It’s like one of those online “LED resistance calculator” thingies, except that you can fold it up and put it in your wallet.

Begin by downloading the pattern here(512 kB PDF File). Print it in high quality mode on a laser printer, preferably on medium-weight “presentation” paper. For it to end up at the original size (that of a business card, 3.5 x 2″), you may need to disable automatic scaling on your printer.

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

There are also five boxes on the larger piece labeled “[REMOVE]” that should be, well, removed.

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.

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

Now that construction is complete, let’s walk through how to use it. Our model circuit is shown in the circuit diagram on the calculator:

We assume that the system voltage (Vcc) is known. We further 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.

Let’s work through a full example. Suppose that we want to run a red LED from a 9 V battery. Then Vcc = 9 V, and our red LED wants to run at 20 mA, and has forward voltage Vf = 2 V. Given that, let’s figure out what resistor should be chosen.

The first thing that we 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.
(If you’ve already done this calculation in your head, you can skip right ahead to the part where we flip it over.)

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

Next, we flip the calculator over.

Our next step is to find the resistor value. To do this, we’ll move the slider to line up our desired current (I = 20 mA, in this case) beneath our 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 we need to know to pick out the resistor. We also need to know the power that is dissipated by the resistor, to make sure that we pick a resistor big enough to handle the load. That calculation is done in the lower half of this side of the calculator:

First, we 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 our case):

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

If you liked this project, you may also like our South Indian Restaurant Menu Decoder Ring, which you can find here, or possibly to make your own 1952 Fraction-of-an-inch Adding Machine here.