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I have watched some youtube videos about capacitive touch screen

http://www.youtube.com/watch?v=ZHzaVzYEZbw

http://www.youtube.com/watch?v=GmCE18RMEeQ

The videos show that a conductor(finger), in which charges can be induced, can be used to trigger the capacitive touch screen. However, I tried to wear a plastic glove to hold a metal rod to touch the screen and no detection is triggered.

I really want to know the physics theory about the capacitive sensing.

Can someone tell me what physical properties or shape trigger the capacitive touch screen?

Is there any examples that something WITHOUT conducting to human body can trigger the screen?

(P.S. I have read other posts about capacitive touch screen here and do not find a good answer so this is not a duplicate)

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  • $\begingroup$ Here is an example of something that works without being connected to the human body. $\endgroup$
    – N. Virgo
    Commented Mar 6, 2013 at 15:13
  • $\begingroup$ What is the difference between sausages/fingers and metal rod for capacitive touch screen? $\endgroup$
    – Timespace
    Commented Mar 6, 2013 at 16:28
  • $\begingroup$ I'm not an expert, but are you sure the screen you tried was the capacitive design? After all, there are touchscreens that work by circuit completion, pressure, or heat. $\endgroup$
    – user10851
    Commented Mar 6, 2013 at 16:58
  • $\begingroup$ Yes. It is a ipad $\endgroup$
    – Timespace
    Commented Mar 7, 2013 at 2:06

2 Answers 2

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Capacitive touch screens use a grid of transparent conductors on the inside surface of the glass to sense position. Each conductor has some series resistance connected so that the capacitance from each conductor to ground can be measured by applying a current pulse to that conductor and measuring the rise and fall times of the voltages at these conductors. By scanning the x conductors one at a time, the point of contact can be interpolated between the conductors with the slowest rise and fall times to get a precise y position, and the y conductors are similarly scanned to get the x position. Because capacitance is proportional to the area of intersection between two plates (one being the conductor on the back of the glass, the other being the user's finger), pointed objects, even if good conductors, do not make good surrogates for fingers. Try the same experiment with a rod of larger diameter (best if it's close to the size of a fingertip, but some touchscreens respond to smaller contact area than others), or some conductive material that deforms as it is pressed against the glass to cover more area, and it should work. The conductivity of the material is much less important than the area it covers.

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The basics of a capacitive touch screen are well covered on the web, you could also look at Wikipedia. But essentially, touch screens on phones and tablets measure the actual difference in capacitance, rather than just detecting any change in capacitance.

The dielectric constants of the two materials have to be larger than a certain amount for it to register as a 'touch'. And since they're mostly calibrated for skin, it'll expect a capacitance similar to that induced by skin within some tolerance. I suspect this is why sausages work, since it's a (somewhat) similar material that's coating them.

This helps reduce the wrong detection of touches from moisture sitting on the screen or any other dielectric causing a change in capacitance, and helps with your silky smooth iPad user experience. ;)

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  • $\begingroup$ It sounds like you are saying the following. 1. Touching the screen changes the capacitance at a point on the screen. 2. The magnitude of the change in capacitance depends on the dieletric constants of the screen and the touching material, which are the two terminals of the capacitor. 3. Human skin has a characteristic dieletric constant, and thus produces a characteristic change in capacitance. 4. Modern touch screens are tuned to respond only to that change in capacitance, in order to detect only skin. Correct? Sounds great! Any sources besides Wikipedia? $\endgroup$
    – algal
    Commented Oct 18, 2015 at 17:54

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