Step 2: Solar Cells
As I mentioned on the parts list, you can use any sort of solar cell for this project, however, there are some things to keep in mind.
While you can find heaps of different types of solar cells on the internet these days, not every cell is equal.
As we're using little motors, we're going to need a certain amount of power. I've found that most of the little vibrating motors you buy online need at least 1.5V and 30mA to trigger. Now, if we were using batteries, we could just find a battery that has those exact specifications, but as we're using solar we have to go above and beyond. When solar panels are rated, they're given a rating at their max power level which usually means noon on a sunny day. As these conditions don't exist all the time, we have to expect that our solar cell will usually be operating at a lower power level than what it's rated for.
The second thing to keep in mind is the size of the solar cell in relation to your motor. The bigger and heavier the cell the stronger the motor you'll need to get it moving.
The key is to find a solar cell that is both small and has a good kick behind it. Salvaging solar cells from old garden lights has worked for me in the past, though often times they lack the current to get the motor started. The ones I prefer to use are some long and skinny 2V 50mA solar cells. They're cheap and easy to find online from quite a few places.