Conny likes to fall asleep with some light on. I dislike having to wait until she's gone late at night, just to switch off her bedside light. I dislike having an energy-wasting halogene 25W burning all night long even more, not to mention the temperature problems with the transformer base buried in stuffed animals.
The solution: teach her something! So we repurposed an old broken desk lamp carcass, I taught her how to solder, programmed a 12f629 PIC and we combined the above with sufficiently many white LEDs and some recycled laptop Li-Ion cells into an auto-off bed light: Press the button when off, and you get 18 min of light. Press the button when the light is on, and the light goes off. Simple, neat, efficient. As a bonus the lamp body is black, Just Like It Should Be.
The diagram is not complete in two particulars: I used a 4.5mm plug with a builtin bypass switch to isolate the battery when charging (don't want to blow the LEDs and/or PIC when my intelligent charger feeds the LiIon), and I repurposed the original lamp switch as an extra "general disconnect". BSTS.
Great care should be taken to avoid shorting or annoying the three 2000+mAh cells in any way - unless you like to play with fire extinguishers.
Conny did all the soldering apart from one or two small fixes and the LED interconnections. Well done.