I remember that I used some recycled parts, also a tranformerless power supply. This circuit should be completely covered by a plastic container with the appropiate ventilation. Here is the schematic:
A small sealed maintenance-free battery is perfect, it can provide up to 4 hours of light. If you want someting smaller and easy to install, you can replace the battery and lightbulb with a super bright white led (12,000 mcd at least) and a 1F supercapacitor. Here is the circuit:
When the circuit receives power, it acts like a trickle charger for the lead-acid battery. When the power fails, the relay is off and it sends the battery power to the lightbulb.
I'm not sure if this circuit can work with ni-cad batteries, but I think it may work without modifications. The zener diode must be 13V for the battery-version and 5 volts for the LED version.
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