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When I do the led calculation I found 150 ohm resistor is enough for 3mm red led. If so why all arduino tutorials, like this one, are suggesting 330 ohm resistor?

I assume V = 5 Volt, diode forward voltage = 2V and diode forward current 20mA

  • I guess most people are using 1% resistors these days, but if someone does happen to use a 10% resistor rated at 150 Ω the actual value could be as low as 135 Ω. The forward voltage can vary, too; a red LED I was using the other day had close to a 1.7 V forward voltage. Under those conditions, you'd be putting almost 25 mA through the LED, probably exceeding the recommended maximum forward current by a good margin. – cjs Apr 21 '17 at 23:54
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150 Ohms may be "enough", but it's often still far too bright, so many tutorials keep a lower limit of around 10mA to help the LED live longer without affecting the application.

You'll probably find that red LEDs are still visible at 1mA and a 330 Ohm resistor with a 5V supply will give about 8.5mA (assuming a 2.2V drop), which is plenty.

It's really a "lazy" choice that will work well in "most" situations.

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  • True. Almost anything up to 1K will work. I always use 470 Ohm resistors just because I have a lot of them. – Axeman May 6 '15 at 10:38
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An important consideration is that the chip cannot necessarily source 20 mA on all output pins at once. In fact pins D5 to D13 on the Uno (PD5 to PD7, and PB0 to PB5) cannot source more than 150 mA combined. Thus the maximum you could source on those pins, if you are driving them all at once, is 150 / 9 = 16 mA.

So, designing for 10 mA per LED is allowing a safety margin, possibly to save tutorials the complexity of explaining that 20 mA is OK for one pin, but not for all the pins at once.

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