I couldn't find any details on how LED rope lights are constructed, so I took the plunge and bought a 12ft. blue one. After I ripped open the package, admired the glow, I decided to take some measurements to see what I was dealing with. I used a multimeter to measure the DC voltage coming out of the converter and got 110VDC. That's when I knew I was in trouble. That's going to take a lot of batteries.
LED Rope Light
4 3-Battery Holders
12 AAA Batteries
4 10 Ohm Resistors
Rubber Bands made from old bicycle tubes
Hot Glue Gun
Needle Nose Pliers
I took apart the end of the rope and found that under the outer cover there was another plastic section that insulated a postive and negative wire and third wire in the center that strung the LEDs together in series. It emerged from one of the side wires, traveled about 3ft down the rope then connected to the other side wire. After every 4-5 LEDs there was a resistor. The LEDs had about 2.8V across them and the resistors had 4.6V. I cut the wire and ran it thru my multimeter to measure current and got 13.7 mA.
At this point I was stuck and considered giving up. I slowly realized I had to rewire the whole thing to make it work with batteries. I thought about using a 9V or 2 AAA batteries, but decided on using 3 AAA batteries to power the rope. By using a battery pack on opposite sides of the wheel it should balance out.
I decided to place the LEDs in a parallel circuit without a resistor on each LED. It's generally good practice to place a resistor on each LED when used in parallel, but it would be just too much work soldering all those resistors. So instead I used one resistor on the whole thing. I'm not sure if using only one defeats the purpose of having a resistor in the first place, but lets just go with it.
I used an online resistor calculator and entered 4.5V (3 x 1.5V) as the source voltage, 2.8V for the LED voltage, and 178 mA (13 LEDs x 13.7mA) for the total current. It recommended a 10 Ohm resistor.
To double check I use Ohm's Law which I remember as V.I.R. or Voltage = Current x Resistance. (Current is always an 'I', I don't know why.)
| So if I have: ||V = I x R|
|Then:||R = V / I|
|Numbers:||R = (4.5V–2.8V) / 0.178 A|
|and I get :||R = 9.5 Ohms|
I weaved the rope into the wheel and end up up cutting a 33 in. section for my 26 in. wheels. I couldn't pull the innards out so I had to cut the outer section to take it apart.
Once I removed the inner section I began cutting the insulation off every 2.5 in. Having the LEDs close together looks cool, but it's more work. Every 2.5 in. seemed good enough. Removing the insulation was harder than I expected. I used wire stripper, a razor blade, and needle nose pliers.
I used a battery pack I had lying around to test each LED to see which lead was positive. I pushed the led into the plastic part and wrapped the LED wires around the main wires. Then soldered away. To solder the battery pack to the rope, I double checked which wire was positive, soldered on the resistor, then soldered the red battery pack lead. I connected the black battery lead to the the other wire.
I used hot glue to insulate and attach the wire to the battery pack. I also injected hot glue into the rope where the soldering was done to insulate the wire and help keep it all together.
To finish up, I used wire zip ties to hold the battery pack onto the LED rope. I made one tie really tight and left one loose enough to slide over so I can add and remove batteries. To keep the battery pack from flopping around on the wheel I used some rubber bands made from old bicycle tubes to attach it to a spoke.
Once all four sections were complete, I weaved the rope into place, and wrapped the rubber band around a spoke to secure the batteries. It's turns out to be really easy to add and remove the lights.