Help with AT42QT1012-TSHR - Page 1
Forum section: electronics / beginners

author: Njackste
author_rank: Contributor
timestamp: May 17, 2023, 12:57:47 am
content: I'm designing a touch circuit that lights up an array of LEDs using a MOSFET but the board also has a USB hub built in.I've been reading the datasheet of the AT42QT1012-TSHR and I can't understand how the Cx capacitor works, I'm designing in kicad so how do I calculate the load capacitance. I also see that it connects to GND, so do I need to put a via on the touch pad going to GND? Also as a USB hub is on the same pcb it's a 4 layer board and the middle 2 are GND so do I have to remove them in the area where the touch pad is? Lastly I need the chip to be sensitive enough to be able to be activated through plastic but not too sensitive.Thanks

author: ataradov
author_rank: Super Contributor
timestamp: May 17, 2023, 01:27:40 am
content: Don't bother calculating it, just make the electrode the size you need in your application with no grounds around including other layers.You will have to tune the sensitivity for your specific board and dielectric material using external components anyway.Cx is not a real capacitor, it is a capacitor formed by the ground and the electrode. But you don't want to have ground plane under it, because in practice the capacitance would be too high. In some cases you may need to place a sparsely hatched ground fill under the electrode. But in reality the only way to know is to make the board and see how it performs in your specific environment.

author: Njackste
author_rank: Contributor
timestamp: May 17, 2023, 04:02:28 pm
content: Thanks so much for your help but can you please elaborate on this "In some cases you may need to place a sparsely hatched ground fill under the electrode"

author: Njackste
author_rank: Contributor
timestamp: May 17, 2023, 04:04:05 pm
content: Also why should I not calculate it, as kicad has a simulation software built in and I need to use it to calculate crystal load capacitors

author: ataradov
author_rank: Super Contributor
timestamp: May 17, 2023, 04:27:49 pm
content: Quote from: Njackste on May 17, 2023, 04:02:28 pmIf you find that you need to increase Cx to meet your sensitivity requirements, you may need to place a ground plane on a layer below the electrode. But solid ground plane will usually add way too much capacitance, so the solution is to use a hatched plane. But I would not worry about that at first. Just be prepared that it may not work perfectly on the first iteration of the board.Quote from: Njackste on May 17, 2023, 04:04:05 pmOscillator loading capacitance calculation are just well known formulas.To calculate capacitance of this electrode you need to have completely routed board (since routing would affect it) and a completely different type of simulator, which KiCad does not have.But also, even if you calculate it, what are you going to do with that number?

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 17, 2023, 06:03:06 pm
content: I'm just a hobbyist, but interested in all kinds of sensors, including touch sensors. To understand the various electrode designs, I searched for and found a few relevant application notes:Atmel Touch Sensors Design Guide(via Farnell)Microchip AN2934 - Capacitive Touch Sensor Design GuideNXP AN3863 - Designing Touch ElectrodesSilabs AN0040 - Hardware Design for Capacitive TouchI hope you find these as informative as I did.I also foundCole, et. al, "An Analysis of Electrode Patterns in Capacitive Touch Screen Panels"very informative, in the sense that it gave me a better real-world grasp of what kind of effect e.g. the electrode shape has in a panel or matrix (surprisingly little!).For a simplified short description on the effect of the electrode size and distance to a ground plane, seeEDN: Electrode design in capacitive sensor appicationsarticle.

author: floobydust
author_rank: Super Contributor
timestamp: May 17, 2023, 06:28:52 pm
content: OP how thick is the plastic and airgap to the PC board, for the touch sensor?SparkFun Capacitive Touch Breakout - AT42QT101xthey mention can detect through 1/8" acrylic. You can also see the mesh ground-pour they use.I find if you have a mesh ground-pour behind the electrode, you don't get much range and most applications don't want false triggering from being close to the electrode verses touching it.

author: Njackste
author_rank: Contributor
timestamp: May 17, 2023, 10:44:04 pm
content: Thanks everyone for the help I've finished my design now