Introduction to Capacitive Sensing

AN1101 Introduction to Capacitive Sensing A SHORT HISTORY Author: Tom Perme Microchip Technology Inc. While capacitive sensing has been around for more than 50 years, it is becoming increasingly easier to implement and more popular. A classic example of a INTRODUCTION capacitive switch is the Touch Lamp. The Touch Lamp has been around for a long time, and it is a simple, Capacitive sensing is becoming more prevalent and in capacitive switch that turns a light bulb on, off or dims it. demand for consumer applications. This application note introduces one solution for capacitive sensing using sev- New technology allows much more sophisticated eral Microchip parts. The targeted parts of this application control of touch buttons. A key to this has been micro- note are the PIC16F616 family, the PIC16F690 family controllers with mixed signal peripherals. They provide and the PIC16F887 family, representing low-to-high pin the ability to perform capacitive sensing, decision count, 8-bit microcontrollers. making, responsive actions and other duties pertinent to the system as well. Several techniques for capacitive sensing are currently present in industry. Many are based on measuring a frequency or duty cycle which is changed by the intro- CREATING A GOOD SENSOR duction of additional capacitance from a person's finger A quick background on the natural capacitance of a to ground. Some other methods use charge balancing PCB pad, and the process that occurs when pressing a or rise and fall time measurements. This solution finger near a pad, will help clarify how to make a good measures frequency using a free-running RC oscillator. sensor. FIGURE 1: CAPACITANCE ILLUSTRATIONS Nominal Capacitance Introduction of Finger Equivalent Capacitance C C p p C f Glass Cu FR4 C C p f FIGURE 2: SIMPLE RC CIRCUIT TABLE 1: GLOSSARY OF TERMS Acronym Description ROSC ? Permittivity of Free Space o C s ? Relative Dielectric Constant r d Distance Between Capacitor Plates