/ /How to put it together

1. Start with print out
   
   
   

   ---

2. Cut out the pattern with scissors or a hobby knife, following the outline of both pieces. A straight edge may come in handy.
   
   

   ---

3. Remove the sections labeled [REMOVE]
   
   
   

   ---

4. There are two crease lines on the large piece (the housing), and one on the smaller piece (the slider). Use a straight edge to assist with starting the folds. Firmly crease the fold on the slider, but leave the housing folds flexible for the moment.
   
   

   ---

5. The housing piece has a fold-over flap at the top, as shown here. Insert the slider into the housing, fold the flap over, and then firmly crease both folds of the housing. The flap can be held down by a single piece of tape (or glue if you're ambitious).
   
   
   

   ---

/ /How to use it

The model circuit is shown in the circuit diagram on the calculator:


Assume that the system voltage (Vcc) is known and assume that you know the characteristics of the LED: what current you want to operate at, and the forward voltage (Vf) of the LED when operated at that current. Once you have those numbers, the calculator can help you figure out what value and size resistor are needed.

Suppose that you want to run a red LED from a 9 V battery.

Vcc = 9 V
red LED wants to run at 20 mA
forward voltage Vf = 2 V

Given that, figure out what resistor should be chosen.

The first thing that you need to calculate is the voltage Vr across the resistor. To do this, line up your forward voltage (Vf = 2 V) directly beneath your system voltage, Vcc = 9 V.



The voltage Vr across the resistor then appears in the lower right hand box for you to read out: 7 V.



Next, flip the calculator over.



The next step is to find the resistor value. To do this, move the slider to line up your desired current (I = 20 mA, in this case) beneath your resistor voltage (Vr = 7 V):



Once that's lined up, the resistor value is indicated by the arrow on the left side of the sliding scale, in this case about 350 ohms.



That's half of what you need to know to pick out the resistor. You also need to know the power that is dissipated by the resistor, to make sure that you pick a resistor big enough to handle the load. That calculation is done in the lower half of this side of the calculator:

First, you need to dial the resistor voltage (Vr = 7 V) into the center of the little box:



Once that's lined up, the power dissipated by the load resistor is indicated directly below the chosen current value (20 mA, in this case):

Directly below 20 mA, the power dissipation is roughly 140 mW, so you should pick a 350 ohm, 1/4 W (or larger) resistor.