I need to make a battery monitor with minimal parts. A green LED should be lit during normal operation. The device has two batteries (designed for 9V, but it could probably use others) in a split supply (-9, 0, +9). I need to monitor BOTH batteries, and have the green light go out and a red LED come on when ONE of the batteries drops below a certain level. Because digikey makes you buy small parts in bunches of 10, it would be really great if I could use the 2 or 3 transistors left over to build this thing with. It seems like a really clever person could figure out a circuit with only two transistors and 5 or 6 resistors, but I am not that person. I'm more of an op-amp kind of guy. I understand the basics, but I am not too good at predicting complete transistor behavior, as evidenced by burns on my thumb and forefinger from a hot transistor earlier today. I was thinking something based off of OK/Low battery indicator, but when I tried it, it didn't work. Apparently a small enough battery means there is not enough voltage to light both the red and green at the same time, so only the red lights? That's not how it worked when I did it, though. Both lit. I don't understand. Any ideas? And when I say "minimal parts", I mean BJTs, resistors, and the LEDs.