Projected Capacitance Touchscreen, confused by how the capacitance is formed.

In summary, the conversation discusses the function of projected capacitance touchscreens and the physical properties of the finger that allow it to function. It is explained that the finger is able to register input on the screen because it is earthed and can hold a larger amount of charge without changing potential. The role of capacitance between the finger and the screen, as well as the capacitance between the body and the device, is also mentioned.
  • #1
eventnick
2
0
Hello,

I have a question that's really bothering me. So I've been reading about projected capacitance touchscreens (the kind that's in so many cell phones, the iphone etc) and I don't understand what the physical property of my finger is that allows the screen to function. From what I've been reading, which is really hazy incomplete information, I am kind of stumped. I've read that any conductor should be able to work but when I place a penny (copper-plated) on the screen and move it with eraser on the back of a pencil, I get nothing. If I am touching the penny with my finger though it works.

The description on wiki does not seem ...complete or seemingly correct in some instances including this specific sentence
"Bringing a finger or conductive stylus close to the surface of the sensor changes the local electrostatic field. " ...since I can't seem to make that work.

http://en.wikipedia.org/wiki/Touchscreen#Projected_capacitance

I have several questions but I would like to see the response to this before I ask them. So yeah if anyone can explain in a technical manner what property my finger has over the penny that allows the screen to register the input I would appreciate it.

I speculate that my finger is holding a charge and that's the difference but I rubbed the penny on the cat haha trying to put some charge on it to get it to work that way and I got nothing.

Thank you,
E.
 
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  • #2
Your finger is earthed.
The penny simply rises to the electric field of the screen - you + the Earth resists changing potential, so a potential difference exists which the screen can sense
 
  • #3
mgb_phys said:
Your finger is earthed.
The penny simply rises to the electric field of the screen - you + the Earth resists changing potential, so a potential difference exists which the screen can sense


Interesting...

But my finger is not technically earthed, of course so is it then that my body is just a much larger conductor than the penny relative to the charge on the screen so I can hold a much larger amount of charge without changing my potential?
 
  • #4
Your finger is earthed, unless you are wearing very good shoes.
Even if it wasn't, yes you are still a large enough source not to change potential much
 
  • #5
Your finger doesn't need to be earthed. But your body has an appreciable capacitance to Earth (i.e. the case of the device you are holding or is on your lap etc. . This is in series with the small capacitance between your finger and the particular area of the screen and this constitutes a loop with a lower impedance than any other path involving other parts of the screen.
 

1. How does a projected capacitance touchscreen work?

A projected capacitance touchscreen works by using a grid of electrodes that generate an electric field above the surface of the screen. When a conductive object, such as a finger, comes into contact with the screen, it changes the capacitance at that point and the location can be detected.

2. What is capacitance and how is it formed in a projected capacitance touchscreen?

Capacitance is the ability of a material to store an electric charge. In a projected capacitance touchscreen, the capacitance is formed by the electric field created by the grid of electrodes. When an object touches the screen, it changes the capacitance at that point and this change can be measured.

3. Why is the location of touch accurately detected in a projected capacitance touchscreen?

The location of touch is accurately detected in a projected capacitance touchscreen because the grid of electrodes can sense changes in capacitance at different points on the screen. By analyzing these changes, the exact location of touch can be determined.

4. How is the sensitivity of a projected capacitance touchscreen determined?

The sensitivity of a projected capacitance touchscreen is determined by the size and spacing of the electrodes in the grid. The smaller and closer together the electrodes are, the more sensitive the touchscreen will be in detecting changes in capacitance.

5. What are the advantages of using a projected capacitance touchscreen?

There are several advantages of using a projected capacitance touchscreen, including high sensitivity, accuracy, and durability. They also do not require pressure to be applied to the screen, making them more user-friendly. Additionally, they can support multi-touch gestures and can be used with a variety of objects, such as fingers, styluses, and gloves.

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