Analog Tach for HT?

Discussion in 'General Questions' started by WolfByu, May 17, 2011.

  1. WolfByu

    WolfByu Member

    I know you can get the digital tachs for these engines but I cant find anything on a analog tach for a HT... has anyone tried this or know where a person could buy one?

  2. Wheres my dog

    Wheres my dog New Member

    In years of MB'iing now myself, I have never heard of or even seen one ever, analog style that is

    I would think analog would have a mechanical fitting at the end that mates with the engine, such a specialized device may not exist for an HT

    I like the thought of it for a vintage look on an MB
  3. WolfByu

    WolfByu Member

    OK what about a electric analog tach? I mean one where the the gauge still has a needle dial but you have wires that hook it up. One ground, one 12v, and a switched 12v for light
  4. Wheres my dog

    Wheres my dog New Member

    Can't say I've ever seen one of those either...

    My next question is how much would that thing cost???
  5. WolfByu

    WolfByu Member

    They are very common just about every aftermarket tach for automobiles is built this way. They start right arounds 30 bucks and go up from there. The thing is you would need one for a one cylinder engine and know where to hook the sending wire to.

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    Last edited: May 17, 2011
  6. Wheres my dog

    Wheres my dog New Member

    I kinda knew that already, but there is most likely no way to modify that without knowing electronics and circuitry
  7. WolfByu

    WolfByu Member

    yea your right about that. I wonder about a scooter tach? I will do some checking..
  8. loquin

    loquin Active Member

    Typically, with an aftermarket analog tach, the input comes from a wire coil wrapped so that a spark plug wire goes through the coil. The coil picks up enough voltage to to be used as the signal input.

    That input pulse is fed through a shaper circuit, to limit the voltage to safe levels and polarity. Then, the shaped pulse is used to trigger a fixed width pulse. (The width of the pulse is the critical thing you would need to change.)

    After this, the fixed width pulse is fed through a signal diode, and used to charge a Capacitor/Resistor filter circuit, which averages the voltage, so that the voltage across the capacitor is what is displayed on the analog volt meter.

    Suppose you have a 10 volt analog meter. (and 12 volts available on your vehicle.)

    The 12 volts would probably be regulated to 10 volts for the tach. If you want a tach to display up to 10000 RPM, that means that you need to supply 1 volt for each 1000 RPM.

    A HT engine fires on each revolution, so 10000 revolutions in one minute is the same as 166.667 revolutions in one second. If you divide one second by 166.667 revolutions, the result (.006 seconds, or 6 milliseconds) is the time that each pulse needs to be, for a 10K RPM to equal 10 Volts.

    If the motor is running at less than 10000 rpm (let's assume 6000, for giggles,) you would have 6000 RPM/60 Secs/Min pulses, or 100 pulses every second. Since each pulse is .006 seconds long, the total time in one second that there is a pulse is 100 times .006 seconds, or 0.600 seconds total, and that is 60 percent of the possible time. Since the 10 volt pulses are fed through an averaging circuit (diode/capacitor/resistor) you would have 10 volts * 60 percent, or 6 volts across the capacitor, and that is the scaled voltage that would cause your 10V meter to show 6. If you've changed the meter face to be labeled 'RPM x 1000' rather than 'Volts' you have a working, scaled tach.

    In the after-market tach, for a V8, you would have 8 pulses every two rotations, or 4 per rotation, rather than 1. If you had the same 10V meter we talked about earlier, the pulse width you would need is 1/4th of what was needed for 1 pulse/Rev, so the pulse width would be 0.0015 seconds (1.5 mSec)

    To modify the tach, you would need to get someone with an oscilloscope to locate the pulses on its circuit board and verify pulse width. And, you'll need to locate the Pulse circuit (it's a 1-shot, also known as a monostable multivibrator circuit) and adjust the R and C values for that circuit so as to make the pulse 4 times longer. (3 times longer for a 6 cylinder tach/2 times longer for a 4-cylinder tach.)

    I've thrown together a block diagram of an analog tach, below, to help explain what's going on inside.

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    Last edited: May 17, 2011
  9. WolfByu

    WolfByu Member

    wow you really do know your stuff! Maybe you should start making and selling them? Seems like a untapped market lol.
  10. Ozi

    Ozi Member

    1-2 ratio 2.5 inch cable tach could be fitted if you knew what you were doing re: drive gear.... :D

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  11. MikeJ

    MikeJ Member

    Loquin is right about the shaping circuit, filter, and all that. He does know his stuff. I just want to add another $.25 of input.

    I built this same circuit in 1979 using individual parts for monitoring the engine speed of a Volkswagen diesel engine. There are no spark plugs. (It had a few other quirks, but that is for VW trivia seekers.) To sense rotation, I used a light bulb shining onto a photo resistor. The light beam pulsed onto the photo resistor when the engine camshaft gear rotated. (The cam gear had wide spokes, like a bicycle bottom bracket gear.) Then the rest of the circuit shaped the pulses and drove the needle of the analog tachometer (something from a local auto store). (The circuit came from a "cookbook".)

    I calibrated the tach by pointing the photo resistor at a florescent light. Those lights pulse at 120 times per second, or the equivalent of 7200 rpm. After carefully removing the glass face of the tach, the readout paper was renumbered using Labelmaker tape.

    It worked as long as the incandescent bulb did not burn out. If burned out, the needle swung all over the place until I replaced the bulb.

