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Voltage controlled relay driver help..

Discussion in 'Electronic Basics' started by Bluexcell, Apr 21, 2004.

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  1. Bluexcell

    Bluexcell Guest

    Can someone point me to a circuit that i can use to turn on a 12V DC relay at a
    preset voltage input, i want to monitor the variable DC tacho drive signal in
    my car and turn on the relay to operate a solenoid. Something that can be set
    with a potentiometer would be ideal.. ive looked on the web , i cant find a
    specific circuit for a novice like me..

    Your help is appreciated.
  2. The component for this job is called a comparator. It is essentially
    a 1 bit analog to digital converter. It has two inputs (labeled + and
    -) and one output. When the + input is more negative than the -, the
    output (a transistor connected to the negative supply pin) turns on
    and pulls down. They can typically pull down with a few milliamps, so
    you will probably need to add a transistor to the output to beef up
    the current capability. You also need something to provide the
    reference voltage that it uses to compare to the tach voltage. The
    simplest might be just a pot across the supply rails. But using a
    zener diode across the pot (and a series resistor to the positive
    supply, to limit the zener current would provide a much cleaner
    reference voltage.

    Take a look at this typical data sheet for a dual comparator and see
    if any of this makes sense to you.

    The limit comparator example on page 10 shows how to boost the output
    current with a transistor. You would also need to connect a diode
    across your relay coil ot prevent it from generating a large voltage
    spike when it is turned off.

    On page 9 they show how to use positive feedback to provide hysterisis
    to keep the relay from chattering if the inputs stay very close to the
    same voltage.
  3. Rob Paisley

    Rob Paisley Guest


    A voltage comparator circuit can do what you want.

    Here is a link with further information.

  4. Bluexcell

    Bluexcell Guest

    Thanks.. that did the trick.. i just wasnt looking hard enough i guess..:)
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