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Thread: Gauge Cluster Tester Mark II

  1. #11
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    One big dial is still missing, the speedometer. The exact design of the corresponding signal can be found in this forum entry.

    Utilizing the new hardware, a separate driving circuit is no longer necessary. One of the output drivers that provide ground can be used to simulate the VSS (Vehicle Speed Sensor). It was a little surprise to me that everything worked right away:



    Testing the speedometer should not take too much time as the odo and trip meter starts running, too. All bench testing combined already put about 20 km on the car without moving it an inch ..

    The number of cables running towards the gauge cluster in above video increased significantly. A nice example why bus communication became, at some point, necessary for modern cars.

    All main functions are running now but three minor issues have been detected and need to be corrected in a new PCB design. We'll talk about those in the next days.
    Last edited by Heineken; Yesterday at 08:48 PM. Reason: Typos
    1997 JDM Custom Order AT VIN 1400005 - Stock
    Heineken's Garage

  2. #12
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    This is the final update to complete the circuit and PCB design. Two issues have been observed while prototyping the tester:

    • The output drivers are too weak to supply the gauge cluster illumination (driver chip overheats)
    • The output drivers internal voltage drop is too high for the fuel gauge (doesn't reach maximum)
    • Not enough ground pins

    There has been some delay as the parts required to solve above issues where stuck in transit for almost two weeks, sorry for that.
    Let's have a more detailed look at the problems and their solution.

    Weak Output Drivers
    The cluster's minimum current usage is ~1 A (at 12 V). Enabling all warning lamps pushes it over 3 A, adding illumination results in more than 4 A. Summing that up we are looking at ~50 W being used by the cluster (with illumination switched on). As most of that current is distributed across a large number of pins it's not a problem for the tester but illumination draws ~1 A across two pins with the left connector being responsible for 2/3rd or 0.75 A.
    Each driver output pin can provide ~0.1 A. Enough for most of the cluster but not for those 0.75 and 0.25 A illumination pins.

    Driver chips with similar characteristics but higher output rating aren't available so it becomes necessary to add more punch to the affected illumination pins by means of a discrete circuit. Out of curiosity, the decision was made to realize it with MOSFET transistors. Those are more and more common today as they typically have very low resistance, fast switching and are often controlled digitally with high frequencies.
    Only the first of those main advantages serves as a bonus in this application but the others won't hurt either.

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    The circuit ..
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    .. the wire connected MOSFET ..
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    .. and the result

    The internal resistance of the MOSTFET is so low (0.05 Ω in case of the IRF5305 used here) that it could handle up to 30 A and therefore didn't even get warm during testing. Note that another pull-down resistor is required to avoid the problems observed during RPM gauge control. Case closed, on to the next change.


    Too high internal Voltage Drop
    Driving the fuel gauge is done by periodically pulling down the reference voltage from the cluster to ground (PWM control). If it's permanently pulled to ground the gauge exceeds the "full" level. Unfortunately, the output drivers have a minimum voltage drop associated with them that prevents a complete pull to ground. The needle stops a few milimeters before hitting "F" so that gauge is not fully testable.

    As mentioned above, MOSFETs feature a very low internal resistance (and a corresponding low voltage drop) so using one of them instead of the driver IC should solve the issue. As the order of the was delayed by several weeks it took a while until the circuit could be tested.

    Click image for larger version. 

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    The circuit ..
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    .. more extra wires ..
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    .. and it works

    The selected IRLD014PBF can be directly controlled by the Arduino and provides an internal resistance of 0.2 Ω. As the fuel gauge is specified to display "F" at 3 Ω, that is indeed low enough as it can be seen above.

    Final location of the two MOSFETs and the one pull-down resistor was decided to be between the two green connector sockets. The 3D view of the PCB editor provides a nice preview of them on the final PCB:

    Click image for larger version. 

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    Not enough Ground Pins

    After one more careful review, new PCBs are going to be ordered. These will include a few more improvements (especially related to layout and pin numbering) and an additional ground pin. Most of the changes can be covered in software but one or two wires need to switch positions on the connector to make everything work.

    Remaining Steps
    Several items remain on the To-Do-List before the project can be considered done. One of them is to be able to return my cluster to the car which means the connection cable to the cluster needs to be documented and a PCB created that allows to check copies of that cable without connecting it to a real life cluster.
    One more important step is to provide a cable supporting the <1995 clusters. As no such cluster is available to me, Drew from NSX Prime is gonna help with that.
    To be able to continue working on the software without risking to break functionality an old Legend cluster (374,000 km) was acquired and needs a matching cable set, too
    A significant amount of work but very much looking forward to it
    Last edited by Heineken; 14-01-2021 at 08:20 PM. Reason: Typos
    1997 JDM Custom Order AT VIN 1400005 - Stock
    Heineken's Garage

  3. #13

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    As always, full of professionalism, I look forward to the end result of your wonderful work

  4. #14
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    Quote Originally Posted by ozon02 View Post
    As always, full of professionalism, I look forward to the end result of your wonderful work
    Thank you very much, that keeps me motivated
    1997 JDM Custom Order AT VIN 1400005 - Stock
    Heineken's Garage

  5. #15
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    looking good, nice to see useful stuff being designed and made
    aka Jonathan!!

    '92 charlotte green auto.... as a daily
    '37 Ford Y street rod......... something for the weekend!

    ...... if a photobucket pic is foggy, click it, and it'll take you to the clear version, yes, it's a clicking faff....

  6. #16
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    Thanks Jonathan
    Double checked the changes to the PCB and layout this evening, correct some labels and going to order the new PCBs on Monday.
    1997 JDM Custom Order AT VIN 1400005 - Stock
    Heineken's Garage

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