    The spark plug wire is one way of picking up engine activity, there are other ways.

    I looked high and low for analog tachs for bicycles. In the long run, they would probably cost more than the TinyTach or SenDec that I use. Solid state equipment is also almost always more durable than fragile moving needles.

    But if anyone finds an analog tach for bicycles, please post details!
  12. BikerInFla

    BikerInFla New Member

    Could you pull one from an old Honda or Kawasaki MC basket case?

  13. loquin

    loquin Active Member

    Well, yes, you ... could. But, the issue with this approach is that they are typically cable driven, mechanical speedo's. So, you would need to get the engine rotation (before the clutch) transferred to a speedo cable, and at the right RPM. Gearing to get 6000 RPM engine speed down to apx. 1500 RPM, AND get a new dial face behind the needle on the speedo.
    Last edited: May 18, 2011
  14. MikeJ

    MikeJ Member

    Hi Everyone -

    I think an analog tachometer for a Chinese engine can be done using off-the-shelf parts. I will spare everyone the long, gory details, but the bottom line is you have to use that one controversial component of every Chinese two-stroke...

    The White Wire.

    That's right, the White Wire. Disregard trying to use the spark plug wire if you want to use an analog tach. You need an analog signal, specifically the sine wave voltage that appears on the white wire. The sine wave frequency and voltage varies with engine RPM. Those are the keys to using an analog tach.

    Get the tach of choice at the local auto supply store. The tachometer must say to connect it to the distributor or coil of the car; most tachs do. I purchased one that would look good on a bicycle for about $32 from an auto parts chain store and I was out the door. It even has a light to illuminate the face of the meter for night riding.

    The provided diagrams should be fairly self-explanatory. The sine wave voltage coming out of the magneto gets rectified into TWO positive-going pulses. The tach has a selector switch for 8, 6, and 4-cylinder engines. Set the switch to only "4". This allows the tach to sense the pulses fed into it and display RPM directly onto the meter face. There is no other alteration to the purchased tachometer.

    The battery is a 3.3 Amp-hour battery from a local hardware store. A charger had to be purchased as well. If you have a 5000 cranking amp 12 volt truck battery, you can use it. The tach draws only 35 milliamps; it won't hurt either battery if wired correctly.

    My test frequency of commercial electricity is available at 60 Hz (common knowledge). That is also the same frequency as a Chinese engine magneto output at exactly 3600 rpm. By using this, I proved that the circuit you see worked at this one frequency, simulating an engine spinning at 3600 rpm. The voltage being fed to the rectifier was 10 volts AC, right in the middle of the magnitude range produced by the magneto at operating rpm.

    There is still testing to be done, to include with a real engine. Just because it works on a test bench does not guarantee success in the field. But it is a good indication of success.

    If you are interested in pursuing this, please do so. Share your findings with the rest of us; I probably will need a few weeks or so of more time for testing.


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    Last edited: May 22, 2011
  15. DuctTapedGoat

    DuctTapedGoat Active Member

    Umm. For a digital analog tach, wouldn't you just hook it up to the sparkplug wire?

    Though, when I think "analog", what I'm really thinking is "not digital".

    For a cabled tach, I would go off the flower nut (which is threaded in the center) and gear it to match or off the smaller bevel wheel, and just drill a small hole in the center of the bolt to mount to, which is really the true RPM of the motor.

    Or is this not right?

    :::EDIT ADD:::

    Found em : "Inductive Tachometers" or "Inductive Hour Meter" is what they're called, they do exist, and there's no work to be done, as it just counts a spark as a rotation.
    Last edited: May 22, 2011
  16. loquin

    loquin Active Member


    Wanted to being up a couple of points:

    first, if you us a bridge rectifier with the white wire output from the HT, you must use an isolated ground on any circuitry after the bridge - else, half of the bridge gets shorted. So, your battery/tach circuit must use a separate, isolated ground.

    Second; I believe that you'll want to add a 12.6V zener diode across the 3.3K resistor. Since the mag voltage increases with RPM, it's possible that the voltage could damage the tach at high RPMs. The zener would eliminate this possibility. I 'marked up' your first schematic to show this diode...

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    Last edited: May 22, 2011
  17. MikeJ

    MikeJ Member

    Hi Loquin -

    Thanks for responding. Yes, you are correct that the engine and tach circuit must be of separate grounds. I tried what you noted here, tying grounds of the source and tach together. RPM dropped to exactly half immediately, indicating rectifying diodes got bypassed. No harm was done, I just noted "Do not do that again!"

    The Zener diode addition is a good idea; I did not think of limiting voltage input to the tach product. Someone posted in another thread that they observed +18 volts after rectifying and filtering the white wire output at high rpm. I would think that tach manufacturers put in overvoltage protection, but I have yet to see proof. An extra Zener would be trivial to install.

    IF manufacturers use the 555 timer chip to make a one-shot circuit to drive the D'arsonval movement with PWM, the trigger fires at around 1/3 battery voltage. Trigger voltage probably should not exceed battery voltage, so I would recommend going for a 9 volt Zener. That still gives lots of room to trigger an output pulse. I have to play with that and observe if the engine performance is affected.

    But first, put priorities in line: 1) Replace engine (wimpy at this >5500 ft high altitude; is good at below 3000 feet altitude) with something much bigger, 2) shake out bugs, 3) put on a few hundred miles. Interleave the analog tach effort when not riding.

    Will catch you later!