Human body model, capacitive sensor

I don't know how you drive it but could it be that some charge bleeds off or onto the plate?

One thing that does have such an effect is accumulated charge on the body. Happens a lot on winter days and when wearing shoes where the underside isn't leather but some type of plastics or rubber. The first touch bleeds that off and when you don't move around too much between that and the following touches the next ones won't register a surge.

Is the average value of the 5V square zero? Or is it 2.5V?

...Jim Thompson

I guess you are getting a charge out of doing that, right?

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The output is actually driven hard with a 2MHz square wave 10% dutycycle for only 5us (ten pulses). Furtheremore the sensor is only activated once every second. So normally the output is 0V hard driven (push-pull output drive) and once every second some fast pulses are present on the sensor plate. Output to the plate is isolated with some Y-capacitors to provide protective impedance for the user.

Therefore it is likely the finger is discharged to ground when it touches the plate (sensor normally at ground), so I would assume the initial charge of the body would be insignificant which is why I have this question of the human body model (maybe some sort of dielectric absorbtion??). Of course charge may accumulate on the Y capacitors.

see simplified schematics at:

Thanks

Klaus

electrostic charge on the human ?

decrease in capacitance by some electrolytic effect, 5v is enough to dissociate water?

if you mean on the second touch maybe touching the plkate dries the finger and thus increases resistance?

stick a scope on it.

Bye. Jasen