Vriska Serket's vision eightfold, part 2: Mapping out the circuit

I knew I needed seven LEDs for Vriska's vision eightfold glasses. Now I needed to figure out how best to wire them up.

All posts in this series:
Part 1: Deciding what we want! Planning the project
Part 2: Designing the circuit (what you're reading now)
Part 3: Building a test circuit
General electronics info: LED basics

Every LED needs to have a connection to power (+) and ground (-), and that path typically needs to include a current-limiting resistor to keep from burning out the LEDs. (If you're not already familiar with power, ground, and resistors, check out this article on the basics of wiring LEDs.)

I'd already decided to use rechargeable AA batteries for this project, but how many would I need?

Alkaline batteries provide 1.5 volts out of the package, but the voltage drops off as they're used. Rechargeable batteries start around 1.3 volts, but they stay at that level for longer. (This is nice, because it means your LEDs won't dim out over the course of the day.)

My LEDs didn't come up with a datasheet, but when I look up red LEDs, I know they have a typical voltage drop of 1.8 volts.

LED colorVoltage drop
Red 1.8
Orange 1.8
Yellow 1.8
Green 3.2
Blue 3.2
White 3.2

Since the red LED needs 1.8 volts, and a rechargeable battery provides 1.3, we obviously need more than one battery1. So let's start by considering two rechargeable AAs as our power source, giving us 2.6 volts.

Ohm's law lets us calculate what resistor value we'll need. We can also use an online calculator; I like this one, which can handle multiple LEDs.

We enter the values, and get ...
... this suggestion

As you can see, the picture has all the LEDs in a neat line, which isn't the arrangement we'll want. Here's why that's not a problem.

When I first moved to Washington, DC, I decided to go out for sushi. I looked up the restaurant online. The Metro station nearest to the restaurant was Farragut North, the Metro station nearest to me was Rosslyn, so I took the Orange Line to Metro Center, changed to the Red Line, and rode to Farragut North.

Then I left the station and realized how silly that was.

Farragut North and Farragut West are several stops apart, but physically, they're almost next door to each other. I could have gotten out earlier and not even had to wait for a second train.

Electrical circuits are like the Metro map. They show you what connects where, but you can put your components wherever you want as long as they all attach to each other correctly. Lengthening a wire here or there to arrange our LEDs in a circle won't change the underlying circuit2.

The LEDs in the diagram are connected in parallel — meaning there's each LED is on a separate branch of the circuit, with its own resistor. So we have to find room for seven resistors3, either on the eye piece — where space is limited — or in the section with the battery pack. I'd rather just run one power and one ground wire to the eyepiece, and branch out to the LEDs from there, instead of splitting into separate "branches" farther away and having to run more wires.

With 2.6 volts from the batteries, we have to place the LEDs in parallel. 2.6 volts won't power two 1.8 LEDs. But if we use four 1.3 volt batteries, for a total of 5.2 volts, we get more options.

With a 5.2 volt input voltage we can still put all our LEDs in parallel (if we use larger resistors), but we can also put pairs of LEDs on the same branch. Multiple LEDs on the same branch are in series, and you only need one resistor per branch.

In fact you could put all seven of the LEDs in series, if you gave them enough juice.

But that meants toting around ten rechargeable batteries, so let's not be ridiculous.

I went with this solution:

Since resistors come in standard sizes, and the online calculator gives you the value of the smallest acceptable resistor, it's possible that the LED that's alone on its own branch will be a little brighter or dimmer than the rest. If one of the LEDs around the perimeter looked different from its neighbors, people might notice — so we'll put the central large LED on its own branch, since it's visually distinct from the rest in other ways as well.

Next time we'll talk about turning this diagram into a real working circuit!

1. Actually that's not the only option — for example, you can use an inductor to increase the voltage. But let's keep things simple. (back to article)

2. If you were using conductive thread (which has a higher resistance per inch than copper wire), or if your copper wires were hundreds of feet long, you'd have to allow for them in your diagram. But at the scale we're working at, you can add a few inches to your wires without changing your schematic. (back to article)

3. You might be wondering if you can just use one resistor and branch off all seven LEDs from it. Here's why that's a bad idea. (back to article)

Written on April 22, 2016