Capacitive Pads

[wuchy143]

Hello,

I attached a pic of two half moon pads on a printed circuit board. You can think of these two pads having an equivalent capacitance.(as in real life they do have capacitance) The question I have is when you place a circular piece of foil directly over the pads(but not touching) why does this increase the overall capacitance of the pads on the PCB? I see by experiment that it increases the capacitance by about 20nF...which is considerable considering my application(capacitive touch).

If you have equations that's cool but I'm really struggling to get a clear idea of what's going on in my head. So sometimes words help the best. Whatever you got I'll take though

-mike

 

[MikeMl]

Think of it as two capacitors in series, where the foil is a floating node. You usually use an AC signal to sense the capacitance change...

 

[wuchy143]

ahhh. hmm...I knew it had to be simple. I attached a drawing showing what you have shown me to make sure I understand it correctly. You're right. When you add that foil it's as if you are taking that one cap and turning it into two caps in series where the internal plates/node is floating. Thank you very much!

You are right. I'm pretty sure AC is the only feasible way to sense capacitance. To sense we put a 2kHz signal onto one of the pads and that spike in current is sensed by some additional circuitry.(muxes/caps/r's)

cheers

-mike

 

[crutschow]

20nF is a huge change. It should be more in the pF range. Are you holding the foil with you hands when you place it over the pads?

 

[misterT]

You are right. I'm pretty sure AC is the only feasible way to sense capacitance. To sense we put a 2kHz signal onto one of the pads and that spike in current is sensed by some additional circuitry.(muxes/caps/r's)
-mike

Capacitive touch sensors (charge transfer sensors) measure the capacitance by first charging the capacitor (with series of pulses, like charge pump) and then measuring the time it takes to discharge the capacitor (through a big resistance.. say 1 Mohm).

Human touch capacitance to earth is around few picofarads.. so 20 nF change is huge.

 

[wuchy143]

oops I was off by quite an order of magnitude. The foil is floating(my hands are not touching it) and it's 20pF not nF. And the pads are around 60-80pF depending on the length of the traces for each of the pads.

misterT,

Do you have any examples or could you elaborate a bit more on how you would implement the cap touch sense through the cap and 1Mohm resistor. That sounds like a much more elegant way of doing things than how we do it.

 

[cobra1]

how many buttons are you needing, i have just finished a project using these sort of touch pads.

 

[wuchy143]

Hey Cobra,

My application is a keyboard with up to ~121 keys. Our technology is outdated and would like to make it cheaper/easier to produce and more reliable. I'd love to take a look at your project or at least hear how you went about it. Thanks for the reply!

-mike

 

[cobra1]

wow, 121 keys, mine only has 2, i used a dedicated chip made by quantum atmel.

not sure how you will go about 121 keys though, maybe a 128 pin arm chip will be able to do this