Around here we get the odd brown out because the local transformer station is inadequate and hasn't been updated since the 50's so obviously it can't handle the fact that houses have more than one electrical appliance. anyway I made this low power device to be put with my torch so I can find it when all the lights go off.
after you build it - you adjust VR1 so that the LED blinks when it's dark and it should last quite a while with 2 1.5v batteries powering it.
try and get the lowest power LED you can find - remember, it will also blink all night.
