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.
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.
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.
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".