Filtering noisy 12V to a logic level

Discussion in 'Electronic Design' started by Miles, Sep 23, 2003.

  1. Miles

    Miles Guest

    Hello,

    I built a circuit for a motorcycle which cuts off power to the
    ignition coils for several milliseconds when a switch is pressed to
    fire an air shifter. It works well, but sometimes it triggers for no
    reason (ie. switch was not pressed).

    The circuit is powered by a LM2937 automotive regulator. The timming
    is done with a 555 in one-shot mode. I think the problem is lack of
    filtering on the trigger wire. The trigger wire will be switched from
    an open, to 12V.

    Any ideas on a really solid noise immune way of converting that noisy
    12V line to a 5V logic level for the 555? My revised idea is to use
    the following signal path:

    1. 12V unregulated
    2. 20V TVS
    3. LC low pass filter
    3. Voltage divider (Divide signal by 4)
    4. Pull Down resistor
    5. 5.1V Zener
    6. Schmitt Trigger output to 555 timer trigger pin

    Do you think this will work? What values for my low pass filter
    should I be looking at?

    Thanks for your help.
     
    Miles, Sep 23, 2003
    #1
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  2. On 22 Sep 2003 21:22:31 -0700, (Miles)
    wrote:

    >Hello,
    >
    >I built a circuit for a motorcycle which cuts off power to the
    >ignition coils for several milliseconds when a switch is pressed to
    >fire an air shifter. It works well, but sometimes it triggers for no
    >reason (ie. switch was not pressed).
    >
    >The circuit is powered by a LM2937 automotive regulator. The timming
    >is done with a 555 in one-shot mode. I think the problem is lack of
    >filtering on the trigger wire. The trigger wire will be switched from
    >an open, to 12V.
    >
    >Any ideas on a really solid noise immune way of converting that noisy
    >12V line to a 5V logic level for the 555? My revised idea is to use
    >the following signal path:
    >
    >1. 12V unregulated
    >2. 20V TVS
    >3. LC low pass filter
    >3. Voltage divider (Divide signal by 4)
    >4. Pull Down resistor
    >5. 5.1V Zener
    >6. Schmitt Trigger output to 555 timer trigger pin
    >
    >Do you think this will work? What values for my low pass filter
    >should I be looking at?
    >
    >Thanks for your help.


    A dedicated 12 volt line leaves your circuit and goes to the switch.
    The signal from your switch goes back into your box. First to a
    pulldown resistor and possibly diodes to VCC and GND to clip any
    spikes, then it passes through a 1K resistor, then there is a 1uF cap
    to ground, and then possibly through another series resistor to
    prevent latchup if using a cmos 555.

    Or just use a coaxial cable to feed the switch and forget all the
    other stuff.
     
    Stepan Novotill, Sep 23, 2003
    #2
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  3. In article <>,
    (Miles) wrote:

    > Hello,
    >
    > I built a circuit for a motorcycle which cuts off power to the
    > ignition coils for several milliseconds when a switch is pressed to
    > fire an air shifter. It works well, but sometimes it triggers for no
    > reason (ie. switch was not pressed).
    >
    > The circuit is powered by a LM2937 automotive regulator. The timming
    > is done with a 555 in one-shot mode. I think the problem is lack of
    > filtering on the trigger wire. The trigger wire will be switched from
    > an open, to 12V.
    >
    > Any ideas on a really solid noise immune way of converting that noisy
    > 12V line to a 5V logic level for the 555? My revised idea is to use
    > the following signal path:
    >
    > 1. 12V unregulated
    > 2. 20V TVS
    > 3. LC low pass filter
    > 3. Voltage divider (Divide signal by 4)
    > 4. Pull Down resistor
    > 5. 5.1V Zener
    > 6. Schmitt Trigger output to 555 timer trigger pin
    >
    > Do you think this will work? What values for my low pass filter
    > should I be looking at?
    >
    > Thanks for your help.


    The inputs have a very high impedance. You could probably trigger it by
    swatting a cloth against the wire (electrostatic generation).

    An RC filter and pull-down resistor will do the trick. Of course you
    don't want to pull the trigger higher than pin 8, so your pull-down
    resistor and RC filter might need to combine into a voltage divider.


    switch ---- R --+--+--- Trigger
    | |
    R C
    | |
    +--+--- GND


    I'm not sure exactly what your regulation is. A simple snubber won't be
    enough. Negative spikes in the input power will trigger the timer too.
    Another RC filter is the simplest solution and it works quite well too.
     
    Kevin McMurtrie, Sep 23, 2003
    #3
  4. Miles

    Boris Mohar Guest

    On 22 Sep 2003 21:22:31 -0700, (Miles) wrote:

    >Hello,
    >
    >I built a circuit for a motorcycle which cuts off power to the
    >ignition coils for several milliseconds when a switch is pressed to
    >fire an air shifter. It works well, but sometimes it triggers for no
    >reason (ie. switch was not pressed).


    Your problem could be the vibration sensitive switch.

