On Wed, 13 Jun 2018 19:41:02 +0100, druck  declaimed the
following:

>If you measure between 2 pins and get 0V it /could/ mean they are at the
>same potential, or it could mean there is *NO* connection between them.
>That's why you always measure from the common ground to the pin.
>

 We presume the board is not broken -- thereby all pins do have a shared
common ground level, and a voltmeter connected between any two pins should
report the potential (voltage) difference between those two pins, doesn't
matter what the voltage relative to ground is. Basically, which ever pin
has the voltmeter /black/ (using standard convention for voltmeters)
/defines/ "ground level", and the /red/ lead will report the potential
difference from "ground level".

>Ground to 5V should give 5V
>Ground to GPIO set high should give 3.3V

 Note: unless I misunderstood the situation gravely, the GPIO is being
used as an INPUT and it is the motion sensor/detector that is setting the
pin value. That sensor is powered by the 5V system, which is why we have
the concern that if it is putting 5V on the GPIO, it could be causing
damage to the pin circuits.

>5V to GPIO should give 0V because they aren't connected.

 But they are connected -- by the RPi common ground. If groundGPIO can
be measured and ground5V can be measured, and both are not 0V, then the
differential between 5VGPIO can also be measured directly -- one pin
becomes the effective "ground" and the other will present a voltage
relative to the other.

 Yes, if groundGPIO shows 0V it could mean the pin is really at ground
level (and the differential between it and 5V is still valid), or the pin
is physically isolated from the rest of the circuit in which case it will
never register any non-0V and you might as well toss that pin into the
trash.
.
>
>---druck


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 Wulfraed                 Dennis Lee Bieber         AF6VN
 wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/

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