Touch screen is very amazing invention which reduces the space occupied by the keypad in the mobile gadgets. As a result the gadgets are being more compact in size and the display is being bigger. Touch screen has become so popular that most of the new gadgets being launched is using a touch sensible display.
After using a touch sensible display a tech geek would wonder how is this working? What type of circuitry is used in this display? I would tell you how touch screen works in this article.
A touch screen is a two dimensional sensing device which is made of two sheets of material separated by small spacers. Generally a sheet of glass is used as a bottom layer and a sheet of Polyethylene(PET) is used as a top layer. Polyethylene is a flexible material.
These two sheets are covered with a resistive substance which is usually a metal compound named Indium Tin Oxide (ITO). Thin layer of ITO is uniformly spreaded onto each glass and PET layer. Small bumps named spacer dots are added onto the glass over the resistive coating. This spacer dots creates space between two layers and prevents accidental or a false touch.
When someone touches the display, the PET film is pressed down so the two resistive surfaces touch each other which completes the circuit. The position of this touch can be calculated by a touch screen controller circuit.
The controller circuit uses two dimensional system to determine the point of touch. The basic two dimensional system is shown in the figure.
By measuring the value of X and Y you can determine the position of touch.
In a touch screen display there is two different circuit for measurement of X and Y position. The circuit for measuring X position is shown in the below figure.
As you can see from the figure that there is different value of resistance for each point on the X line. By measuring the value of this resistance you can determine the X position.
The circuit for measuring Y position is shown in figure.
The function of this circuit is also same as the circuit for X position. As you can see from the figure that there is different value of resistance for each point on the Y line. By measuring the value of this resistance you can determine the Y position.
Thus the controller circuit can determine the position of touch in the display.