Tachometer Voltage Divider Resistance & spurious pulses

Discussion in 'The Projects Forum' started by bill2009, Aug 10, 2009.

  1. bill2009

    Thread Starter Active Member

    Apr 17, 2009
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    I'm pulling a signal from my motorcycle tachometer circuit. it seems to be a 50% duty cycle square wave at battery voltage.

    I needed to bring it down to 5v and, not wanting to interfere with the ignition i used 1M & 470K resistors. It works great but I find i get occasional spurious pulses, maybe once in 20 revolutions.

    They currently get filtered out by the software but I wondered if they might be a result of using too high a resistance in the interface - there's presumably lots of EMF floating around the coils and maybe the 1M resistor lets it show up as detectable voltage.

    I'm re-doing the circuit so I thought I would ask for advice - go down to 100K/47K? lower? put in a small cap on the microprocessor side of the divider?
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a schematic of what you have upto now?
    How did you "clip" the signal at the input of the mcu?

    Greetings,
    Bertus
     
  3. bill2009

    Thread Starter Active Member

    Apr 17, 2009
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    Yes sure. The signal's not really "clipped", it's just taken down by a factor of 3 which keeps it in the 5v range of the mcu.
    [​IMG]
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    How many volts do you expect at the mcu input?
    You could lower the 1M resistor to 10K and replace the other resistor with a zener-diode of the wanted input voltage.

    Greetings,
    Bertus
     
  5. bill2009

    Thread Starter Active Member

    Apr 17, 2009
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    i want either 3.5-5v when the input is 11-14 V, or <3 when it's 0.

    The 10k resistor would only pull 1ma or so, so that's presumably still ignorable by the ignition circuit and a 4 or 5 v zener would be good protection - right? Is this less likely to suffer interference from the ignition?
     
  6. bertus

    Administrator

    Apr 5, 2008
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