I designed this circuit based on Daftmike's low-battery indicator circuit. If you're thinking of using his low-battery indicator and don't mind giving up the green LED in exchange for some parts savings, use this one.
All the info is right there. This circuit uses only two transistors as opposed to five. This saves you some parts and space as well as a bit of battery life. I took out the green LED. So, there is no indicator when the battery is OK but the LED turns on once it goes below the set voltage.
Here is his original thread:http://forums.modretro.com/viewtopic.php?f=7&t=255