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Tachometer Signal Modification Circuit Help

Discussion in 'General Electronics Discussion' started by pHoyt, Jun 24, 2012.

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  1. pHoyt


    Jun 24, 2012
    Hi there,

    I am nearly finished with converting a 1991 Geo Tracker to an all electric vehicle. Unfortunately, I recently discovered that the tach sensor, buried deep in the motor, isn't producing the expected signal. I've been told that the distance between the sensor and the motor shaft is too large so rather than seeing a signal going positive between an expected 4 and 10 VDC, it is going negative by .5 VDC.

    Taking everything apart to adjust that sensor is at least a month long project so I'd rather solve this problem differently.

    I am able to monitor the signal produced on a old 2 channel Kenwood O-scope. The sensor is generating a nice clean square pulse so I'm hoping I can build something that will modifiy the signal to one that the motor controller can use to detect the motor speed.

    Here is a description of the current setup.

    After letting the scope warm up I connect the scope to the tach signal from the senor and the signal ground and adust the trace to show zero volts. I then switch on the EV's ignition which powers up the motor controller and the rest of the EV. Once the controller boots up (it contains a fair amount of computer intelligence) the voltage on the tach sensor line between the sensor and the controller jumps to roughly .5 to .7 positive VDC.

    As soon as I apply throttle and the controller starts to turn the motor I can see the voltage drop to zero with each pulse. In this case, the motor sends two pulses per revolution.

    The controller is expecting a positive going signal between 5 and 14 VDC. The controller supplies a regulated positive 12 VDC to the sensor. The connection between the motor controller and the tach sensor is 3 wires: +12 V, Signal Ground, and the signal wire.

    So my immediate question is if it is possible to design and build a circuit to detect the existing signal, invert it to it is going positive and amplify the voltage at the same time?

    The max rpm of this motor must be kept below 5500 rpm to avoid destroying the motor so having a tach signal that the controller recognizes is critical because the controller will automatically restrict power to keep the motor below the programmed rpm. Once the controller sees an acceptable signal, it will generate another signal to run the original tachomoter in the vehicle's dash.

    Any suggestions? By the way, I'm not an electrical engineer, but I have successfully built various kits and simple ciruits so I can build a circuit if I have a design with a parts list in hand.

  2. Rleo6965


    Jan 22, 2012
    0.5 V was negligible and hard to amplify to larger signal. Noise might distort the signal.
    How about add your own magnetic sensor and place magnet on motor shaft. You might find good magnetic sensor in Ebay.
    Last edited: Jun 24, 2012
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    What information do you have on the sensor?
    If it's powered from a DC supply it is probably a hall effect sensor with a digital output. These are usually 5V-powered; are you sure the controller is providing 12V? Are you sure the sensor is supposed to operate from 12V?
    Is it possible the sensor has an open collector output and needs a pullup resistor before you will see a proper voltage swing?
    It's also possible that there's a problem with the controller board that is preventing the sensor from producing its full output voltage.
    To test this, disconnect the sensor from the controller, independently provide the required power supply to the sensor, connect a 10 kilohm resistor between the signal wire and the positive supply wire, and put the scope on the signal wire.
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