How to use the LED Resistor Selector Dial
You can hold the selector with one hand from the rectangular piece that extends on the left side of the fixed circle. With your other hand you can hold and rotate the rotating wheel:
The fixed wheel (the smaller one) has the LED voltage. The rotating wheel (the larger one) has the supply voltage. You wanna match now the supply voltage with the LED voltage by rotating the bigger wheel. For example, if the supply voltage is 5 volts and the LED voltage is 2.2 volts (typical for red LED), this is how you should align the wheels:
You are now ready to get the resistor value. From the "Resistor Value" window you can read the value for 3 different current rates: 10, 20 and 30mA:
As you can see, for 20mA current you should select a resistor around 140 Ohms...
Finally, flip to the other side to read the power that needs to be dissipated on the resistor:
The power is between 55 and 60 mWatts..
More LEDs in series???
What if you want to use one resistor for 5 LEDs in series? How can you use this tool to calculate the resistor value? Simply, add all the forward voltages of the LEDs and use this value instead. If for example you want to connect 5 cool white LEDs in series and each LED has forward voltage 3.3 volts, then the total forward voltage will be:
Vf_total = 5 x 3.3 = 16.5 Volts
Now, if the supply voltage is for example 22 volts and you want to allow 30 mA of current ...