Capacitive Touch Switch for Automotive Applications 1

Compared to most other industries, people have higher expectations for car design. Last weekend, I went to a computer store with a neighbor to buy a new keyboard. His computer had been in use for two years, and the keyboard no longer worked properly. We got into his car, and he started getting angry because a button on the radio was broken. Please note that this car had already been in service for 12 years, while the computer keyboard had only been used for a mere two years. Yet, he held higher expectations for his car. This is how we all are. For many non-automotive engineers, the design constraints faced by switches and buttons intended for automotive applications are beyond their imagination. The adverse conditions they must endure include:  ● Wide temperature range  ● Wide humidity range  ● Continuous contamination from occupants  Compared to early car models, today's vehicles have far more switches and buttons. Not only are there more of them, but they must also be easily installed into increasingly diverse control surfaces. Additionally, they need to be cost-effective to replace sealed switches. A growingly popular approach is   diode transitioning to capacitive touch switches (CapSense). Since they eliminate mechanical components and can conform to shaped control surfaces, CapSense switches provide the reliability and cost structure required by the automotive industry.  As shown in Figure 1, a capacitive switch essentially functions as a capacitor formed by two adjacent traces; physical laws dictate the presence of capacitance between them. When a conductor (e.g., a finger) is placed near these plates, a parallel capacitance is coupled to the sensor. When a finger is placed over the capacitive sensor, the capacitance increases. Removing the finger causes the capacitance to decrease. By adding circuitry to measure these capacitance changes, the presence of a finger can be determined.  diode