How Do Touchscreens Work? Capacitive Touch Explained
A phone touchscreen does not feel pressure or heat. It senses a tiny change in how much electric charge builds up when a finger comes near. Just under the surface of the screen, clear conductive strips called electrodes run in rows and columns. A chip called the touch controller measures that change over and over and works out where the finger is. This method is called projected capacitive touch.
What you'll learn
- How the two main types of touchscreen, resistive and capacitive, differ
- What changes when your finger touches the screen, and how a chip finds the spot
- Why the screen can tell many fingers apart at once (mutual capacitance and self-capacitance)
- Why water and gloves cause trouble, and how the display and styluses fit in
Prerequisites: none. Reading "What Chips Are Inside a Smartphone?" first helps you see where the touch controller fits.
Where you see it: screens you control by touch
You can control a phone or tablet just by sliding a finger across the screen. Touch screens are also found on station ticket machines, bank ATMs, car navigation systems and control panels for factory machines. The part that senses where on the screen you touched is called a touchscreen, or touch panel.
There are several types of touchscreen. This article covers the two main ones. One is the "resistive" type, which reacts to the force of a press. The other is the "capacitive" type, the main type in today's phones. Some types use sound waves or light, but we leave them out here.
Press to respond: resistive touchscreens
A resistive panel is made of two clear sheets or films, stacked with a tiny gap between them. The two facing surfaces each carry a clear coating that conducts electricity. Many tiny dot-like spacers keep the gap open.
When you press the top film, it bends and touches the coating below. The coating is made to conduct electricity a little poorly. Put a voltage across its two ends, and the voltage on the coating changes gradually from one end to the other. Read the voltage at the touched point through the other coating, and you know how far that point is from the edge. Read it across and then up and down, and you have the position.
Here the voltage splits in proportion to length. You can try this by changing the numbers in the voltage divider calculator.
A press brings the coatings into contact, so the panel responds to more than a bare finger. A gloved finger, a plastic pen or a fingernail also works. The structure is simple, and the panel is cheap to make. That is why it has long been used in business terminals and machine control panels.
It has weak points too. Light must pass through several layers, so the screen looks a little dimmer. The surface is a film, so sharp objects scratch it easily. It also has trouble telling two or more fingers apart at once. Still, some products pair it with a dedicated controller to handle up to two fingers.
Clear electrodes on top of the screen
An electrode is a conductive part that a circuit uses to send electricity in or to read changes in it. A touchscreen places electrodes on top of the display. An ordinary metal film blocks light, so it would hide the picture. Instead, touchscreens use a special material that conducts electricity and also lets light through well.
The best-known material is an oxide of indium and tin. It is called ITO, short for indium tin oxide. In the type of panel common in phones, the electrodes have mainly been made of ITO.
There are alternatives to ITO. One is a mesh of metal lines too thin to see. Another is a film with very thin silver wires scattered across it. For example, Mitsubishi Electric offers industrial panels whose sensors use thin metal wiring instead of ITO.
Your finger acts as a conductor
To understand capacitive touch, let's first look at how charge can be stored. Place two conductive plates face to face with a gap between them, and apply a voltage. Positive charge then builds up on one plate and negative charge on the other.
How much charge can the plates store at a given voltage? This amount is called capacitance. The larger the plates, and the closer they are, the larger the capacitance. A part that uses this effect to store charge is a capacitor.
The human body also conducts electricity. To an electrode in the panel, a fingertip is "a conductor right nearby." The finger is also connected to a large body. When a finger comes close, the electric field around the electrode changes, and the capacitance changes slightly. A capacitive touchscreen measures this change.
Your finger touches the glass surface, not the electrodes themselves. The panel senses the change through the glass, and this type is called projected capacitive touch. It is often shortened to PCAP.
How it works 1: checking where rows and columns cross
In a projected capacitive panel, thin electrodes are laid out in rows and columns. The rows and columns cross each other, with a thin non-conducting layer between them. The electrodes are a few mm apart, closer than the width of a fingertip.
