Understanding LEDs on arduino board

I've been looking into leds that are already on the uno. I've even found the schematic and datasheet for the LEDS.

  • I've found out that the leds are 0805 series with around 2V forward drop.
  • I've found out they are coonected with 5V trough 1k resitor and to TX line for example.
  • In the datasheet for LED i've also seen the current they recommend is 20mA

So now this data together it does not go together. And here is my question. What am I missing?
Because If i try to design this myself I see 5V-2V=3V, so the resistor should be such that would allow 20mA with those 3V, so R=3V/20mA=150Ohm.

So I am pretty sure I calculated 150ohms correctly, and i am also pretty sure i checked all the data correct, and yet we are having 1000 ohms on arduino.

Can anyone tell me why? Did I miss something?
Thank you for all your help.

20 mA is for the maximum brightness. But for an indicator LED that’s the last thing you want. A 1000 ohm current limiting resistor results in an LED that is plenty bright enough for the application.

So how can I know then what is this minimal brighteness for indicator led? If I want to design my own similar to arduino board, how do i choose the current i want? Appearently 3mA is already enough, but how low can I go? I see there is a chart of luminance/current in datasheet, but still, I dont have a feeling how strong that luminance is. Any recommendations about this?

I would just wire up an LED and experimentally determine it. A potentiometer wired as a variable resistor or a resistor decade board makes it easy to play around with different resistor values.

One fixed 1K resistor in series with one 10K potentiometer so you don't accidentally turn the pot in the wrong direction and see a bright flash of burning LED before it all goes dark :slight_smile:

You should experiment under the anticipated ambient light condition. Is your device going to be indoors, under sufficient lighting, or in a dark corner etc.? If you've tinkered with LCDs, most people run their back light so bright that it's pointless. If you have 400 lumen of ambient light, you should actually turn the back light off. If you're reading the LCD in dim light, a very low current just a couple of mA will be enough for reading under dim light to no light.

Also, you could pick the color of your board to contrast with your LEDs to make them more visible. If you still worry about wasting too much power, blink them.