A Solar Engine
is a circuit that takes in and stores electrical energy from solar cells, and when a predetermined amount has accumulated, it switches on to drive a motor or other actuator. A solar engine is not really an 'engine' in itself, but that is its name by established usage. It does provide motive force, and does work in a repeating cycle, so the name is not a complete misnomer. Its virtue is that it provides usable mechanical energy when only meager or weak levels of sunlight, or artificial room light, are present. It harvests or gathers, as it were, bunches of low grade energy until there is enough for an energy giving meal for a motor. And when the motor has expended the serving of energy, the solar engine circuit goes back into its gathering mode. It is an ideal way to intermittently power models, toys, or other small gadgets on very low light levels.
It is a great idea which was first thought up and reduced to practice by one Mark Tilden, a scientist at Los Alamos National Laboratory. He came up with an elegantly simple two-transistor solar engine circuit that made tiny solar powered robots possible.
Since then, a number of enthusiasts have thought up solar engine circuits with various features and improvements. The one described herein has proven itself to be very versatile and robust. It is named after the day on which its circuit diagram was finalized and entered into the author's Workshop Notebook, Easter Sunday, 2001. Over the years since, the author has made and tested several dozen in various applications and settings. It works well in low light or high, with large storage capacitors or small. And the circuit uses only common discrete electronic components: diodes, transistors, resistors and a capacitor.
This Instructable describes the basic Easter Engine circuit, how it works, construction suggestions, and shows some applications. A basic familiarity with electronics and soldering up circuits is assumed. If you haven't done anything like this but are eager to have a go, it would be well to first tackle something simpler. You might try the The FLED Solar Engine
in Instructables or the "Solar Powered Symet" described in the book "Junkbots, Bugbots, & Bots on Wheels", which is an excellent introduction to making projects such as this one.