How does touchscreen work?
In: Technology
Electrically conductive materials that are near each other make a capacitor. If you use a battery you can charge a capacitor up to full charge very quickly. If you put a resistor in the path of the electricity it will slow down the charge up process enough that you can measure the charge up time to see how much capacity it has.
Touch screens have a grid of thin wires. The left-right wires never touch the up-down wires, but are very close. Picking a left-right wire and an up-down wire lets you pick a capacitor at some location on the screen (since you have two electrical conductors near each other.)
The touch screen scans across every “capacitor” in the grid and measures the capacity.
Your skin isn’t very conductive. However, your blood is very conductive. When you bring your finger close to one of these capacitors, the meat-bag conductor increases the capacity nearby, and that will be picked up by the scan. (You can activate a sensitive touch screen without even physically touching it if you have steady hands.)
Fun facts:
The bits and bobs inside your phone and tablet make the capacity of these little wire-pair capacitors vary across the screen, so the screen needs to learn in the factory what no one touching it looks like, generally after the device is fully assembled.
I’ve been told that Apple holds a patent on square wire grids, so everyone else uses a diamond pattern, and does a little extra math to put it into screen positions.
Latest Answers