I'm having a heck of a time with this capacitive keyboard... - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: May 02, 2017, 02:28:07 am
content: I ordered some new "foams" for it from eBay and most, but not all work. The seller is sending me some that are known to work for the ones that don't, but I still wonder if there is something else going wrong with this keyboard electrically that I need to fix.The voltage regulator is getting 12V in and putting out 4.87V which seems a bit low to me as it seems that these 7805 type vregs are usually more accurate than this, but it is quite old (1983?)I tried using an eraser on some pcb pads, no different. I've tried to use some deoxit on the key pads to see if that would help, no difference.If I use a "good" foam, it is very sensitive to where it is positioned on the pad, even 1mm off and it might not work. Some pads prefer the foam disc to be off center. Sometimes it will work and a second later it will not. I tried the keyboard all screwed down and i get the same result. For testing I've been putting a foam on the pad and pressing it with a pencil to see if it will type the key or not.Any ideas on how to make it more sensitive? Should I replace the vreg? Other ideas? I have already used some water soluable flux and rewet/resoldered all the parts at the top before giving it a water rinse/compressed air blow off. That was before the deoxit. Nothing seems to improve it...

author: alank2
author_rank: Super Contributor
timestamp: May 02, 2017, 03:41:01 am
content: More questions - why is it my finger can set each pad off easily, but if I get some foil and tape it on both sides so it won't short out the pads, that it on its own won't activate the key. It needs a bit of distance to a clear plastic disc to work - is this forming a capacitor between the metal disc and the clear disk? Normal pads can sit on a key, but don't activate until you press them? Does pressing them create the perfect capacitance it is looking for? Can I simply make something that doesn't need to be pressed, but is the right capacitance and just move that over the key or remove it?

author: Fungus
author_rank: Super Contributor
timestamp: May 02, 2017, 09:12:49 am
content: Quote from: alank2 on May 02, 2017, 03:41:01 amBecause it needs the capacitance of a human body to work. It's all about mass, half a gram of tinfoil isn't enough.Can I simply make something that doesn't need to be pressed, but is the right capacitance and just move that over the key or remove it?Yes, but it has to belarge.nb. You can connect your small piece of tinfoil to something bigger with a piece of wire.

author: alank2
author_rank: Super Contributor
timestamp: May 02, 2017, 12:18:05 pm
content: So, we have a metal foil that is insulated from the pcb pads, then a foam material, then a plastic disc. Assuming the foil is one side of the capacitor and the plastic disc is the other, what exactly happens? A pulse is sent to one pcb pad and the foil/plastic disc will take it in temporarily and then release it to the other pcb pad where it is detected? It is like a one side capacitor???!?!Q#1 - if the foam were thinner so it didn't need to be pressed, would it set the key off all the time like my finger?Q#2 - if I wanted to make a non-foam version, what materials could I make one that would work out of? Would two metal discs work? Why or why not?