The touch controller sends a rapidly changing electrical signal into one row electrode. Part of that signal passes through the capacitance into the column electrodes that cross it. Measuring the capacitance between two electrodes this way is called mutual capacitance sensing.
When a finger comes near a crossing, part of the row electrode's electric field turns aside toward the finger. Beyond the fingertip is a large body, so that part escapes into the body. It never reaches the column electrode. The signal arriving at the column electrode drops a little. In other words, a touch makes the capacitance at that crossing smaller.
The controller switches the signal from one row electrode to the next and measures the value at every crossing. It repeats this many times a second. The crossings whose values drop show where the finger is. This is only the basic idea. Real chips are more refined, for example by using several electrodes at once.
A finger is usually wider than the spacing between electrodes. So under a finger, the values change on several neighboring electrodes. By comparing how much each one changes, the chip can work out where the finger is, even between two electrodes.
How it works 2: why it can tell many fingers apart
The other way to measure looks at each electrode on its own. It checks how much charge that electrode can store relative to everything around it. This is called self-capacitance sensing. When a finger comes near, a conductor joins the electrode's surroundings, so the stored charge goes up.
A 2017 paper from Mitsubishi Electric says self-capacitance has the edge in sensitivity. Mutual capacitance has the edge in telling several fingers apart. Self-capacitance has a weak point: if rows and columns are measured separately, it struggles to tell two fingers apart.
For example, say you touch two spots at once, apart on a diagonal. The chip learns which two rows and which two columns reacted. But those rows and columns make four crossings. The chip cannot tell the two real points from the two untouched ones. These fake points are called "ghosts."
Mutual capacitance measures the crossings one at a time, so this confusion does not arise. However many fingers there are, each crossing whose value drops marks one finger. Touch sensing in today's phones mainly relies on mutual capacitance.
Real products sometimes combine the two methods. In Mitsubishi Electric's 2017 paper, a combined method tells up to 10 fingers apart.
The touch controller: measure, calculate, report
Sending signals into the electrodes and measuring the tiny changes is the job of a dedicated chip, the touch controller. A finger's effect is very small. It is easily buried in random electrical disturbance, called noise. So the chip uses tricks to reduce noise, such as repeating the same measurement many times and adding up the results.
From the measured values, the controller calculates the finger's position. It then reports it to the phone's main computing chip, the SoC. Reading values, calculating and passing on the result is much like the "read, decide, drive" loop in "What Is a Microcontroller?"
Here is an example of how often it measures. Samsung says its Galaxy S23 series, announced in 2023, checks touch 240 times a second in game mode. That is about once every 4 milliseconds. The more often it checks, the more closely it can follow a fast-moving finger.
Some products put the circuit that draws images on the screen and the touch controller on one chip. In 2015, the US company Synaptics announced such a single-chip product for phones and tablets.
Why water, gloves and noise cause trouble
Water also changes the electric field around the electrodes. So on a wet screen, a water drop may be mistaken for a finger, or the finger's signal may be thrown off. A water drop changes the values in a different pattern from a finger. Some designs use that difference to tell drops apart from fingers.
With gloves on, the screen usually stops responding. Glove fabric conducts electricity poorly. It also puts more distance between the finger and the electrodes. Together, these make the change very small. Even so, electrodes that conduct more easily and a tuned chip can catch the weak change through a glove (according to a Nissha document). Mitsubishi Electric's 2017 paper also reports an industrial panel that works through thick heat-resistant gloves or with water drops on it.
Electrical noise is another headache. Noise from a charger or from the display circuits mixes with the small change a finger makes. So some touch controllers have features that suppress noise from chargers and the display (Microchip's products, for example).
How the display and touch sensor are stacked
The touch sensor and the display that shows the picture (an LCD or OLED panel) are separate parts with different jobs. In the past, the usual way was to bond a glass sheet or film carrying the electrodes on top of the display. The extra layers made the screen thicker and heavier.
