Understanding How Capacitive Touch Screen Works



This video is from a student preparing for?Final project for PHY 12 at Tufts, fall semester 2015.

Video Transcripts

Have you ever wondered how the touchscreen on your phone or laptop works? We're here to explain! These devices use capacitive touch sensors with property of capacitance to detect a conductive object like your finger when it touches the screen. Before we can explain what a capacitive touchscreen display is, you need to have a basic understanding of circuits. Large Capacitive Touch Screen Monitors & Displays ? faytech North America A circuit is a closed loop that charge can travel through at any point in the circuit. The charges have a voltage or potential which corresponds to their electrical potential energy measured from a reference point called ground. A battery creates a potential difference which allows charge to flow around the circuit and through any circuit elements or devices connected in the circuit. A capacitor is a vice that stores electric charge. A common type is the parallel plate capacitor which consists of two conductive plates and an insulator. Between them initially, these are neutrally charged meaning that positive and negative charges on them. When you put a battery onto them, the negative charges accumulate on one side and the positive accumulate on the other. The difference in charge creates a voltage across the capacitor and this voltage can be used to power another circuit element like let's say a light bulb. Capacitance is the amount of charge that something can hold. The capacitance of a parallel plate capacitor is modeled by this equation. C equals epsilon a over D. Epsilon I will get to that later. A is the area of the parallel plate and D is the distance between the two plates so if you make the area bigger you're going to have higher capacitance you make the distance smaller you get higher capacitance. Now epsilon is the dielectric constant which is a characteristic of how difficult it is for charge to move in the insulator. Here we're going to show you how a capacitor can be charged and then used to light an LED. Touch Screen Monitors & Displays for Sale - faytech North America First we're going to check that the LED works by connecting it to a power source through resistor. The resistor slows down the flow of charge. The led doesn't burn out . Now we're going to charge the capacitor by connecting it to the power source. When we touch the capacitor to the LED. The LED lights up notice how the LED fades out. This is because the resistor keeps all the charge from the capacitor from flowing through the LED at once although it doesn't look at the cylinder. The cool capacitor we used in the demo is actually a parallel plate capacitor. It's made of two conductive sheets separated by an insulator and rolled up. You can build a capacitor for materials you can find at home here. We've built one by separating two conductive sheets of aluminum foil with a sheet of plastic. So here we have our capacitor even though it's this big, it actually only has a capacitance of a couple nano farad. That is about a thousand times smaller than what we used to light the LED so we can't use this one to light an LED. We can't show you that it will hold charge. This machine is called an oscilloscope and the line here shows us the voltage on the capacitor over time. So what I'm going to do is I'm going to hook up the capacitor to a voltage source and I'm going to disconnect it. Something interesting is going to happen, you see how this line curves down. That shows us that the charge when we disconnected the battery didn't go away right away. There was something holding on to the charge and that something could only have been our capacitor. Now we can look at how capacitive touch screens work. Let's consider just a single point on your phone screen so when your finger comes in contact with the glass of your phone screen then your finger forms a capacitor with an electrode that is underneath the screen. For now imagine that your finger is actually connected to ground it might be some other voltage but the results are the same so your phone uses a two step process to figure out this capacitance. That will tell how far your finger is from the screen because capacitance changes with distance so the first thing it does is it actually puts a voltage on this electrode and that puts some charge on. A fundamental equation of capacitors is that the voltage across a capacitor is equal to the charge on that capacitor over its capacitance so this puts a fixed amount of charge on because this is a steady capacitance. We want to figure out the next thing that happens is the phone disconnecting this voltage and actually connects this electrode to ground through a second capacitor. This capacitors capacitance is known and since we now have two capacitors going. That is to ground the total of capacitance increases since this increases the voltage right here. It must decrease and that decrease will actually be related to the capacitance here so by measuring the voltage here after making this connection the phone can figure out whether it's being touched or not so a two-dimensional touchscreen is made up of a bunch of rows and columns of these electrodes. It figures out where your finger is in two dimensions by just looking at all the electrodes together and seeing which ones are being touched. This specific type of touchscreen is called a self capacitive touchscreen. It can only accurately detect one touch at a time. When you touch it with two fingers, the controller knows the rows and columns that have been touched but it can't tell whether the contact points are here . Here is a newer form of touchscreen is called a mutual capacitive touch screen which can detect two touches because each row and column intersection has a capacitor multi-touch touchscreens that allow you to do things like zoom in and drag.

For More Information on Capacitive Touch Screen , check out this touch screen manufacturer, Faytech North America

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