The project was to give an audible warning when a LiPo battery approached is discharged voltage, for a three cell battery it's 9v, so this circuit goes off at 9.25v, but the voltage is adjustable to suit other types of batteries.

The circuit consists of a voltage reference, a CA3140 op-amp and a adjustable input voltage.

Click on the diagram to see a large image.

The circuit was made up on to a piece of Veroboard

Then installed in a suitable enclosure.
The circuit has been modified, by removing the Pizo buzzer and replacing it with two LEDs and a small resistor, this gives a visual warning rather than an audible one, which could not always be heard over the noise of the electric helicopter.