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power LED's

greenwald1818

hi,

     I was wondering, im building a raspberry pi laptop, and I need a way to tell if the battery is low. so, what part would I need to get if I get 2 LED'S, and I want the blue one to be on if the battery is full and the amber one to be on if it gets below 4.5 volts (with a 7.5 volt battery) OR if I have a multi-colored one how would I do it if the same thing applies except with a multi colored one. thx in advanced.

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    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. 

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    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. 

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