Tact switch for overlay keypad - Page 1
Forum section: electronics / projects

author: DigitalDesigner
author_rank: Contributor
timestamp: June 10, 2025, 05:50:42 pm
content: Hi,I’ve been searching for tact switches that can be placed under a coverlay to create a 22-key keypad—something similar to a membrane keypad, but without actually using one.So far, I am thinking about the PTS125 series by C&K (screenshots and datasheets are attached):- PTS125 series with "Ribbed Actuator"- PTS125 series with "Square Snap-Fit Actuator"However, I haven’t been able to find any information about the keycaps—no datasheets, and more importantly, no 3D models. Not even on C&K’s official website.Are “Ribbed Actuator” and “Square Snap-Fit Actuator” standard industry terms for identifying cap interfaces? If so, where can I find 3D models and datasheets for the corresponding caps?And just as importantly, are these—either on their own or with caps—the right type of switches for an application like the one I described (a tact switch under a coverlay), or are there other switches specifically designed for such a purpose or design?Thank you for the feedback.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 10, 2025, 10:41:35 pm
content: I cannot provide a link because I haven't found C&K's drawings for their caps - it is indeed irritating as I have been looking for them for exactly the same application as you - vinyl overlays on panels with switches.However, I did manage to find grainy drawings on Aliexpress of all places - but didn't bookmark it :/ Note the round caps that are meant for the switch in your second image aren't always completely flat, plus they tend to be rather wide for overlay panels, the hole required for them is massive, so not great for crowded panesl. I ended up buying a bunch (not from Ali...) and actually measuring them myself.I have been meaning to make up a test panel, with different sized holes and different sized switch tops/actuators to find the best spacing. Something I have used in the past were:https://uk.rs-online.com/web/p/tactile-switches/3786741mounted so they sit flush to the front of the panel. Also, the hole for them was countersunk to reduce the sharp edge around the cutout that is often the failure mode of such overlays. No need for caps.The best panel I've done had an overlay with raised buttons, where the button tops protruded through the front panel hole by 1mm, this meant that the hole in the panel could be very close to the switches actuator diameter, as the overlay wasn't being pressed into the hole, just the raised part was being deformed.

author: ajb
author_rank: Super Contributor
timestamp: June 11, 2025, 02:48:27 pm
content: If I'm interpreting what you're asking correctly, here's what I've done and found to work well. These are just standard 6x6mm tact switches (eg APEM ADTSM66RV) on 10mm spacing with no button caps -- just the stems built into the switches, which extend up to 10mm from the board. The front aluminum plate has 3.9mm holes with a 9mm∅ countersink on the front to give space for the overlay to deflect without weakening the plate too much. (The 3D model in the screenshot is old, and shows smaller countersinks than we put into production.) I would say this is the most compact you'd want to go -- it's about the smallest spacing to easily and accurately hit the right button, if the countersinks were any smaller the buttons would get too hard to press, and if they were any larger without moving the buttons farther apart the overlay wouldn't be supported well enough and prone to fatigue cracking on the edges of the countersinks. Getting the right tactile feel really depends on nailing the height tolerance between the overlay and the board. We had some issues with thickness/flatness tolerances in early production of the plates, so some buttons would be really hard to press or would be held down when the module was assembled, but since we got that sorted out we've consistently gotten a nice tactile feel on them.If you're not as space constrained as we were, spacing out the buttons so you have more room for the overlay to deflect will make everything easier. Note that if your tact switches have a longer actuation stroke, or you use a cap with a larger top surface area, you will need even more room for overlay deflection. If you want more 'give' to the overlay you can add a little rubber bumper between the overlay and the button cap, which can also help with height tolerances. Embossing/doming the overlay over the buttons can also improve the feel of the keypad by giving more material to take up the deflection when the button is pressed

author: thm_w
author_rank: Super Contributor
timestamp: June 11, 2025, 10:22:10 pm
content: Is there a reason you don't want to use snap dome switches? They are quite reliable.Its the plastic dome resistive ones that are trash.https://www.jrpanel.com/product/MetalDomeSwitchArray

author: DigitalDesigner
author_rank: Contributor
timestamp: June 17, 2025, 03:45:55 pm
content: Thank you all for your detailed feedback!@Buriedcode and @ajbyou suggested very similar ideas and after looking into various options, I will go with that. No cap and countersunk hole. But I am thinking of keeping it just 0.1 below the overlay and leaving the overlay "stretch" a tiny amount at the centre which should be ok considering the relatively large hole around the plunger and it also being countersunk to avoid damaging the coverlay. So thank you both!@thm_w,one of the configurations is also the membrane keypad but the client asked for both because the same PCB has to work for multiple enclosures and products.Thank you all!

author: Terry Bites
author_rank: Super Contributor
timestamp: June 28, 2025, 04:37:47 pm
content: If you're going for a high prodction run call in a rep from a panel switch company.DIY designs can be uneconomical lot to prodcuce.

author: DigitalDesigner
author_rank: Contributor
timestamp: July 01, 2025, 10:32:47 pm
content: Thank you Terry, it is only 100 to 120 pieces run.