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| Capacitive touch/proximity buttons implementation in to PCB |
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| 3dgeo:
--- Quote from: blueskull on December 30, 2018, 07:05:43 pm ---Compared with your button layout, I'm more concerned with your NRF module antenna-to-ground-plane clearance. --- End quote --- Will NRF have any impact on touch sensors? NRF itself is optional and will not be in final product, it's just for short range testing purposes. |
| nctnico:
--- Quote from: 3dgeo on December 30, 2018, 06:58:01 pm --- --- Quote from: nctnico on December 30, 2018, 06:48:11 pm ---Look at Cypress' capacitive touch chips. They also have tools to determine the optimal design. For starters you'll need way more clearance around the capacitive touch pads. --- End quote --- Really? I think spacing is fine, this keypad has way smaller gaps and no grounding around and works just fine: --- End quote --- The problem is that the field lines are not going far away from the PCB if the gap is small. If you want to use an overlay which is 3 or 4mm thick (which one of the designs I did has) then this will become an issue. Just look at the design tools Cypress provides. Another issue is that you have to stay within the capacitance of the capacitive sening solution is able to deal with. I rather do some math than randomly smacking some pads on a board and hope a capacitive sensing solution works. Something else I'm missing are ESD limiting resistors. |
| 3dgeo:
--- Quote from: nctnico on December 30, 2018, 07:36:36 pm ---... I rather do some math than randomly smacking some pads on a board and hope a capacitive sensing solution works. Something else I'm missing are ESD limiting resistors. --- End quote --- As I said I'm new at CT buttons, I don't know any math, if I did there would be no need to ask help in forum. According to data sheet "...The VDD periodic voltage ripple over 50mV and the ripple frequency is lower than 10 kHz can cause wrong sensitivity calibration. To prevent above problem, power (VDD, GND) line of touch circuit should be separated from other circuit. Especially LED driver power line or digital switching circuit power line certainly should be treated to be separated from touch circuit..." There are LED drivers on the board, so yes, I should add 200 ohm resistors and 10pF caps (according to data sheet). How should I isolate CT ICs power line? With something like ground loop isolator? Would it be a good match to PCB if I make same size pads from copper tape and test them? |
| 3dgeo:
I did some searching and came across this very useful document: Guidelines for designing touch sensing applications with surface sensors According to it, to have working CT buttons I have to have at least 4 times bigger pad compared to distance from pad to finger, 5.5 mm in my case, so at least 22 x 22 mm pad and distance between pads should be at least 2 times from pad to finger distance, 11 mm in my case, and I have to remove grounding around pads, so yeah, I'm a bit off..... :) Just for learning purposes I probably will manufacture board with current size pads, but it will be very optimistic to hope they will work properly/reliably. Plan B – look for proper standoffs, plan C – dedicated CT PCB just below acrylic. If anyone have recommendation for proper standoffs – good time to share :) |
| 3dgeo:
Little update, I don't know why should anyone care at this point, but here I go: ;D Cos I need more proximity sensor than a touch (4 mm gab below plexiglass) according to info I found I need to keep ground away, so I cleared it as far as I could and for good luck increased pads to 13 mm (was thinking about 15mm, but I think that would be an overkill. Also added bypass caps to LEDs. I didn't mentioned that before, but I was playing with TTP223 cheapo touch sensor modules, and they work fantastic, even at the distance, not at 5,5mm as I need, but at 3-4mm – no problem. So I assumed that IC with greater sensitivity will do the job. To be honest I still do :scared: Yes, I can easily add PCB on top with touch sensors, but it will increase frame manufacturing cost a lot so I want to avoid it if I can. Originally I picked TS20, but I realized that TTP229-LSF has same mother as TTP223, so I have to at least test both ICs. If none of them will wok I'll just go with 9 TTP223, from my tests with 13 mm pad TTP223 will have no problem reliably working even with gap I need. To be continued... :popcorn: Cheers :-+ |
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