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Thread: Simulating the Tach signal

  1. #1
    Senior Member DrJeff's Avatar
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    Simulating the Tach signal

    As part of the TesLorean project (http://dmctalk.org/showthread.php?13316-The-TesLorean) I'm keeping the stock gauge cluster, but attempting to "re-program" the gauges to present useful information from the Tesla drive unit, cooling system, traction battery, etc. The cluster will be driven by a microcontroller.

    The Tach gauge is proving tricky. It takes GND, 12v, and a signal that comes from the coil and runs to the RPM relay and the Idle Speed relay. I've been experimenting with different signals but no sure success just yet. What I'm looking for is a waveform or the electrical characteristics of the tach signal.

    Thanks
    Jeff

    PS. Just in case you're wondering, the plan is to use the Tach to display the "Miles of Range Remaining" The TesLorean has a 21kWh battery and should get about a 70 mile range. Miles Remaining is guestimated from the battery state of charge, the drive unit power consumption, and recent driving habits (miles/kWh).
    Jeff
    #6313 (lic: DMC-EV Texas), 25k miles, 100% leather, touchpad, 100% LED, dimmable LED dash, remote door lock & Elvis mod, all A/C vents in kneepads, wedgectomy, escutcheon velcro fix, GM door chimer, custom arm rest/storage/controls...

  2. #2
    '82 T3 Turbo FABombjoy's Avatar
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    I'm using a relay with the armature removed to drive my tach. 12v on once side of the relay coil and the tach (switched gnd) output of Megasquirt on the other.

    Provided that you have any sort of 5v/12v/24v/GND pulsed output a similar circuit should be usable. You just need the inductive kickback from a coil field collapse to trigger a tach pulse. Depending on the driving circuit, a blocking diode might be needed as well to keep the 100v+ pulse from entering your controller, assuming it isn't designed to handle such a thing.
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  3. #3
    Senior Member Bitsyncmaster's Avatar
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    The RPM relay requires greater than a 12 volt pulse. As stated by FABombjoy, that inductive spike is needed to get higher than the cars 12 volts. The stock idle ECU is the same (greater than 12 volt threshold). Now not sure if the tach works the same since I have not tested that threshold.

    I was thinking of building a test (simulator) circuit. I think a square wave greater than 20 volts would work or create that inductive spike like FABombjoy suggested.

    You would need a voltage of 24 volts (or more) and just use a NPN transistor with a pullup resistor on the collector going to that 24 volts. Then drive the base through a resistor with a TTL level (from your micro). The emitter is tied to ground.
    Dave M vin 03572
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  4. #4
    Senior Member DrJeff's Avatar
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    So the 12v relay with the armature removed is acting like a coil, which when denied access to GND will create an inductive spike. Here's a draft diagram of how I'm interpreting this...

    Tach Signal.jpg

    I'm not sure about the resistor Ohms, probably need to experiment to get it into the right range for the Tach.
    Jeff
    #6313 (lic: DMC-EV Texas), 25k miles, 100% leather, touchpad, 100% LED, dimmable LED dash, remote door lock & Elvis mod, all A/C vents in kneepads, wedgectomy, escutcheon velcro fix, GM door chimer, custom arm rest/storage/controls...

  5. #5
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by DrJeff View Post
    So the 12v relay with the armature removed is acting like a coil, which when denied access to GND will create an inductive spike. Here's a draft diagram of how I'm interpreting this...

    Tach Signal.jpg

    I'm not sure about the resistor Ohms, probably need to experiment to get it into the right range for the Tach.
    That may work. I would have put 12 volts to the coil and the other side of the coil to the tach input and the driver (transistor or MOSFET).
    Dave M vin 03572
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  6. #6
    Senior Member DrJeff's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    That may work. I would have put 12 volts to the coil and the other side of the coil to the tach input and the driver (transistor or MOSFET).
    This might be a better representation (the Tach gets 12v power and has access to GND, as well as the Tach signal wire), plus your suggestion (if I understood correctly).

    Tach Signal b.jpg

    If I understand the theory, its the MOSFET closing that causes the inductive current spike in the coil which then runs through the Tach (on the Tach signal line).

    Jeff
    Jeff
    #6313 (lic: DMC-EV Texas), 25k miles, 100% leather, touchpad, 100% LED, dimmable LED dash, remote door lock & Elvis mod, all A/C vents in kneepads, wedgectomy, escutcheon velcro fix, GM door chimer, custom arm rest/storage/controls...

  7. #7
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by DrJeff View Post
    This might be a better representation (the Tach gets 12v power and has access to GND, as well as the Tach signal wire), plus your suggestion (if I understood correctly).

    Tach Signal b.jpg

    If I understand the theory, its the MOSFET closing that causes the inductive current spike in the coil which then runs through the Tach (on the Tach signal line).

    Jeff
    That looks better. What happens is when current is abruptly stopped (driver turning off), the magnetic field collapses and that induces a voltage in the coil. That voltage can than ring (oscillate until the signal falls to zero volts). That's the main reason I prefer to just use a higher voltage rather than using inductive kick. You would know if it rings by your tach showing double or higher RPM.

    You can use a voltage doubler to get 24 volts from 12 volts or a switching type power supply chip (boost converter). Then all you would need is a resistor (the pull up to 24 volts) and your driver.

    If you need a suggestion for ICs to get higher voltage I can help.
    Dave M vin 03572
    http://dm-eng.weebly.com/

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