Ever heard of capacitive touch sensing? It operates on a fairly simple principle: the human body can store charge. Touch an electrode on your board, which can also store charge, and you effectively change its capacitance, something which can be detected. In recent years, the popularity of this technology has grown tremendously. Atmel, for example, bombards you with their “QTouch” products when you visit their site.
So what’s so great about it? Well, how cool is it to have sensors designed into your own board layout? For starters, it looks amazing! Creativity is your limiting factor here. For example, you could have your robot do something when you touch your logo on the board! There are advantages beyond the aesthetic here though. With enough sensitivity, your pad could work with a nonconductive acrylic or glass panel above it. This saves you the trouble of having to cut out holes for your buttons! Speaking of buttons, these pads don’t wear out nearly as fast as those little tactile switches. As soon as one breaks (they’re usually good for about 50 000 presses), your board can be rendered useless, unless you’re willing to muck about with a hot air gun. Using a through-hole pushbutton? Forget about desoldering it, then! For most people, 50 000 presses is more than enough, but if you’re designing something that’s going to be used often, like a control board for a home-automation system, this could present some problems in the long run.
You can also implement sliders and wheels with capacitive touch sensing. Think of the Freescale FRDM board – that has a slider on it that can control various things, such as an LED’s brightness or a motor’s speed! This opens up a whole new window of possibilities for human-machine interface. Apple got this right in their iPods, with their iconic control wheel.
However, there are some downfalls. For example, the sensitivity of the sensor needs to be calibrated upon startup to account for changes in relative humidity, temperature and other environmental factors. If it’s too sensitive and your hand could be detected before you even touch the pad. If it’s not sensitive enough, touches may not be detected at all! Furthermore, there are quite a few things you need to take into consideration when laying out the board. Ground planes, trace width and length, vias, etc. all need to be of the right size and in the right place for optimal performance. You can get ICs that simplify things greatly for you, but if the design doesn’t really call for a touch pad, you might just be better off sticking with buttons and potentiometers. That said, if you’re up for the challenge of designing such a board, by all means try it out! You might just be amazed.
