For my project, the Lithium Ion Polymer Battery - 3.7v 2600mAh powers arduino, transmitter and a few other parts. My whole design works on 3.3v. I'm trying to build a circuit to indicate the status of the battery using LED. When the voltage of the battery is consumed below 3.0v, I want to light on the Red LED. I found some link and tried to build a circuit based on it. http://www.made-by-bacteria.com/forum/viewtopic.php?f=59&t=479 The first one I built, it does not seem to be working for low Vcc like 3.3v. http://sharingmanual.blogspot.com/2011/03/op-amp-voltmeter-circuit-with-led-bar.html For this one, I didn't know if I should use the zener with 3.3v breakdown voltage. Can anyone suggest me how you can go about this? or suggest me a link? Any tips will be appreciated.