Touchscreens: resistive and capacitive

Try swiping your phone with a woolly glove. Nothing.

Now try a supermarket's old self-checkout with the same glove. It works.

Two different technologies, two different physical tricks. Once you know them, the glove mystery solves itself.

• Resistive: detects pressure. Pressing pushes two layers together.
• Capacitive: detects a conductor (your finger) changing an electric field. No pressure needed.

Both are input devices laid over a display, so the whole touchscreen is input and output.

"Describe how a capacitive (or resistive) touchscreen detects the position of the user's touch." Usually 3 to 4 marks.

Often with a follow-up: "Give a reason why this type is suitable for [situation]."

1. A glass screen covers a transparent conductive layer (a grid of electrodes).
2. A small voltage is applied, creating a uniform electrostatic field across the screen.
3. A bare finger is a conductor, so touching the screen draws a tiny amount of charge and changes the capacitance at that point.
4. Sensors in the controller measure where the capacitance has changed and calculate the x and y coordinates of the touch.
5. The coordinates are sent to the processor, which works out what was touched.

Step 4 is the forgotten one. Saying "the screen detects the finger" doesn't explain how a position comes out.

1. A flexible top layer and a rigid bottom layer (often glass), each with a conductive (resistive) coating, are held apart by an air gap and tiny spacer dots.
2. Pressing the screen pushes the top layer down until it touches the bottom layer, completing a circuit at that point.
3. The voltage measured across the layers depends on where they touch, so the controller calculates the x and y position.