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Homemade 555 Tachometer

Discussion in 'General Electronics Discussion' started by Kris01, Jan 6, 2017.

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  1. Kris01

    Kris01

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    Jan 6, 2017
    Hi everyone, I found this schematic online and I'm curious how I can adapt it to my motorcycle.

    My bike is a 1 cylinder, 4-stroke engine that fires the plug on every stroke (not every other like normal). As I understand it the circuit outputs 0.01 ua for every 1 RPM. 50ua = 5000 RPM. In my case I would need a 100 ua meter to read 5000 RPM since my plug fires twice as much as most engines. The circuit is designed to read a plug that fires every other stroke. My bike has a "wasted spark" ignition system. There also are no points. It is a CDI ignition system. Can I wrap a few turns of wire around the spark plug as input instead of connecting to the non-existent points? Does any of this look out of place or could it be done better?

    [​IMG]
     
  2. duke37

    duke37

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    Jan 9, 2011
    The meter does not need to be different, just adjust RPM CAL to suit. Reduce C2 by half to allow higher trigger speeds if necessary.
    You may be able to get a signal by capacity pick up but will need experiment. I would increase R1 to perhaps 100k to protect the 555.

    Do not use a CMOS 555, they will be more delicate.
    A resistor (100Ω?) and capacitor to earth before the regulator will give some extra protection.
     
  3. Bluejets

    Bluejets

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    hevans1944 likes this.
  4. Kris01

    Kris01

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    Jan 6, 2017
    I'm very new at this so bear with me. Can you give me any idea what value (roughly) that I would need to adjust RPM CAL so that the meter reads correctly? What would the higher trigger speed do? Would the needle just respond quicker?
     
  5. Minder

    Minder

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    Check out the LM2907, it was designed for this application.
    M.
     
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  6. duke37

    duke37

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    Jan 9, 2011
    The 555 produces a fixed width pulse for every trigger pulse. The meter measures the average voltage through RPM CAL. Thus if there are twice as many pulses as required, then the resistor will need to be twice the value.

    If the input pulse comes too soon to get a complete output pulse before the next one then the capacitor needs to be reduced to make a shorter pulse. You should look at the pulse length and calculate the maximum possible input frequency.

    The needle response time depends on the capacitor across the meter and the needle inertia.
     
  7. Kris01

    Kris01

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    Jan 6, 2017
    You're getting way over my head really fast! I know just enough about electronics to follow a diagram. The schematic above that I posted was something that I thought I could build pretty easily. You guys are making me think otherwise now. Is there an easier way to do this? Can someone provide an easy to read schematic?

    Thanks for all the help! Sorry I'm not as experienced in this stuff as a lot of you guys seem to be.

    What you say makes all the sense in the world to me. I just don't know how to go about doing it.
     
    Last edited: Jan 6, 2017
  8. Minder

    Minder

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    The spec sheet for the LM2907 has the application details.
    M.
     
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  9. duke37

    duke37

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    Jan 9, 2011
    If you want something simpler, go for the LM2907, it has a regulator built in so you do not need the 7805. It is also designed to work from the dirty 12V supply of a vehicle.
    The TI data gives application circuits.
    One of the reasons we are confusing you with facts is that you want to change things. To make alterations, you need to understand what you are doing.
     
    hevans1944 likes this.
  10. Kris01

    Kris01

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    Jan 6, 2017
    Agreed. It's just an idea I was playing around with.
     
  11. hevans1944

    hevans1944 Hop - AC8NS

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    Playing is what we do here! All who have responded with advice and links are quite active here and enjoy welcoming and helping others to become more acquainted with electronics. Where you go from here is entirely up to you, but this is a good place to find help when you need it.

    The circuit you found can be easily adapted to a wide variety of analog D'Arsonval meter movements with various full-scale current ratings from "sensitive" microamperes up to several "less sensitive"milliamperes. The less sensitive movements are likely to be more rugged construction and more appropriate to the high-vibration environment of a motor bike. The circuit you posted may have been "optimized" for a one-cylinder 4-stroke engine and a 50 μA movement, but that is by no means a "requirement" for the circuit to work. And, as others have stated, there are "better" circuits you should at least consider.

    Are you interested in pursuing the construction of a tachometer from a hobbyist point of view? Learning how the circuits work? Learning how to adapt circuits to your purpose instead of blindly following "monkey see, monkey do" instructions provided by someone else? If you can answer "yes" to those questions, there is plenty of help available here. Almost everyone who posts useful answers to this forum has "hands on" experience in electronics. You should get some too!

    Can you "wrap a few turns around the spark-plug wire" to obtain a "trigger pulse" for the tachometer? Of course you can. There is a LOT of voltage on, and a strong electrical field around, the wire when the ignition coil fires. A few turns of wire will create enough capacitance between spark-plug wire and the wire wrapped around it to "see" a rapidly decaying pulse each time the coil fires. Problem is, you have virtually no control or knowledge of the pulse amplitude or pulse width, both of which will vary depending on whether it occurs during the power stroke or the exhaust stroke, whether the throttle is open or closed, and the engine rpm. Considering all these factors, you need to create a pulse of the proper level and duration to act as a trigger pulse each time and every time the ignition coil fires. This is where the "hands on" play becomes an important factor.

    I am somewhat known for lengthy replies in this forum, but until you show some more interest, this is as far as I will go. Visit the links others have posted and return here with more questions.
     
  12. Kris01

    Kris01

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    Jan 6, 2017
    I'm learning a little as I go. Like I said, I can blindly follow a diagram but I lack the knowledge to "fine tune" it to my tastes. I'll check out the links too. Thanks!
     
  13. Rleo6965

    Rleo6965

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    Jan 22, 2012
    You can find lower tach signal voltage on driver transistor of coilpack which less damage to input of your tach circuit.
     
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