[MinnowBoard] 1.6V output on gpio pin

[Anders, David]

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...

Dave




-----Original Message-----
From: elinux-MinnowBoard [mailto:elinux-minnowboard-bounces at lists.elinux.org] On Behalf Of Trevor Woerner
Sent: Friday, February 5, 2016 6:31 PM
To: MinnowBoard Development and Community Discussion <elinux-minnowboard at lists.elinux.org>
Subject: Re: [MinnowBoard] 1.6V output on gpio pin

Hi Dave,

On 02/05/16 18:23, Anders, David wrote:
> It depends on how you have it connected

At first I had it in series with a 560ohm resistor but it wasn't really turning on, so I got rid of the resistor and am just putting the LED between the GPIO pin and ground.

> and what the current draw is.

No idea :-) It's just an LED I had lying around in my parts drawer. Is there a simple way to determine it?

>   Like many dev boards, the Max/Turbot is only designed to provide a very minimal current on the GPIO pins, generally around 7mA. You'll need to use a low current LED along with a current limiting resistor to make sure you don't exceed that value.

How many amps do the +3.3V and +5V pins provide? Putting the LED between either of those and ground works great. I just wish I could get the same results using a GPIO too.

Best regards,
     Trevor
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