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Thread: Car Speed Sensor

  1. #1
    Junior Member
    Join Date
    Oct 2017

    Car Speed Sensor

    Hello, i need the wisdom of the smart electronics people. The project is im building a speedo corrector for a car (im basically putting a focus 2ltr zetec engine into a car built in the 70's).

    The car uses a 3 wire sensor which outputs 10v when in front of metal and 0v when non present.
    The issue is i need to take this 10v signal from the sensor get it down to a safe 5v to feed into the MCU (top circuit) then in the Teensy mess with the signal timings and then out put it back to the ecu (bottom circuit) which im guessing is expecting 10v as normally the 10v signal from the speed sensor goes straight to the ecu.

    I got the idea for this project from this site ( but this one uses a Schmitt Trigger which im not sure i need

    Click image for larger version. 

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    Does the structure of the circuits look good to do the job? i chose to use opto isolators to protect the microcontroller.

    Many thanks


  2. #2
    Senior Member
    Join Date
    Apr 2017
    looks reasonable. not sure how you calculated R3 R4 and CS. The chosen values may filter out fast pulse counts. I've got a similar application where my sensor can't ready above ~2300 pules per min with and 1K and 0.1 uF as filtration is too strong. Hence you may need to experiment with those values.

  3. #3
    The issue could be with what kind of output your sensor has, I'd note in the project linked the sensor output is connected to +5V with a pull-up indicating that it's an open-collector output. is your sensor the same/similar?

    Opto isolating is fine, but probably overkill
    If your sensor drives an output to 10V, you could just whack two resistors to divide the voltage down to 3V3, then add a 5V Zener in for protection, that reduces your first circuit to 3 components.
    On your way back to the ecu, you could use a MOSFET rather than an isolator, though you'd have to invert the signal in logic, reducing your second circuit to two components.

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