I learned the hard way recently that my attempt at visualizing pin states on prototype boards using LEDs without current limiting resistors was a bad idea: it caused the LED to light up when the output pin was turned HIGH, but the downstream receiver of the pin didn't register the HIGH because the LED stole too many volts
I only figured this out after building and tearing down my prototype and reassembling each of the modular bits, verifying they worked, and then noticing it didn't work after integration until I removed the LEDs. I have since learned I also could have used a buffer driver or LED driver, alkthough that adds more complexity to my simple prototype.
(in case you're wondering, yes, I know you're supposed to use a current limiting resistor, but I've also observed that my 5mm LEDs work just fine when given regulated 5V, they end up dropping 4.6V and consuming 40mA, which is about double the current they are rated for.)
How bright do you need the LEDs to be? Even 20 mA is a huge amount for modern electronics. On my boards I have 100 kohm resistors with green LEDs and they are very visible even at some 20 uA of current
Not really bright at all (sitting next to the device in a room where I control the lighting). My current LEDs are 5mm white, Vf 3V, max 20mA and I just hooked one up to my variable power supply and it looks like I can run it at 3mA and it's still super bright.
At times I have run them much lower, to the point where the light is just barely visible. If I set my power supply to cap out at 2.6V instead of 3, the current reading is 0.000, which I think must be below 1mA, and it's still quite visible.
(i'm not an ee expert so I frequently make thinkos related to voltage and current, but I think I've mostly got LEDs down).
I only figured this out after building and tearing down my prototype and reassembling each of the modular bits, verifying they worked, and then noticing it didn't work after integration until I removed the LEDs. I have since learned I also could have used a buffer driver or LED driver, alkthough that adds more complexity to my simple prototype.
(in case you're wondering, yes, I know you're supposed to use a current limiting resistor, but I've also observed that my 5mm LEDs work just fine when given regulated 5V, they end up dropping 4.6V and consuming 40mA, which is about double the current they are rated for.)