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.
Step 1: Easter Engine Circuit
Here is how the Easter engine works. Solar cell SC slowly charges up the storage capacitor C1. Transistors Q1 and Q2 form a latching trigger. Q1 is triggered on when the voltage of C1 reaches the level of conductance through the diode string D1-D3. With two diodes and one LED as shown in the diagram, the trigger voltage is about 2.3V, but more diodes can be inserted to raise this level if desired.
When Q1 turns on, the base of Q2 is pulled up through R4 to turn it on also. Once it is on, it maintains base current via R1 through Q1 to keep it on. The two transistors are thus latched on until the supply voltage from C1 falls to around 1.3 or 1.4V.
When both Q1 and Q2 are latched on, the base of the "power" transistor QP is pulled down through R3, turning it on to drive the motor M, or other load device. Resistor R3 also limits the base current though QP, but the value shown is adequate to turn the load on hard enough for most purposes. If a current of more than say 200mA to the load is desired, R3 can be reduced and a heavier duty transistor can be used for QP, such as a 2N2907. The values of the other resistors in the circuit were chosen (and tested) to limit the current used by the latch to a low level.