As an electronics teacher, I spend a lot of my time teaching repetitive products from kits - mostly, that I've had little input with and am really just making sure they understand the components. I decided it was about time I challenged myself, so designed my own project, found the components, and did the maths.... Luckily, being in school, I also have a whole load of unwanted projects, which I rip apart to reuse - all the components in this project are recycled.
For a chain of 3 StarLights, you will need:
1 2xAA battery case
Battery Clip (PP3 clip)
15 3mm LEDs - 5 yellow, 5 red, 5 green
3 10ohm resistors
2.5m red wire
2.5m black wire
Step 1: Do the Math, Test the Circuit...
I started out by testing my circuit using breadboard - you don't need to do this step. I used R = V/I to work out the resistor values for LEDs in parallel, like this:
V = 3 (supplied by battery) - 2.2 (forward voltage of LED) = 0.8
I = Combined current required by the LEDs = 0.02 x 5 = 0.1
0.8 / 0.1 = 8ohms of resistance
I tested the maths on the breadboard, and thankfully, it worked!
Step 2: Create the Stars
To solder the stars together, I used bluetack as a base to keep things in place. I pushed the negative legs of each of the 5 LEDs into the bluetack, and then carefully pulled, bent, and pushed the positive legs together into a point, and soldered. It wasn't the neatest piece of soldering I've ever done for the first one, but it got easier!
Give the star a second or two to cool down, then flip it over, using the bluetack again to hold it in place, and repeated the process for th negative legs, pushing them together and soldering into place.
The process was repeated with the other coloured stars.
Step 3: Attach Leads and Resistors
To connect the stars to a battery pack, I then soldered a red wire to the positive side of the star, and a black wire to the negative side of the star. So that I could hang them at different lengths, I cut the wires as follows:
Star 1: 100cm black, 98cm red
Star 2: 80cm black, 78cm red
Star 3: 60cm black, 58cm red
The red wires are shorter to accomodate the resistor, which I have soldered at the opposite end of the red wire to the star.
Step 4: Connect Together and Test!
Grab the 3 resistors (one on each red wire) together, and hold them steady in the blue tack. Wrap the red wire of the battery snap around the 3 ends, and solder in place - all 3 wires should now be securely attached to the battery connector.
Then take the 3 black wires, twist the ends together, and wrap with the black wire around the twisted ends. Solder this too. Everything should know be attached to the battery snap.
Plug in the battery case with 2AA batteries, and test!
Step 5: The Pretty Bits
In an attempt to diffuse the light from the LEDs, and to make them more visually appealing, I made covers for each star out of tracing paper. Having access to a Laser Cutter at school, I decided that was the easiest way to produce my stars, however, a pair of scissors/craft knife would have been just as effective (and probably quicker, but I'm lazy!)
I produced a simple cutting file on 2D Design (TechSoft), and using a load of leftover tracing paper from old GCSE projects, I cut them out on the laser.
I placed the tracing paper star on the desk, put the LED star in the middle, and glued a second paper star on the top of it, making sure the glue was only around the edges. I then repeated the process for the other two LED stars.