This one started as a five-minute detour and turned into an evening. I was wiring up a strip of blue LEDs for a small enclosure and reached for the same resistor value I always use for red ones — and immediately cooked the first LED.
Turns out blue (and white) LEDs have a noticeably higher forward voltage than red, yellow, or green — usually somewhere around 3 to 3.4V, compared to roughly 1.8 to 2.2V for red. Run the same current-limiting resistor math you'd use for a red LED and you're under-limiting the current by a wide margin, which is exactly how you end up with a very bright LED for about half a second.
The fix is just doing the resistor math properly for the diode you're actually using: R = (Vsupply − Vf) ÷ If. For a 5V supply, a blue LED at roughly 3.2V forward voltage, and a target current of 20mA, that comes out to about 90Ω — so a standard 100Ω resistor is a safe, close-enough choice.
Small lesson, but the kind that's easy to forget between projects: always check the datasheet's forward voltage for the specific LED color before reusing an old resistor value. I'll be swapping in the real photos and a proper writeup of the enclosure build here soon.