From the MAKE Flickr photo pool

Flickr member Kc7fys built this fine little voltage meter to indicate battery level using a bi-color LED and an LM358 op-amp chip –

I could measure the resistance values and just use regular resistors to make it smaller. It tells me when my radio power supply (battery pack) is 12V or above, 11-12V, or below 11V with three colors in the LED.

KD1JV Voltage Meter Works!

Battery Voltage Monitor Circuit