On 12/06/18 12:17, Mark J wrote:
> In message 
>           mm0fmf  wrote:
>
> [snip]
>
>>>>>
>>>>>     Uhm... 0v between "motion pin" and 5v pin means the "motion pin"
>>>>> is at
>>>>> 5v also...
>>>>>
>>>>>     BUT! RPi GPIOs are 3.3V inputs...
>>>>>
http://www.mosaic-industries.com/embedded-systems/microcontroller-projects
>>>>>
/raspberry-pi/gpio-pin-electrical-specifications#rpi-gpio-input-voltage-an
>>>>> d-output-current-limitations
>>>>>
>>>>>
>>>>> A 5V signal on that "motion pin" could easily result in burning out
>>>>> the RPi
>>>>> GPIO (if you are lucky, only the one GPIO will be killed)
>>>>> http://www.notenoughtech.com/raspberry-pi/rpi-gpio/
>>>>>
>
> [snip]
>
> The use of even a high impedance voltmeter between 5v and 3.3v GPIO pins
> worries me - maybe this has already been covered, but...
>
> If that meter is registering 0v between 5v and a GPIO pin, then that
> implies /both/ leads are at 5v, so the 3.3v pin has 5v on it, all relative
> to 0v. Can the use of a voltmeter in this way kill an IO? My feeling is
> that a voltmeter should always be used from 0v to 5v or 3.3v, to avoid
> this issue. Maybe I'm wrong...
>

It could be that the 3V3 pin is just open circuit, and is not programmed
as an output.

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