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How would I create a circuit which turns on an LED when the voltage goes below 2.5V? I am using 2x AA batteries

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    \$\begingroup\$ How "sharp" do you want the "knee" to be - ie how sudden the transition?. What standby current is acceptable? Why 2.5V - non-rechargeable batteries have some energy left at 2.5V (but of course the operated equipment may care. Softish knee with 2 transistors. Super sharp knee with TLV431 and 1 transistor (maybe 2) or TLV431 and LM339 comparator or ... . \$\endgroup\$ – Russell McMahon Nov 4 '12 at 12:13
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A simple comparator circuit is one way of doing it, something like this:

Low Battery Detector

You will have to adapt it as necessary for the lower voltage. Use a low voltage comparator (if not open drain output then switch the LED/resistor between output and ground) and change the zener for a low current voltage reference (TL431 would do okay).

You can use the TL431 mentioned directly as in this circuit (does high and low levels):

Low Battery Detector 2

The TL431 conveniently has a reference voltage of 2.5V so you don't need the R1A/R2A divider, you can just tie the ref input to Vbatt. If you want an even lower reference, there is also a 1.24V reference version, the TLV431.

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