Quantum Research Group
makes these chips that are designed to detect proximity. They're used
in control panels to replace mechanical buttons, fluid level sensing
applications, and anywhere else you might want to sense proximity. They're
basically capacitance sensors, and the company has an easy
to understand document
about how they work.
On the right is a qt310 which has a single channel. [Quantum makes
devices that support up to 64 channels.] You can see from the picture that
the only external components that were required for this thing to work
were two resistors, and two capacitors. I was using that green coiled
jumper as my sense electrode, and you can see the probes of the meter
connected to ground and the output pin of the chip.
I powered everything up, and it worked on the first try. The meter
reads ~5V normally, and when I bring my finger close to the coil it drops
down to 0V.
I bought a handful of these chips. The six channel version I'm planning
on using to build a control panel for the office. The first application of
the single channel version is going to be detecting the presence of
someone sitting on the couch. Anything conductive can be used as a sense
electrode, and since the couch has a pull-out bed I'm hoping to connect
the chip to the steel frame and turn on the touch screen on the end table
when someone sits down.
Programming the qt310
The chip has many user-programmable features that relate to how the
chip senses, the behavior of the output pin, how and when the chip
recalibrates itself, whether or not it outputs a heartbeat signal...
One of my problems is the chip's Max On Duration feature. If the chip
senses for longer that this duration it recalibrates itself. This is a
problem for me because I'm assuming that people are going to want to sit
on the couch for longer than this. Fortunately this feature can be turned
off with PC software, but it requires an interface board. [For the
adventurous, Quantum provides information on how to program the chip
without this board.]
I'll update this when I get the board and actually get the couch sensor
Update: Well, I got the board. It's very well made [it had
better be for $50], and works great. But I can't seem to be able to get
the chip to work with the couch frame as the sensor. I'm not going to put
much more effort into it as I'll be replacing the couch soon and I'll need
to come up with a different sensor for that. Probably something like a
window screen under the cushions.