need some comparator advice

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japcarmad

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hi,
just joined up to get some advice on a simple circuit that require for an automotive project

my goal - to build a simple device that can trigger a relay at a given voltage for example. my range of input is 0 - 5 v. i need to turn on a relay at 1.5v, 3v and 4.5v. possibly 3 identical circuits to operate 3 devices each at these given voltages. i need it turn on at the given voltage and off again when it drops below it.

i have been looking for a while on the net and think i need to use a LM2903 comparator IC to control a transistor and in turn operate the relay.

i have fundamental electronics knowledge but need further help to figure out where to go from here.

many thanks in advance
 

Which type of relay you use? write its specification? I suggest you should use LM324 in which three comparator are present. comparator just compare two voltage level and according to that level give high voltage and low voltage as output. Which is not enough to operate relay. first connect potentiometer(variable resistor) at one terminal(inverting or non inverting based on your requirement) which give the reference voltage and at another terminal your device which voltage will change. then connect output of the comparator to relay driver(ULN2003 or ULN2803). which pull up its voltage level. then connect the relay to o/p of relay driver.
 

the relay will be switching on a 12v source
 

then the given ckt is best for you. you can use three comparator using single IC. refer to its datasheet.
 

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