I am looking for any idea's / circuits on a battery low indicator the battery will only be 3v need the indicator to activate at 2.5v. Can any one help thanks.

 

Do you need it to light an LED or just give a voltage level? A simple op am with a couple of resistors and diodes would do the latter just fine.

 

Just looking to light up an LED

 

OK, it gets a bit tricky, but is doable. The problem is the voltage needed to light up the LED. Older LEDs use less dropping voltage, so I'll assume that. Someone else may come up with something better, but I'll see what I can draw up in a bit.

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OK, I've never built this, but I think it should work. I was aiming for as low power as possible.

 

The LTC1440C works down that low and has a typical application schematic similiar to what you want in the datsheet.

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1002,C1463,P1172,D2764

Worst case current consumption is only 4.4uA you will also have the sense divider but it should only consume about 10 to 14uA altogether.

 

Here's another option:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1273

Works on a supply voltage down to 2v, quiescent current of 5µA, low output of 0.7v, and hysteresis to prevent oscillation.

Ken

 

Can either one of those drive an LED?

 

The MAX8211 should, if his battery cutoff is 2.5v. Its sink current is 4mA (min) and output saturation is 0.17v (typical...I was wrong earlier...0.4v Max)...that leaves at least 2.1v for a high brightness red LED and current limiting resistor.

Ken

 

It can swing to Vol+0.4 Vcc worst case. They spec 0.8mA for a 3V rail. More then enough to drive a LED. I usually only use 20 to 30 uA.

Ive never tested it down to that low a voltage though but according to the datasheet it is doable.