Capacitive sensor wiring

Question

I attached some wires to a microcontroller for capacitive proximity sensing of fingers or hands. The sensitive areas are of several shapes.

For a compact shape, I just put some wire in spiral fashion to get a larger sensing area.

But there is a long linear shape (about 400mm) too, so I just spread out the wire this way. Now it has a large idle capacitance, is not very sensitive, captures a lot of noise and is not more sensitive than the attaching wire, so the detection area for a certain threshold expands to the wire between the sensor wire and the controller.

Is there a simple way to get the linear area more sensitive and defined?

Answer 1

For good sensitivity I'd recommend a specialised IC sensor.

The Atmel QT1010 is popular and very sensitive.

Adafruit do a breakout board with a 5 channel version, the QT1070 that looks ideal for you. It provides a corresponding output each time one of the 5 channels is pressed; but does not allow multi-touch (several at the same time).

Atmel's datasheet provides good info on how to make custom sensors that warns against the perceived wisdom of adding a ground plane, for example:

"Ground planes around and under the electrode and its SNSK trace will cause high Cx loading and destroy gain. The possible signal-to-noise ratio benefits of ground area are more than negated by the decreased gain from the circuit, and so ground areas around electrodes are discouraged. Metal areas near the electrode will reduce the field strength and increase Cx loading and should be avoided, if possible . Keep ground away from the electrodes and traces. 2.4.3 Decreasing Sensitivity In some cases the QT1010 may be too sensitive. In this case gain can be easily lowered further by decreasing Cs. 2.4.4 Proximity Sensing By increasing the sensitivity, the QT1010 can be used as a very effective proximity sensor, allowing the presence of a nearby object (typically a hand) to be detected. In this scenario, as the object being sensed is typically a hand, very large electrode sizes can be used, which is extremely effective in increasing the sensitivity of the detector. In this case, the value of Cs will also need to be increased to ensure improved sensitivity, as mentioned in Section 2.4.2 . Note that, although this affects the responsiveness of the sensor, it is less of an issue in proximity sensing applications; in such applications it is necessary to detect simply the presence of a la rge object, rather than a small, precise touch."

Answer 2

I would start my answer in reminding that the capacitance of a parallel-plate capacitor constructed of two parallel plates both of area A separated by a distance d is approximately equal to the following:

• When you put some wire in a spiral fashion you are somehow increasing the area A of one plate of the capacitance, which increase the capacitance.
• When someone approaches his finger, the distance d is lowered, which increase the capacitance.

I could not validate my solution because I have not the required hardware, but I think that replacing the long wire by an aluminum ribbon, or something similar, would increase the area A, thus the capacitance.