This basic and versatile 5V supply uses a zener diode and an emitter-follower transistor.
http://www.edn.com/article/CA92007.html
When the battery voltage drops below a certain point the circuit flashes the LED. In the off state the circuit draws only 1uA.
http://www.imagineeringezine.com/PDF-FILES/undervol.pdf
It draws power from the 120vac power line and uses a small 6KV camera flash trigger coil. Can produce dangerous shocks.
http://www.imagineeringezine.com/PDF-FILES/12kv.pdf
To stabilize the voltage, I used 7805 and 7812 regulators. These are easily connected and used. They also allow the external transformers to be approximately +12 - +30 volts.
http://metku.net/index.html?sect=view&n=1&path=mods/modlab/index_eng
http://www.repairfaq.org/sam/samschem.htm
produce a fully isolated and regulated 5 volts @30ma from the 120vac power line.
http://www.imagineeringezine.com/PDF-FILES/acdc1.pdf
LED will light when battery is over 42 volts
http://www.jensen-transformers.com/as/as038.pdf
http://www.edn.com/article/CA217665.html
basic structures of power supplies
http://home.eunet.cz/rysanek/sup_en.html
http://www.electronicsforu.com/electronicsforu/lab/ad.asp?url=/efylinux/circuit/jul2000/cir2.htm&title=