So, building the sensor into the display itself became common. Forming the electrodes directly on the top surface of the display panel is called on-cell. Samsung Display builds touch sensors into its OLED panels. In 2023, it announced it would extend this technology to large laptop screens.
Going further, putting the sensor inside the display is called in-cell. This design uses layers already inside the LCD for touch sensing as well. No separate touch panel has to be bonded on, so the screen can be thinner and lighter. Japan Display and other companies make such LCD panels.
How do styluses work?
Some simple styluses for phones have a tip made of conductive rubber or fiber (such as Sanwa Supply's products). The tip connects electrically to the hand holding the pen. It then changes the electric field around the electrodes in place of a finger. Such pens use no battery and are called passive styluses.
Active styluses run on a battery, often rechargeable, and send their own electrical signal from the tip. The panel picks up that signal, so even a fine tip is easy to locate. In Wacom's capacitive method, one controller handles both pen and finger touches.
Wacom also has a method in which a separate sensor board behind the screen sends power to a pen with no battery. This method does not use capacitance. It relies on the link between electricity and magnetism, called electromagnetic induction.
A short history of finger-touch screens
In 1965, E. A. Johnson of the UK's Royal Radar Establishment published a paper on a screen that sensed a finger's touch. Thin wires on the screen sensed an approaching finger as a change in capacitance. It is often cited as the first example of a capacitive touch screen.
In 2007, Apple announced the iPhone. Its key feature was a large multi-touch screen, operated with fingers. Multi-touch means you can use several fingers at once. A 2010 article in the trade magazine Information Display shows what followed. Projected capacitive touch was almost unknown in 2006. By 2009, it had shot up to second place among touch technologies.
Common myths
Myth: A touchscreen senses the pressure or heat of your finger
In fact, a phone's capacitive screen measures a change in stored charge. That is why a light touch is enough, while pressing hard through an ordinary glove often does nothing. Resistive screens do use pressing force, which makes the two easy to mix up. Some models did add a separate way to sense how hard you press. One example was "3D Touch" on the iPhone 6s in 2015.
Myth: A touchscreen responds to anything that touches it
In fact, a capacitive screen responds only when a conductor comes near. A plastic pen or a fingernail usually does nothing. Responding to anything is a feature of resistive screens, which use pressing force. The myth often comes from using a pen on older devices or ticket machines.
Myth: The display and the touch sensor are one and the same part
In fact, the display that shows the picture and the sensor that detects touch are separate parts with different jobs. Even in in-cell designs, which share layers, showing the picture and measuring touch are separate functions. Because the jobs are separate, one can fail while the other still works. A screen that still shows the picture but no longer responds to touch is one example.
By the numbers
| Number | What it is |
|---|---|
| 240 times a second | How often the Galaxy S23 series (2023) checks touch in game mode, according to Samsung |
| 10 fingers | How many fingers the industrial panel reported by Mitsubishi Electric in 2017 can tell apart at once |
| 1965 | The year E. A. Johnson published his paper on a finger-touch screen |
| 2006 → 2009 | Projected capacitive touch went from almost unknown to second place among touch technologies (Information Display, 2010) |
The Galaxy S23 screen redraws its picture up to 120 times a second, according to Samsung's specifications. So in game mode, it checks touch twice for each redraw. Checking touch more often than the picture changes lets the phone react faster to finger movement.
Learn more
- See the differences in a table: Capacitive vs piezoresistive MEMS sensors (a sensor that uses the same idea of measuring a change in capacitance), Series vs parallel circuits (the capacitance of connected capacitors)
- Terms for the next step: capacitive sensing (finding motion or position from a change in capacitance), time constant (how quickly a capacitor charges up)
- Try the numbers: the RC time constant calculator (how a capacitor charges through a resistor)
- Related articles: "How Do Image Sensors Work?" (the phone's other sensor, which turns light into electricity), "What Is an Integrated Circuit?"