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ME TOO!! (A "what is it?" thread...)

Discussion in 'General Electronics Discussion' started by Six_Shooter, Nov 6, 2013.

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  1. Six_Shooter


    Nov 16, 2012

    I have a 1973 Datsun 240Z, these used a current triggered tachometer. A popular upgrade or swap is to use a tachometer from a 1975 to 1977 280Z, since they are nearly the same physically and are points triggered.

    So I have a 1977 280Z tach, that works great, when I use a single coil based ignition system (with a distributor), however I have a DIS (Distributorless) system, and the tach works fine up to about 500 RPM, at which point it just dropped to zero, until RPM came back down below 5000 RPM. I've been using an aftermarket tach for now to get by, but would really like to get the original tach working again.

    So I have a few ideas on how to make it work and I know the problem is caused when the Toff time of the signal drops below 1.48 ms. This Toff time is a pretty consistent for the tach dropping to zero at any RPM. I tested this using a 555 astable circuit with adjustable duty cycle.

    What I'd like to do is modify the original circuit by reverse engineering it, and see what I can use to make the it more tolerable of the input signal, if I can.

    So, on to what I need identified:
    It is a maroon device with 6 pins that looks a lot like a mylar capacitor, just with those 6 pins.
    I have tried running the Hitachi part number through Google but have come up empty thus far.

    So... What is it?

    Attached Files:

  2. davenn

    davenn Moderator

    Sep 5, 2009
    its what they call a hybrid circuit
    its sorta an inbetween discrete components and an intergrated circuit
    inside the ceramic coating there will probably be a ceramic plate with circuit tracks etched onto it, these will be interconnecting a bunch of components resistors capacitors transistors etc.
    It will (most likely) be manufactured specifically for that particular product

  3. Six_Shooter


    Nov 16, 2012
    That's not what I wanted to hear... but also explains why I would get no results when searching Google. :(


    I wonder if it's worth reverse engineer the circuit and trying to figure out what those might do, there's two of them. Capturing the input and output signals might help...

    I'll likely have more luck either making an an interfacing circuit that will make sure the Toff time stays above 1.48 ms. Interestingly the tach works perfect with a Ton time of around 1ms, at any input frequency (RPM), so I'm thinking of some sort of edge detection trigger, that triggers only on the rising edge. So far I've been thinking of a 555 timer in monostable mode, using the signal from teh ignition as a reset. I want to look at some of the CMOS stuff again, to see if it would be better to utilize something in that realm. Last year we discussed an edge trigger device in digital electronics class, but the details are fuzzy. lol

    I've also been thinking about making a new circuit to replace the existing one. The signal to the actual movement seems to be very square pulse like, but never actually reaches 0 volts. I can grab some screen shots and post them, if anyone is interested.

    More testing and experimentation will be needed...
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Yes I would go for an external monostable, to provide a pulse stream that the tachometer will always be able to use.

    You say the tachometer needs a Toff of 1.48 ms or longer, and works with a Ton of around 1 ms. What is the minimum total cycle time at the maximum RPM displayed on the tachometer? I assume it's more than 2.5 ms?

    Google 555 monostable. You'll find thousands of circuits with explanations. The 555 is a reasonable fit for automotive applications, with a maximum supply voltage of 15V. But you need overvoltage suppression, and preferably a voltage regulator (e.g. a 78L08 or similar), to prevent damage from load dump, which can produce a voltage of 70V, sometimes more, on the 12V supply. Have a look at my design at and duplicate the right hand part, up to and including C7. It may be overkill but better safe than sorry IMO.

    Is the old tachometer supposed to get a pulse for every spark? If it's designed to work with a signal from a magnetic pickup, it may be designed to have a pulse every revolution of the crankshaft, which would be half the frequency of a signal picked up from the coil or ignition system (for a four-cylinder engine, and if my recollection of the Otto cycle is right...)
    Last edited: Nov 8, 2013
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