Tachometer circuit needed

Discussion in 'The Projects Forum' started by jddavis, Jun 11, 2008.

  1. jddavis

    Thread Starter New Member

    Jun 11, 2008
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    Been building auto DC monitors with LM3914 .
    Want to use LM3914 or LM3915 for 8 cylinder engine tachometer .
    Maybe use LM2907 ?
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Do you have any kind of sensor to sense the speed of the axis and you want to amplify its signal?

    or Do you want to build a tachometer entirely from the beginning and you want some ideas?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    An LM2907/LM2917 seems like a good starting point.
    You'd need at least a couple of decades of LM3914's just to get a rough idea of the engine's RPM range. If the RPM range of the engine was 0 to 7000, two decades would give you a resolution of 350 RPM, which is pretty doggone rough. Three decades would give you a resolution of 233.3 RPM per segment.

    What's your RPM range, redline, and what resolution do you need?
     
  4. jddavis

    Thread Starter New Member

    Jun 11, 2008
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    0
    I'm looking at National Semi May 2003 LM29x7 pdf page 10 top :
    Voltage Driven Meter Engine RPM .
    Has anyone built similar ?
    Apparently one wire from "ignition" to pin 1 of LM29x7 .
    Zener in LM2917 improves 0.3% accuracy ?
    First breadboard for 4 cylinder 2002 Ford Escort ; max RPM 5000 ?
    Why 400 Hz ~ 6000 ERPM - 8 cyl ?
    Will decide LM3914 or LM3915 , 1 or 2 decades later .

    Thanks , Jim D
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hi Jim,
    An LM3914 will produce a linear display, which is what you want.
    An LM3915 will produce a logarithmic display, which would be of limited usefulness for your application.

    The "ignition" lead would be from your ignition system's tachometer lead.

    Don't know offhand why a Zener diode would improve accuracy.

    For your engine specifications, consult your owner's manual, a repair manual, or your local Ford dealer.

    Estimated RPM: In 4-cycle Otto engines, the crankshaft makes two revolutions per ignition. So, 6000/2=3000. Now we convert from RPM to RPS. 3000/60=50. In the example, 8 cylinders are given - so we multiply by 8. 50 x 8 = 400 Hz.

    In the LM3814 datasheet that National Semiconductor provides, there are a number of examples of connecting the IC for display of multiple decades of LEDs.
     
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