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Help adding delay to reduce false trigger

Discussion in 'General Electronics Discussion' started by maxedmini, Nov 26, 2010.

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  1. maxedmini


    Nov 26, 2010
    hi there this is my first attempt at creating a circuit so keep it simple for me please :D:D

    A quick description
    the circuit is used to warn of low coolant on a car
    the green led is lit when the ignition is turned on and the red led and buzzer sound when the float closes the switch this all works ok BUT i need to add a delay to reduce a false trigger as i have found the water movement sometime closes the switch for a second and i get a quick buzz and a flash of the led

    so what i require is a way to add a delay so if the switch is closed only for a second or so the buzzer and led do not trigger???

    the components used are
    red LED = 12v 8.5mA
    Green LED = 12v 8.5mA
    Buzzer = 3 to 24Vdc 5mA

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Everything is OK except what you have done with the LEDs. (Adding a delay is non-trivial)

    Perhaps those LEDs have an integral series resistor. Otherwise they would have given one bright flash before expiring.

    Using a time delay relay may be an option. It has a delay down to 1 second. It may be a little pricey though.

    I've tried finding a simple circuit that I would be happy placing in a car, but I've not been successful. I don't want to suggest something that is too hard to build, nor something that will not cope with voltage fluctuations that are common on car electrical systems.
    Last edited: Nov 27, 2010
  3. maxedmini


    Nov 26, 2010
    yes the leds are 12v ones

    I have built the current circuit and all works fine except for the false alarms

    can a 555 chip be used some way to prevent this??
  4. neon


    Oct 21, 2006
    USE AN LM555 as one shot for one sec use it as a one shot using pin 2 as triggering or tbreak the line from switch red cathode install a big cap in series to the buzzer and that will spike the buzzer to ground when the switches closes only.
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