Discrepancy in the Arduino's internal LED resistor value?

[TheAliw1]

Hi everyone,

I noticed that when looking at a couple Arduino Nano schematics there seems to be a discrepancy in the RP2 resistor array value!

Take for example this schematic and this schematic.

In the former RP2A/B/C/D has a value of 330 ohm and the former 1K ohm.

I stumbled across this difference as I was trying to explain for myself why a 330 or 1K ohm resistor was used here.
The resistance value difference seems quite big!

I am guessing this difference has to do with the LEDs used which are unfortunately not specified in both schematics and are probably different.

But lets imagine they used these 0805 SMD LEDs. From the data sheet. My attention was drawn to the following table:

In which some values are nicely provided for operation under 20mA at 25 degrees C.
Typical forward voltage of the LED = 2.0V

R=U/I
the Arduino runs on 5V
the Arduino digital pin has a maximal output of 40mA

5V - 2V = 3V
3V/0.02 = 150 ohm

So resistor with a minimal resistance of 150 ohm should be used to achieve this.
Now let's look at the power rating this resistor should meet.
P = U x I
3V x 0.02A = 0.06W = 1/16,666W
So choosing a 200 ohm resistor with a power rating of 1/10W to be on the save side would be good in this setting?

PS I have no idea if the typical luminous intensity of 38mcd of the led is too bright or dim as an indication LED. Any ideas?

[CrossRoads]

38mcd is pretty dim for 20mA.,
More modern LEDs are much brighter at lower currents.
Brighter means 1K resistor would still make the LED very viewable.

https://www.digikey.com/products/en/optoelectronics/led-indication-discrete/105?FV=ffe00069%2C400006%2C1f140000&quantity=0&ColumnSort=-206&page=1&stock=1&k=led&pageSize=25&pkeyword=led

(forum may add some extra characters to the start & end of that)
You can see there are 0805 size LEDs with up to 2490 mcd at 20mA.
With Vf of 3.3V, then I think as small current would still be really bright:
(5V - 3.3V)/1000 ohm = 1.7mA.

[IamFof]

@TheAliw1

A few things to note.
The schematic showing 330R, is for a Nano 2. It says so on the drawing.
The other is an Arduino schematic for a Nano 3.3.
The resistors are not individual, the are part of a resistor pack, typically 4 resistors in a 1206 footprint. Their component designations, RP2B & RP2D indicate this.
Using 330R, and a Vf of 2v, each LED will draw 9mA. Using 1K will reduce the current to 3mA.
Pretty well all LEDs will light at 5mA, and any difference in intensity, you wouldn't notice. In any case, do you need a high intensity?

Hope this helps.

Fof

edited: got my maths wrong. Whoops

[TheAliw1]

@IamFof

So from this we can conclude that the Nano2 and Nano3.3 must have used different LEDs right? (Or they might have used the same LED but in which case they are more dim on the Nano3.3)

IamFof:
In any case, do you need a high intensity?

No not at all. I am more concerned by the LED's potentially being too bright. I just want to be able to tell if the LED is lit or not when looking at it in normal room lighting without it blinding me It just serves as an indicator LED like on the Arduino. Nothing functional besides that. Any idea of a mcd value I should be aiming for?

Your calculations seem to be matching mine after the edit I just wanted to make sure I am doing these calculations right and choosing the right resistance values and power rating value of the resistor for this application. Which seem to be right if I may say so myself.

Let's move on to RP1A and RP1B. Why are there 1K ohm resistors on these Rx and Tx lines to the FT232RL? I couldn't find them mentioned in the ATMega328P data sheet and in the FT232RL data sheet there is some mention of a 1.5K ohm resistor in relation to pin 15 and 16 in section 3.2 on page 7. However I don't think this is related to RP1A and RP1B on the schematic. My question: Why did someone (smarter and with more knowledge than me) decide to put these 1K ohm resistors there?

[Paul_B]

TheAliw1:
So from this we can conclude that the Nano2 and Nano3.3 must have used different LEDs right? (Or they might have used the same LED but in which case they are more dim on the Nano3.3)

I can't really recall to what extent LEDs developed over the lifetime of the various Nano designs (and do not fancy researching it). The fact is that with current quality LEDs, it is wasteful to use more than a couple of milliamps.

TheAliw1:
No not at all. I am more concerned by the LED's potentially being too bright.

And indeed, they are too bright by and large.

TheAliw1:
I just want to be able to tell if the LED is lit or not when looking at it in normal room lighting without it blinding me It just serves as an indicator LED like on the Arduino. Nothing functional besides that. Any idea of a mcd value I should be aiming for?

How long is a piece of string? I find the "pilot" LED in particular, annoying most of the time. Sure, for simple experiments it may be nice to know the power is getting in but beyond that, it just becomes a nuisance but not worth vandalising the board.

I cannot imagine having anything less than 1k. You of course do not use a Nano (let alone a UNO!) if you are concerned about minimising current consumption but on a Pro Mini you would choose to disable the pilot LED and possibly the pin 13 one if necessary.

TheAliw1:
Let's move on to RP1A and RP1B. Why are there 1K ohm resistors on these Rx and Tx lines to the FT232RL? I couldn't find them mentioned in the ATMega328P data sheet and in the FT232RL data sheet there is some mention of a 1.5K ohm resistor in relation to pin 15 and 16 in section 3.2 on page 7. However I don't think this is related to RP1A and RP1B on the schematic. My question: Why did someone (smarter and with more knowledge than me) decide to put these 1K ohm resistors there?

Pins 15 and 16 of the (ugh!) FT232 are the USB interface. I shan't even bother to look that up.

These 1k resistors are there to isolate the USB interface chip so that an alternative serial data or even general input can be applied to pin 0 via the module connections, overriding the output from the FT232. Since these are all CMOS inputs, the resistor can be a relatively high value - 1k or more - without limiting the response speed.

Of course, since pin 1 is always an input the the FT232 and again, CMOS, it should never be a load so there really is no reason for the 1k resistor there that I can see. Anyone else know?

I think it just "seemed a good idea at the time" to make the circuit look symmetrical when they designed the original "Arduino USB".

[IamFof]

@TheAliw1

Just one other little comment.

The RP is in a 1206 footprint. Your idea of replacing with the linked 200R, would require fitting 4 x 0805 resistors. 0805 are approx. half the size of a 1206, so you would need, including the clearences around them, at least 3 times the board area, for fitting.

Fof