Amazon Fairy-Light Mod

At the beginning of this year I was a brand new first-apartment owner. As such, I bought a pair of 30ft battery powered fairy lights. Lo and behold, the batteries ran dead after a week; so they stayed dead for months.

Flash forward, I visited Chau’s house and she had a USB version of the fairy lights that plugged into an outlet. They looked good and worked well too. Thick enchanted forest vibes.

I didn't even check the price on Amazon because WHY WOULD YOU PAY FOR SOMETHING THAT WILL TAKE YOU A MEASLY 5+ HOURS TO DO WITH WHAT YOU HAVE? That's how you gotta think around here. (Actually though, I knew it was an opportunity for me to learn and practice skills that I don’t often use).

So I got the recently torn-to-pieces fairy light strip (a visiting cat thought it was a toy) and cut off the battery carrier/circuit board with 3 fairy lights still attached. I figured, just give it the voltage it needs and turn it on. Turns out, I'm a genius: it worked. The lights lit up instantly - once I gave it the correct voltage. 

I built a little voltage divider with a breadboard. Here is a pic:

I used an online voltage divider calculator to figure out which resistors to use to convert my 12v in to a 4.5v out. Theoretically, I should have gotten 4.5v; I ended up with 2v. I iterated with the calculator and different resistor configurations several times without much luck. Just then, the brilliant idea of understanding the voltage divider equation just above the calculator came to me. Here it is:

R1 is isolated to the denominator of the Voltage Divider equation. Changing R1’s value reliably and predictably changes Vout:

• Increasing R1’s resistance → decreases Vout’s voltage
• Decreasing R1’s resistance → increases Vout’s voltage

Understanding this allowed me to use the almighty Process of Elimination to find a resistor combination that would give me a voltage close to my desired Vout.

I finally found a resistor combination that brought my 12v Vs down to 5V Vout. 5V is within 10% of 4.5V which seemed to be a tolerable discrepancy, so I continued with 5V. After soldering wires to the positive/negative terminals that the batteries touch, I pressed the on button and the lights lit up. 

To turn it off, I just yanked the power.

Thankfully I was using the test module and not the working, untorn, module. Yanking the power fried the circuit board. I learned that quality circuits likely have components that protect against an operator yanking power all of a sudden. This circuit does not have that feature. 

I transferred the voltage divider setup to the functioning fairy light module as Chau soldered the extra strip of lights from the broken module onto the end of the working module. I now have 60ft of fairy lights that I can plug into the wall 🙂