Let’s look at a simple electronic “LED recipe” without a lot of theory. Later we’ll show the “why,” working from scratch to expand one’s repertoire.
Start with one of these 3-cell battery holders with switch: AAA will usually suffice…but if you have a lot of LEDs or want extended run time, AA provides about three times the capacity. The switch is handy, one less part to add ourselves.
Our recipe requires 3 cells. Do not start out with a 4-cell (or larger) pack!
We’ll use common alkaline (single-use) batteries. There are “better” batteries, but for cosplay stuff I really like alkalines…traveling to conventions afar, you can get spares anywhere, just visit the nearest drugstore or the hotel’s traveler shop.
This battery pack is much bigger than the coin cell in the LED throwie, so never connect the LED directly. That’s like drinking from a firehose…the LED will quickly burn out, even pop! Adding a resistor pinches off some of the electrical current to keep things in check.
The resistor must be matched to the LED type and battery. Resistance, measured in Ohms (Ω), is marked with colored stripes. (Way back when, there was no way to print tiny numbers on them, and this system stuck around.)
Adafruit doesn’t sell these resistors. They can be found at Radio Shack (if they still exist when you’re reading this), or places like Fry’s or Micro Center if you have one locally. Otherwise, search online.
If you can’t locate these exact values, that’s okay! Use the next available size up. The LED will be slightly dimmer, but still totally usable. 1/4 Watt reistors are ideal, but 1/2 Watt is fine too.
The goal then is to create a circuit - a loop linking the battery, led and resistor in series (one following the other around the loop):
Recall from the throwie discussion that LEDs have polarity; the “+” (longer) leg goes to + on the battery pack.
Resistors don’t have polarity. It’s shown on the + wire, but they’re not picky and can go on either side of the LED, + or –. It can be inserted anywhere along that wire, but usually easiest to install close to the LED (or even joined directly).
The wires can be as long as needed…even several feet, say if battery and lights are at opposite ends of a staff. There’s a convention of using red wire for + and black for –, but this is totally for human convenience and doesn’t affect the circuit (a very common misconception, so don’t feel silly for asking). If you only have one color of wire (or want to use all black because it hides well), that’s fine, just keep track of your connections.
For multiple LEDs (such as glowing eyes), connect + and – to each one, with a separate resistor for each LED. You can mix and match LED colors in the same circuit, just make sure each has the correct value of resistor (e.g. red and blue will be different).
That's the basic idea. Now…how to build it?