[MinnowBoard] 1.6V output on gpio pin

[Trevor Woerner]

Hi Dave,

On 02/05/16 20:30, Anders, David wrote:
> Trevor,
>
> So you can do some basic math on the LED. Assuming it is a RED LED (other colors have different values), typically these will have a forward voltage rating of around 1.8 to 2.2V. we are going to assume 2V. also typical 3mm and 5mm LEDs have a max rating of 20mA, so we are going to assume we want to drive it at 10mA which should be sufficient. With these assumptions in place we can do some math using ohms law.
>
> The gpios are at 3.3V, the LED is going to drop 2V, so that leaves us with with an excess of 1.3V. from our assumptions we have decided to operate at 10mA, so using ohm's law we divide 1.3V by 10mA:
>
> 1.3 / .010 = 130
>
> So in order for this circuit to work and drive the LED at 10mA at 2V, we would need to use a 130Ohm resistor in series. Now, 130 Ohm resistor isn't really one of the standard values that you can get at radio shack or other hobby shops, it's more typically a 120 Ohm resistor  (for more info on this visit http://www.resistorguide.com/resistor-values/ ). You should be able to easily find a 120 Ohm resistor in assortment packs and such. We can double check our math using the 120 value:
>
> 1.3/120 = 0.0108
>
> So we are still basically at 10mA.
>
> This should help get things working for you...

Thanks!

I've put a 120 Ohm resistor in series with the LED (I think I have a 3mm 
red LED) and it works... but it's so very faint. If I disconnect the 
lead from GPIO 21 and stick it into the +3.3V pin the LED lights up so 
much nicer. I wish I could get the LED to light up nice and bright using 
the GPIO the same way it does with the +3.3V.

I tested it and see that between pin 21 and ground is 3.3V (the same as 
it is between the +3.3V pin and ground), but (obviously) the amount of 
current is different. Is the amount of current on the GPIOs programmable?

Best regards,
     Trevor