On 21/01/18 19:24, Dennis Lee Bieber wrote:
> 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).
>
>

I don't know what the difference between active low and active high wiring.

The voltage source for the led is from the Raspberry pi GPIO 17
I have been following this youtube video:

https://www.youtube.com/watch?v=OEilz2Cq_xY.

I've just done some checking of some of the GPIO pins with a voltmeter,
and got this:

GPIO 17 0.06 vdc
GPIO 27 0.01 vdc
GPIO 22 0.07 vdc
GPIO 23 0.05 vdc
GPIO 24 0.02 vdc
GPIO 08 3.25 vdc
GPIO 07 3.25 vdc
GPIO 25 0.01 vdc

Now if GPIO is only measuring 0.06 Volts, that is not enough to power up
the led. Should it be connected to a 3.25V pin instead.

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