# LED resistor from assumed forward drop and current
supply = 5 V in 3..12
forward = 2 V "Assumed LED forward voltage"
current = 20 mA in 5..40
resistor := (supply - forward) / current [ohm]
resistorPower := (supply - forward) * current [W]
check resistor minimum: resistor >= 100 ohm
scenario higher supply: supply = 9 V