Capacitive touch/proximity buttons implementation in to PCB - Page 1
Forum section: electronics / projects

author: 3dgeo
author_rank: Frequent Contributor
timestamp: December 30, 2018, 12:27:48 pm
content: Hello,I want to implement 9 capacitive touch/proximity buttons in this specific PCB position (picture below – "TP1-TP8", "LOGO").I'm noob what comes to capacitive touch buttons (I know working principle, but no practical implementation).I need it to detect finger from 5.5 mm (4 mm air, 1.5 mm acrylic) and have RGB LED in the middle of the touch pad (again, picture below) – is it possible? What I should expect if I manufacture PCB in the picture (note that I will connect all ground islands).Powered from PC USB, so power ripple shouldn't be an issue.Can NRF24L01 radio module or OLCD/IPS display interfere? What other components can interfere with touch/proximity buttons?ICs I looked to for this job:TS20TSM12TTP229-LSFI picked TS20 for integrated sensitivity adjustment (no extra components needed), 5V logic, comprehensive data sheet and I2C registers list, tho I don't have a clue how it will work.Sorry if my post/questions isn't very well written, as I said I'm new what comes to capacitive touch buttons.Any advice or insight will be greatly appreciated.

author: james_s
author_rank: Super Contributor
timestamp: December 30, 2018, 06:05:20 pm
content: I loathe capacitive touch buttons, they provide no tactile feedback at all and never seem to work reliably with my fingers. I think you'll have difficulty making them work with such a thick gap too, the ones I've seen have only a thin sticker or layer of plastic over the sensor pads.

author: 3dgeo
author_rank: Frequent Contributor
timestamp: December 30, 2018, 06:37:01 pm
content: Quote from: james_s on December 30, 2018, 06:05:20 pmOh, I know it will not be easy, but I need them there and I'm willing to put some effort. If distance will be a problem I have other solutions – add extra PCB on top with pads or add SMD springs/standofs.Tho I would like to find out all potential issues before I order PCB samples, cos they do cost...

author: nctnico
author_rank: Super Contributor
timestamp: December 30, 2018, 06:48:11 pm
content: 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.

author: 3dgeo
author_rank: Frequent Contributor
timestamp: December 30, 2018, 06:58:01 pm
content: Quote from: nctnico on December 30, 2018, 06:48:11 pmReally? I think spacing is fine, this keypad has way smaller gaps and no grounding around and works just fine:

author: 3dgeo
author_rank: Frequent Contributor
timestamp: December 30, 2018, 07:14:37 pm
content: Quote from: blueskull on December 30, 2018, 07:05:43 pmWill 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.

author: nctnico
author_rank: Super Contributor
timestamp: December 30, 2018, 07:36:36 pm
content: Quote from: 3dgeo on December 30, 2018, 06:58:01 pmThe 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.

author: 3dgeo
author_rank: Frequent Contributor
timestamp: December 30, 2018, 08:02:36 pm
content: Quote from: nctnico on December 30, 2018, 07:36:36 pmAs 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 cancause wrong sensitivity calibration. To prevent above problem, power (VDD, GND) line of touchcircuit should be separated from other circuit. Especially LED driver power line or digitalswitching 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?

author: 3dgeo
author_rank: Frequent Contributor
timestamp: December 31, 2018, 08:07:33 am
content: I did some searching and came across this very useful document:Guidelines for designingtouch sensing applicationswith surface sensorsAccording 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

author: 3dgeo
author_rank: Frequent Contributor
timestamp: January 03, 2019, 11:56:32 pm
content: Little update, I don't know why should anyone care at this point, but here I go: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 doYes, 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...Cheers