I picked up a battery powered "beckoning cat" on a to Shanghai, and it has consumed 3 "AA" batteries over a period of 36 months. Seems like a silly waste of batteries, right?
With spring upon us and some sunny days in Toronto, I decided it was time to put one of our Adafruit Solar Badges to work. These little badges output 40 milliamps @ 5 volts in direct sunlight, which seemed like enough for our cat.
The cat was originally powered by a single AA battery, which provides 1.5 volts. Being a good engineer, I thought it would be best to give it around the same voltage. One way accomplish this is with voltage dividing resistors.
I won't get into the theory here, but I knew that using lower resistance would consume less power, so I went with 10 ohm and 18 ohm resistors. This would provide 1.79 volts.
You can learn more about this calculation at LearningAboutElectronics.com
It turned out that my efforts to match the voltage were in vain, as with the resistors the cat's arm barely moved in direct sunlight!
Resistors dissipate energy, and it turns out that even these low impedance resistors were sapping too much energy.
I thought about it, and realized that the reason the cat had an AA battery probably wasn't due to delicate electronics that could only tolerate 1.5 volts...rather, it was for cost savings and packaging. So I thought, why not just hook the solar cell directly to the cat.
It turns out that this is all the cat needed!
The cat even works in lower levels of sunlight:
Here's a picture showing how I soldered the wires to the AA battery terminals: