That is how my wondering started. Capacitive sensors are technically feasible, but probably not in the average hobbyist lab. I have always tried to build circuits that could be easily reproduced with simple tools and little cost. Not counting that I am not yet able to draw a PCB and get it printed: I "burn" my computer time at work and in the free time I prefer staying away from it (a partial lie, since I have to do firmware coding...).

techie said...

magnetic reed switches and a magic wand.. engrave the labels from the back, and backlight or side light. key lights to magic wand storage.