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Thread: Creating a log marker using the MVPI Pro A/D input

  1. #1

    Creating a log marker using the MVPI Pro A/D input

    My MVPI is semi-permanently installed and I cannot get to it while driving for sure. Does the schematic below seem to be OK for a log marker? I am basically just inputting a 5v signal from a 12v/5v converter. I am going to put in a 5v circuit anyways for adding additional sensors.


    I've heard that the A/D inputs have a floating voltage when disconnected, which is why I am switching it to ground when the switch is off. I am not sure if the resistor is needed but seems to be added insurance in case something goes awry.

    I initially thought of using a momentary switch just to mark a spot, but thought a regular switch provides more flexibility in case I find the need to identify a section of the log. It would be more easily incorporated into a chart, or using a filter.

  2. #2
    Sorry, don't know why it posted the picture upside down, it is rightside up on my computer!

  3. #3
    I did some experimenting and thought I would post my findings in case someone else is interested. I hooked it up as shown in the picture above, EXCEPT that I had already wired in the 5v supply and the switch, so the resistor ended up being put between the switch and the MVPI. Tried various resistors and with the 5.2V output of my power supply:

    1k ohm showed about 4.85v on the scanner.
    20k-40k showed high 3.??v.
    100k ohm was above 3v (3.5?).
    510k ohm, which is what I settled on, shows 2.99v on the scanner. That is enough for log marking or using as a filter in a chart, with some safety built in for any problems with the 5v supply.

    Other notes.
    - With nothing attached to the MVPI at all, logging that channel showed around 0.45 - 0.5v.
    - My switch when "off", which grounds the circuit, drops the voltage on the scanner instantly to 1.??v, and then it slowly bleeds down to below 1v. There must be some capacitance in the reference voltage in the MVPI. I suspect if I had put the resistor between the 5v power and the switch as was originally intended (as the diagram above shows), it would go instantly to 0v. Maybe next time.