Putting objects on capacitive touchscreens

[PorridgeBear]

I am trying to understand capacitive touch screens and what kinds of objects can be placed on their surface to register much like a finger.

I have done some experiments this evening. I made a quick iOS app that shows a message when a touch is registered and then changes when the touch ends.

First, I hold my finger on the display and the message stays the same "object dectected" so long as my finger remains. On removal the message is "object removed". This is as expected.

Now I glued some tin foil on the end of a plastic bottle. I am able to get this to register as "object detected" if I rotate the tin foil on the screen a little bit. It's not instant but it does fire. I notice however that the "object removed" fires after about 5 seconds. This seems repeatable. I never touch the foil with my fingers. I thought perhaps this could be static from rubbing the foil on the screen.

Next I put a metal spoon on the display. This immediately registers. I leave it in place and it takes a good 10 - 15 seconds but also then fires the removed message. I am now thinking metal conductor objects are storing part of my own charge and then they lose the charge.

To test this I decide to wait for the spoon to register as removed then I pick it up with a tea towel so I am not handling it directly. I am insulating myself from it so as not to pass charge. However the spoon still reliably registered as detected and then fires undetected after the same time. I don't understand this - the tea towel should not be passing charge?

Then I try a 50 pence piece coin. This does not register at all no matter what. How is this possible? Are coins not conductive?

I then tried a large metal garlic press - 3 times more metal than the spoon and this does not register as detected either. Not sure why.

Finally a very small metal washer does not register either but does if i touch one side as the other is on the display.

So really a mixed bag here and I don't feel much wiser. I read about capacitive screens. I read that when a conductor comes close that it draws charge away from the screen which is detected. However by that definition would not every conductor in my list above reliably trigger detected - and importantly why do all those that do register detected "time out" and fire undetected after some seconds - can't they draw charge anymore?

Thank you!

 

[Runei]

Well, there are several parts where things can be tricky here. I'm not expertly familiar with the iPhone touch-screens.

First off, the the capacitive touchscreens rely on the capacitive coupling between the detection grid in the screen and the objects coming close to the screen. What happens for the touchscreen circuitry, is that it measures a change in capacitance at some point on the screen, using this grid.

Depending on the object used to touch the screen, the capacitive coupling will be different. As it is now, it's hard to say how much of a capacitive coupling is needed for the circuitry to detect a "touch". To know that you would need more specific specs on the screen.
The capacitive coupling is in general dependent on the conductance of the object, but also the 'geometry' of how it touches the screen.

Next off, the software that is set to detect the touch might have built in algorithms to discriminate certain things, it's hard to know what is going on without a detailed specification.

If you want to do some further experimentation you would need to somehow fix some of your parameters and vary others controllably.

For example:
- Whatever object you use should have a specific dimension (say a circle of diameter 0.5cm or thereabouts).
- Place the object on the screen with a plastic tool
- Find the conductance of the given material tested (eg. what material is the 50 pence coin made of?)

That way you might be able to figure out how the technology Apply has built into their touch screen discriminates between what is a "touch" and what is not. And remember they might have done a lot of things to make sure the screen detects "fingers".