2024-04-29
From a LED's datasheet, you will find two attributes:
Forward voltage Vf. Typically 2V-3V.
Forward current If. Typically 10mA - 30mA.
You want to avoid any current above the value of If to make sure your LED
does not burn. For that, you will need a resistor connected in series.
In terms of minimum current to pass through the LED for indoor use, high-efficiency LEDs can light up with as little as 1 mA.
So the equation to find a resistor is basically:
R = (Vin - Vf) / (If)
For common values of Vin = 5V, Vf = 2V, and If = 20mA:
R = (5 - 2) / (0.02) R = 150 Ohms
With lower voltages you will need less resistance, and with higher voltages you will need higher resistances.