Monitoring Lipo Voltage

Ductapemaster

I own an R/C helicopter and it uses a 3 cell lipo battery pack for power. I need some way to monitor the battery voltage in flight so that when it drops past a certain point (about 3 volts per cell), I get an indication that I need to land. There are many commercially available products for this purpose, but being a DIY person, I want to build one myself. I am designing mine off this: http://www.tcdynamics.com/prod04.htm

The circuit watches the voltage of the battery pack while under load and it has set points where it lights the RGB led different colors. I don't need something quite this fancy (I was thinking a bi-color LED) but if its easy to do I don't mind programming it.

So, I need some help designing this. To keep things simple I would like to use the SX's built in ADC, if it's accurate enough. The voltage measured will range from 12.5+ volts to a bit below 9 volts. As far as I know I will need some sort of voltage divider to bring it down to a level that the SX can understand.

Has anyone done anything like this before? Any suggestions?

Thanks!

-Dan

EDIT: I just noticed that the website says it has an "All Analog Design"...anyone have any idea how they did this? Especially with the multi-color LED's...

Bean

A 7.5 volt zener diode and a resistor should work.

(12.5 - 7.5) = 5 V (If you want to use ADC)
(8.9 - 7.5) = 1.4 V (threshold of TTL trigger level, if you want to just read HIGH/LOW)

Bean.

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