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Speed switch using LM2907

Discussion in 'Electronic Design' started by Udo Zaydowicz, Nov 20, 2004.

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  1. Hello,

    I've got a problem with the above-mentioned f/U-converter (8-pin
    version) using it as a speed switch.

    Given is a car's speed signal (symetrical rectangular signal). Switching
    should occur at a frequency of appr. 36 Hz. The speed switch is build
    iaw the example in the datasheet/application note. The speed signal is
    connected to the IC-input through ac coupling as described in the
    application notes.
    Schematic: http://www.zaydowicz.de/Forum/Schaltung.jpg

    Well, it is switching as designed. However, if the car is decelerated to
    a stop (frequency -> 0), the converter switches sometimes just after the
    car stops.
    Quite obvious, this happens each time when the positive flank of the
    speed signal is 'caught' and the car stops before the negative flank.
    The speed signal itself is clean, not spikes, no disturbences etc..
    The picture at http://www.zaydowicz.de/Forum/Signal.jpg should
    illustrate this (purple: speed signal, yellow: LM2907 f-input).

    Can anybody help me with that?


    Many thanks in advance, Udo
     
  2. GPG

    GPG Guest

    Possibly interference from the supply getting to the input via R7, C4.
     
  3. Hello GPG,
    Rather not, I think.
    1. The supply is stabilized and protected against the typical 'dirt' in
    the car, and I could not measure any.
    2. It only happens in the situation I described above. If the car is
    stopped between the negative and positive flank of the speed signal,
    everything is working fine.

    Udo
     
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