power LED's

Hi Alex,

This type of thing is possible to implement using comparator ICs and a voltage reference. For example, see the datasheet for LM3915 which has a block diagram of the type of circuitry that would do what you need. Another alternative - if you're using something like an Arduino as part of your build - would be to use one of the ADC channels to monitor a fraction of the supply voltage and control the LEDs.

can you give me links to all that stuff?

Hi Alex,

I don't know of any links to do exactly what you wish to achieve, but the LM3915 datasheet has a block diagram to give you an idea of how to achieve it with a comparator. You'd still have to design the circuit. Similarly for an ADC method, you could search for potential divider to see how to divide the input voltage to a range suitable for the Arduino ADC (assuming this is what you choose - just a suggestion in case you were already using it as part of your design). Code to read the ADC can be found by searching for 'arduino adc' - it is an AnalogRead function. To make an LED turn on, the DigitalWrite function can be used I believe (I'm not knowledgeable on Arduino, but I know it can be done with it - the arduino.cc website has example code).

Hi Alex,

I have found the circuit that will help you , it uses The LM 3914 it has ten comparators, which are internally assembled in the voltage divider network based on the current-division rule. So it divides the battery level into ten parts !!

you can read more about this circuit at http://www.electronicsforu.com/electronicsforu/circuitarchives/view_article.asp?sno=137&article_type=1&id=433&tt=unhot&b_type=new

what is the ic1 LM3914

The LM3914 is basically a Dot/Bar Display Driver used for indication of voltage levels, You can read more about it HERE...

One can (okay, I am showing my age) build a comparator or schmidt trigger from discretes.  If one want's high Zin, one constructs an inverter w/ a fet (or mosfet) and follows it with a bipolar inverter.  Schmidt action is cast by using a very large positive feedback resistor from the output to the input, just sayin', these things are mighty fast, not that we need fast here.  If one want's low Zin, one can start with the bipolar inverter and finish with the FET inverter.  Use switching Qs, if poss.  You could probably just hook up a (backwards, of course) Zener to the front end of this version, as it is current-sensitive.  Alternately, get a potential (voltage for the illiterate community) comparator chip.  I'd get an OC (open collector) version, if using a single led, hook it up and it's ballast to the output.  Else, if you insist upon two LEDs, use a non OC comparator or one with a PU (pull up) resistor, appropriately scaled from the (data-sheet) DS.  Feed this into a CMOS gate, in order to get push-pull action.  Hook this output to the middle of an led (plus ballasts) string.  Choose resistors to balance subjective output.  At the front end of the comparator are two inputs, one is fed by your battery, the other by a reference, I would use a cheap, tiny voltage-regulator here, preferably.  You can use a string of a backward Zener plus forward diode(s) to balance the tempcos, again, showing my age.  Interchange of the inputs complements the sense, equiv, presuming equal ballasts, of swapping LED position.  Using a R-string from the supply would be obviously problematic.  Similarly, in many situations using the processor itself would be undesirable. 

Nowadays they have comparators w/ complementary-symmetry outputs.  I know what I am doing, but I was born awhile ago!

It's all about what is in your drawer for a one-off.  Another way to get the two LEDs thingy going would be to spring for a dual OC (open collector, open drain) type comparator.  One comparator energizes LED-and-ballast A, on the basis of battery level, the other comparator senses the absence of drop across load A and, in response, energizes LED-and-ballast (load) B.  Also if your design has an unused CMOS gate locally, you might want to go with the earlier solution.  EEs are an adventitious lot.

There is an episode of the Ben Heck show where he builds an alert system for a mailbox that is set up to warn him inside the house when the batteries are starting to get low.  Would probably be a good episode to watch.