Our grandson Matthew had long outgrown the Buddy L Toy School Bus that we picked up at a garage sale years ago, so it was languishing in the basement buried in the basket of neglected toys. Coming across it recently, my wife suggested to me: "...why don't you make it solar powered or something..." The result now daily makes its rounds on the living room floor - rain or shine.
This toy is about 10" long with a metal body, so is rather heavy for powering by mere light. And of course, a school bus must be able to make its runs on cloudy days as well as sunny. So it was clear that the bus would have to incorporate a so-called "solar engine". This is a circuit that allows the electrical energy produced by solar cells to be stored up until enough has been accumulated to start the motor and move the bus along a fair distance.