Here is a totally different take on the Joule Thief (JT) circuit commonly found in garden lights. Instead of charging a 1.2v battery directly from the solar cell and converting the power to run a 3-volt LED, we'll be using the JT to convert the output from the solar cell and charging a Lithium battery first. Then when night falls, the battery is used to drive the LED directly.
This method has some advantages: (1) the Lithium cell that was chosen here (and avialable for $2 here
) has an output of 3-volts, which can drive a White LED directly; it also has a huge capacity (800mAH) and very low leakage. (2) The solar cell normally can only charge the NiCd battery in full, direct sunlight, but, with the JT circuit, it is able to deliver power to the Lithium cell even on overcast days.