[sdiy] LED current source needed for 'digital' outputs?

[MTG]

Well Vref wouldn't be my first choice. Do you have an positive voltage 
available that's not a reference?

GB

On 3/1/2014 5:20 AM, Justin Owen wrote:
>>> For an LED displaying a simple on/off digital signal - e.g. a logic output - is there any reason to not just use: Source > LED > R > GND?
>
> From: Gordon JC Pearce [gordonjcp at gjcp.net]
>
>> Most digital outputs pull down rather better than they pull up, so you'd be better to wire it between +5 and the pin.  Put a 150 ohm resistor in series (or really anything you have within about 100-470 ohm, it's not at all critical) and you'll be fine.
>
> From: Tom Wiltshire [tom at electricdruid.net]
>
>> That's why you sometimes see LEDs hanging off the +5V rail and giving an inverted output.
>
> Inverting and hanging them off a Vref is great idea - thanks guys.
>
> - J
>
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