Vehicle Shift Light

Discussion in 'General Electronics Chat' started by bluebrakes, Dec 17, 2009.

  1. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
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    Somebody was asking me today about making an RPM shift light that flashes when you reach peak power on a vehicle.

    I was wondering if anybody had any ideas where to begin?

    I had thoughts about using a frequency comparitor.


    Some seem to pickup the alternator signal from the power lines. How can i do this?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, peak power and maximum RPM are two different things.

    You'd always want to shift before you got into the redline area unless you wanted to risk engine damage. That could be done with just a tachometer circuit.

    But trying to figure out when the peak power was reached would be a bit more complicated. You'd need to look at throttle position and engine vacuum (manifold absolute pressure), too.
     
  3. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
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    Well, ideally it would need to be adjustable to any RPM. So you could adjust it accordingly.

    But I do know what you mean though. That has kinda thrown a spanner in the works. So I would want to keep it simple, as it in it flashes when a certain RPM has been reached.
     
  4. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
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    the lm2917 looks a good ic to use. What do you think?
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    Ford Motor Company had a light on the dash to indicate the most economical shift points on their straight transmission vehicles (Escort was one of them). It was operated from a vacuum switch, if I recall properly..
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    LM2907/LM2917/LM2907-8/LM2917-8 - ugh. It's a math nightmare trying to figure them out. I still have a fellow waiting for some help, but I don't have any of them onhand to experiment with to test the math against, and I don't have a valid model to use to simulate one either.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Every auto store these days (ie the ones that sell go-fast goodies) has a rnage of shift lights and dash-mount tachos that have shift lights etc. They are not real expensive, maybe you should have a look at what's currently available?

    SgtWookie- I think I remember the thread you are talking about. Couldn't you just use a pot on the LM2917 and then the guy could just turn it to the adjustment he wanted? Why the need for the math and commitment to fixed value parts?
     
    Last edited: Dec 18, 2009
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Well, you CAN use a pot, but you need to start off with values close to what is optimal for the particular application; otherwise you wind up with strange problems - like excess ripple on the output, slow response time, etc.

    There are four different "flavors" of the same basic IC.
    The LM2917 and LM2917-8 have built-in Zener diodes.
    The LM2907 and LM2907-8 require external regulation.
    The internal voltage follower transistor saturates at Vcc/2, which limits the output range of the Zener versions.

    The -8 versions are 8 pin ICs, which are suitable for use with variable reluctance transducers; they can detect input transitions above and below ground, and the inputs are protected against such excursions.
    The non-8 versions are 14 pin ICs which are suitable for processing tachometer signals.
     
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