the resistance and current examples are calculated as follows:

:Example 1 with of 12 and 9 V, = 1.8 V, = 5 mA, calculate :

:: = (12 V - 1.8 V) / 5 mA = (12 - 1.8) / 0.005 = 2040 ohm, maybe round down to a 2K ohm resistor (common resistor values).

:: = (9 V - 1.8 V) / 5 mA = (9 - 1.8) / 0.005 = 1440 ohm, maybe round up to a 1.5K ohm resistor.

:Example 2 with of 5 and 3.3 V (digital logic voltages), =