Capacitive Touch Technology

We are all familiar with the Touch Screens. The simple easy to use input interface for users to input their choices as per the options presented to them using the underlying programming. But how do these touch inputs work? In this article, we will look at Capacitive touch technology, the type of touch input which is mainly used in today’s touch display devices, mainly smartphones and tablet PCs.

Touch input technology was first introduced to bundle the output display with the input touch screen for a more easier graphical user experience. Along with Capacitive Touch technology, there are various other technologies which use various other principles of electronics to acquire user input from the touch screens. In Capacitive Touch Screens, as the name suggests, the principle of capacitance is used to determine the presence of human touch.

A simple Capacitor when supplied with a specified voltage, takes time to charge to its full capacity and subsequently takes time to discharge when the voltage source is removed and the capacitor is connected to a sink. This time duration for charging and discharging is noted, and remains fairly constant for no changes in the circuit. This charge-discharge time changes when the capacitance of the circuit changes. This is the main working principle behind the capacitive touch screens.

When the human finger touches the capacitor in the circuit, it acts as another capacitor added to the circuit. The human body acts as a Dielectric. This increases the overall capacitance of the circuit and interferes with the charging and discharging times of the circuit. Thus, the difference in the charge-discharge time across the circuit will signify the presence of the touch by user.

Figure – Capacitive Touch Sensing Circuit

The hardware involved in this is most of the time a dedicated microcontroller which charges the capacitive screen and looks for differences in the charge-discharge times of the circuit, and when this value deviates from the normal, it sends a signal to the main controller about the input from the user. The clear transparent touch screen display are a result of insulator layer of glass and a conductive layer of Indium Tin Oxide (ITO). This forms a capacitor and when a human finger touches this, the human flesh acts as a dielectric changing the overall capacitance of the circuit.

Capacitive Sensing like this can be utilized in measuring object properties without actually interacting with the object as a close presence of any dielectric act as capacitance for the circuit. Thus, this principle is utilized when the object in study cannot be made contact with.

Thus, the capacitive touch screen is basically a charging-discharging capacitor circuit looking for change in the charge-discharge times.