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Tachometer circuit

Discussion in 'General Electronics Chat' started by BioniC187, Sep 18, 2012.

  1. BioniC187

    BioniC187 Member

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  2. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    I havent used that circuit but we could do a simulation (myself or someone else here).
     
  3. Jaguarjoe

    Jaguarjoe Member

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    I don't know about the 555 ckt, but I do know the LM2917 works well. I had one in the trunk of a car for many years now helping control a lock up torque convertor.
     
  4. dave

    Dave New Member

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  5. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    I did a quick test of the 555 circuit and it does seem to function as a tachometer.
    I varied the input pulse width and the output stayed the same pretty much.
    But increasing the frequency by a factor of 2 resulted in an output average of close to 2 times the original frequency output.
    So it does function as a tach at least basically.

    It would have to be tested next on the car it was going to be used with to see how it performs over the whole range of rpm's possible.
    I do wonder if that input 1k resistor should be a bit higher though. That value seems kind of low.
     
  6. alec_t

    alec_t Well-Known Member Most Helpful Member

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    I agree. The points voltage can go quite high if the distributor capacitor is poorly. That 1k resistor could be dissipating ~ 1W.

    I've run a sim. Although the tacho functions, the power supply for it is badly designed in that the 9V zener is passing ~ 280mA and dissipating ~ 2.4W. The 15 Ohm resistor also dissipates ~ 1W. Perhaps it was intended to be 150 Ohms.
    (The remainder of the circuit draws only ~ 5mA.)
    I suggest either increase the 15R to ~220 Ohms or else use a 78L09 or other 9V regulator IC in place of the 15R and 9V zener.
     
    Last edited: Sep 18, 2012
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  7. debe

    debe Active Member

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    This is fairly old ETI March 1977, It works as ive used it before.
     
  8. BioniC187

    BioniC187 Member

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    Thanks for replies :)
    I can do a sim as well, i just don't trust sims that much. It is always different than real life.
    I also had concerns of that 1k resistor. I don't want it to take out too much current from my coil, then i'll start having car trouble haha
     
  9. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    alec:
    Good catch there as that resistor is way too low.

    The second circuit posted by debe looks better.

    BTW it would also be possible to use an averaging circuit on the output and use a volt meter instead of an analog current meter.
     
    Last edited: Sep 18, 2012
  10. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Agreed. And unless the OP already has a milliammeter a voltmeter looks a better/cheaper option.
     
  11. dr pepper

    dr pepper Well-Known Member

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    My triumph had a tacho in I built from everyday electronics, it didnt connect to anything, the pickup was just a bit of wire wrapped around one of the ht leads a few times, the chip itself was a lm2917 this is a dedicated f to v chip and recomended if you want an accurate rev counter.
     
  12. debe

    debe Active Member

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    I agree that the LM2917 is a far better option & more versatile. I used one driven by a Hall device as the pulse generator on a Diesel engine for a tacho. Just stuck a small magnet on the front pulley of the engine.
     
  13. BioniC187

    BioniC187 Member

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    Alright, thanks all. I'm gonna hold on this a bit, as i am looking into other possible methods :)
     
  14. Dipak

    Dipak New Member

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  15. Chinoi

    Chinoi New Member

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    Replace 200k variable resistor in series with 50 ma meter into 200 ohms to make it work.
     
  16. Jaguarjoe

    Jaguarjoe Member

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    Make sure whatever supply regulator you use is sufficiently protected from the vagaries of the car's 12v system. Spikes to almost 60v can occur. Look at an LM2940, it is internally protected.
     
  17. BioniC187

    BioniC187 Member

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    I have an lm2940 in place. A 5V and a 10V
     

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