Game Development Reference
In-Depth Information
constant. As the controller measures the voltage, V , at pin 1, we can now solve for
resistance:
R = V/I
With our constant current, I , and our known voltage, V X , we can calculate the resistance
of the circuit by inputting (5 - V X ) for V as follows:
R = (5 - V X ) / I (1)
Note that we have to have the change in voltage across our resistor (the wire), so be sure
to use the difference between the two values. Every conductor has an inherent internal
resistance, and through testing we can determine what the resistance is, measured in
ohms per unit length, and use that to determine our total resistance described by:
R = 2rL
where r is the aforementioned ohms per unit length and R is the total resistance of our
circuit. Note that we have multiplied L by 2 to account for both the wire run to the point
of contact and back. If we substitute this for R in our earlier equation, we now have:
2rL = (5 - V X ) / I
And finally:
L = (5 - V X ) / (2rI)
where L is the only unknown. To illustrate, let us assume the measured voltage is 4.95V
and the wires are 24-gauge copper wires. A quick look in a standard electrical engi‐
neering book gives the resistance as 0.08422 ohms per meter. When we designed our
constant current source, let's say we picked 50 milliamps:
L = (5 - 4.95V) / ((2)(.08422 ohms/meter)(.05A))
L = 5.9 meters from the controller
As you can see, the material's resistance per meter, the constant current supplied, and
the sensitivity of the voltage-sensing circuit must all be finely tuned to ensure that the
controller is capable of sensing touch events in the appropriate dimensions. In a resistive
touch screen, the wires are microscopic so that the resistance per meter is much higher.
This allows the screen to detect smaller distances.
Four-wire resistive touch screen
With some modification, we can expand our previous model to two dimensions. In the
four-wire touch screen, there are four basic layers and four wires, three of which will be