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Topic: Capacitive sensing and grounding (Read 5730 times) previous topic - next topic

DrDiettrich

Consider the strength (potential) of the EM field. It's very high near the tiny sensor plate, while it is very weak acrossAC all connected parts (PC...), due to their big surface. That's why your finger has a strong effect just near the plate. But if you power the entire circuit from a battery, and disconnect it from other big circuits, the field potential increases on that smaller surface, so that your finger will also have a strong effect when touching the battery or some other parts of the circuit

If two plates are used, the EM field exists almost only between those plates, with a strong effect near the plates, and almost nothing when approaching or touching anything else.

wvmarle

On the other side powering the TTP223 from 2 (or 3) AA batteries led to "strange behavior".
Not surprised as you lost grounding.
First you were grounded firmly through the USB to the computer to the power socket (even if you don't use a grounded socket, it's still a decent ground). A battery powered device is free from grounding, so even touching its "ground" changes the circuit.
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Smajdalf

Consider the strength (potential) of the EM field. ...
Thank for the explanation (and patience), I think I got some insight now.

Not surprised as you lost grounding.
First you were grounded firmly through the USB to the computer to the power socket (even if you don't use a grounded socket, it's still a decent ground). A battery powered device is free from grounding, so even touching its "ground" changes the circuit.
DrDietrich's explanation is better: disconnecting laptop for mains made no noticeable difference. So large area of ground inside whole laptop is more important than its grounding. But I guess if I were brave (stupid?) enough to connect the battery powered circuit to mains ground the power wires would be much less sensitive and only electrode would react to touch.
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Grumpy_Mike

All capicitive sensors work on the same effect, what differs is the way the capacitance is measured. With a battery operated device you need a ground electrode close to your touch surface. In the case of the iPad I am typing on at the moment this is the metal back shell.

A lot of beginners confuse signal ground with mains ground. In English English we differentiate these two by using the term ground and Earth, with earth being the mains ground.

jadonmiller

Have you tried using ADCTouch? I know capsense is known for grounding issues, but I've never had problems with this library. It's a one-wire, no-resistor setup, and their example auto-calibrates upon reset; a great idea!
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