The math formula is Vsource - VLED / ILED = R. A typical 5mm red LED may be 1.8v and 20mA so running them on 5v circuit: 5 - 1.8 / 0.02 = 3.8/0.02 = 190 ohms. (probably 200 as the nearest common value resistor) But that only works if one wanted to have LED at max rated spec or to drive LED at speci…

Computing resistor value for LED

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