    --

    Regards,

    Boris Mohar

    Got Knock? - see:
    Viatrack Printed Circuit Designs http://www3.sympatico.ca/borism/
    Aurora, Ontario
     
    Boris Mohar, Sep 23, 2003
    #4
  5. Miles

    Terry Guest

    Boris Mohar threw some tea leaves on the floor
    and this is what they wrote:

    > On 22 Sep 2003 21:22:31 -0700, (Miles) wrote:
    >
    >>Hello,
    >>
    >>I built a circuit for a motorcycle which cuts off power to the
    >>ignition coils for several milliseconds when a switch is pressed to
    >>fire an air shifter. It works well, but sometimes it triggers for no
    >>reason (ie. switch was not pressed).

    >
    > Your problem could be the vibration sensitive switch.


    It could also be spikes from the ignition, so check that its not
    sensitive to that kind of thing ?


    --
    Kind Regards from Terry
    My Desktop is powered by GNU/LinuX, Gentoo-1.4_rc2
    New Homepage: http://milkstone.d2.net.au/
    ** Linux Registration Number: 103931, http://counter.li.org **
     
    Terry, Sep 23, 2003
    #5
  6. Miles

    Jim Thompson Guest

    On 22 Sep 2003 21:22:31 -0700, (Miles)
    wrote:

    >Hello,
    >
    >I built a circuit for a motorcycle which cuts off power to the
    >ignition coils for several milliseconds when a switch is pressed to
    >fire an air shifter. It works well, but sometimes it triggers for no
    >reason (ie. switch was not pressed).
    >
    >The circuit is powered by a LM2937 automotive regulator. The timming
    >is done with a 555 in one-shot mode. I think the problem is lack of
    >filtering on the trigger wire. The trigger wire will be switched from
    >an open, to 12V.
    >
    >Any ideas on a really solid noise immune way of converting that noisy
    >12V line to a 5V logic level for the 555? My revised idea is to use
    >the following signal path:
    >
    >1. 12V unregulated
    >2. 20V TVS
    >3. LC low pass filter
    >3. Voltage divider (Divide signal by 4)
    >4. Pull Down resistor
    >5. 5.1V Zener
    >6. Schmitt Trigger output to 555 timer trigger pin
    >
    >Do you think this will work? What values for my low pass filter
    >should I be looking at?
    >
    >Thanks for your help.


    See "NoiseBlank.pdf" on the S.E.D/Schematics page of my website for a
    variety of ways to handle ratty edges.

    ...Jim Thompson
    --
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
    | http://www.analog-innovations.com | 1962 |

    I love to cook with wine. Sometimes I even put it in the food.
     
    Jim Thompson, Sep 23, 2003
    #6
  7. Miles

    John Fields Guest

    On Tue, 23 Sep 2003 12:33:42 -0700, Jim Thompson
    <> wrote:

    >On 22 Sep 2003 21:22:31 -0700, (Miles)
    >wrote:
    >
    >>Hello,
    >>
    >>I built a circuit for a motorcycle which cuts off power to the
    >>ignition coils for several milliseconds when a switch is pressed to
    >>fire an air shifter. It works well, but sometimes it triggers for no
    >>reason (ie. switch was not pressed).
    >>
    >>The circuit is powered by a LM2937 automotive regulator. The timming
    >>is done with a 555 in one-shot mode. I think the problem is lack of
    >>filtering on the trigger wire. The trigger wire will be switched from
    >>an open, to 12V.


    ---
    If that's true, then the first problem is that the 555 doesn't trigger
    on a positive-going edge, but on a negative-going edge, so you're more
    than likely triggering on negative spikes when the switch bounces, and
    every once in a while when the trigger input is driven low by noise.
    The trigger input of the 555 is the floating base of a PNP, so if you're
    not using a pullup on the trigger input, it will normally be off, but
    noise could certainly trigger it.
    ---


    >>Any ideas on a really solid noise immune way of converting that noisy
    >>12V line to a 5V logic level for the 555? My revised idea is to use
    >>the following signal path:
    >>
    >>1. 12V unregulated
    >>2. 20V TVS
    >>3. LC low pass filter
    >>3. Voltage divider (Divide signal by 4)
    >>4. Pull Down resistor
    >>5. 5.1V Zener
    >>6. Schmitt Trigger output to 555 timer trigger pin



    ---
    Unless you have some overriding need to feed the 555 from a 5V supply,
    there's no reason why you can't power it with +12 and momentarily pull
    the trigger to ground when you push the switch. Do you have a schematic
    you can post so we can see what you're doing?

    --
    John Fields
     
    John Fields, Sep 23, 2003
    #7
  8. On 22 Sep 2003 21:22:31 -0700, (Miles) wroth:

    >Hello,
    >
    >I built a circuit for a motorcycle which cuts off power to the
    >ignition coils for several milliseconds when a switch is pressed to
    >fire an air shifter. It works well, but sometimes it triggers for no
    >reason (ie. switch was not pressed).
    >

    Before you go off and flog the input for noise, try an experiment.

    Make all the connections, power and grounds included, but disconnect the
    signal input wire right at the input to your circuit. Just leave it floating at
    the circuit end but connected at the switch end.

    Then run around for a while and monitor the output of your circuit. If
    you still get random actuation, then you have a "ground loop" problem and even
    the most well designed filter circuit in the world won't do Jack.

    Jim
     
    James (Jim) Meyer, Sep 24, 2003
    #8
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