On Sun, 21 Jan 2018 19:10:48 +0000, RobH  declaimed the
following:

>On 21/01/18 18:26, alister wrote:
>>
>> The GPIO ports on a pi are 3.3 V
>> if you have driven them at 5V without protection you may have damaged the
>> port & possibly other parts of the CPU (current limiting with a resistor
>> may not be sufficient protection).
>>
>> this could easily result in erratic or unpredictable behaviour.
>>
>
>I used a web based calculator to calculate the resistor value and I'm
>using a 220 ohm one

 Active High or Active Low wiring?

 IOWs, what is the voltage source for the LED?

 GPIO -> limit resistor -> LED -> ground (active High -- LED is on when
GPIO is high/3.3V)

 V+ -> limit resistor -> LED -> GPIO (active Low -- LED is on when GPIO
is low/ground/0V)

 In the former, the processor GPIO provides the voltage, so you should
be seeing the 3.3V maximum, and need to limit the current to that available
on a GPIO (If the calculator was based on 5V, you should be safe, as you
are running the LED on less than maximum current given a 3.3V source). In
the latter, the voltage is provided by some power supply bus -- and if that
is 5V, and the GPIO is High (conceptually off), you would still have 2.8V
passing through the resistor/LED being sinked by the GPIO. {Note: my use of
source/sink may be inconsistent -- I tend to think in terms of higher
voltage flowing to lower voltage... But electron flow itself is in the
opposite direction).


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

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