DIY tachometer for cnc spindle

Discussion in 'Digital Circuit Design' started by rotccapt, Jul 30, 2016.

  1. rotccapt

    Thread Starter New Member

    Jul 30, 2016
    6
    1
    Im sorry if this is in the wrong forum please move it if it needs to be moved.

    I am working on a tach for a cnc spindle i am making. I am trying to make a ready made unit that is designed for the rc industry that is used to plug in parallel to the hall sensor for an electronic ignition unit. So i am having trouble trying to figure out what i need to do to make the tach work without the ignition unit.

    So the tach is rated for 3.3v to 12v dc. I will be running it on 5v. My trouble shooting thus far has been taking voltage readings to try and figure out what is going on. If i hook the tach directly to the battery and sensor the tach will initialize and show 0 rpm but no mater how fast i spin the magnet over the sensor i do not get a reading on the tach if i hook it back up with the ignition everything works correctly.

    so with the ignition unit powered by a 4 cell nicad:
    v+ is 4.8v
    signal no magnet is 4.8v
    signal with magnet is 0v

    with the sensor plugged directly into the battery:
    v+ is 4.8v
    signal no magnet is 0.7v
    signal with magnet 0v

    plugging my test lead directly into the sensor lead on the ignition unit without the hall sensor i get:
    v+ 4.8v
    signal 4.8v

    i tired applying pos voltage to the signal line but it still did not work. i think i may need an amplifier on the signal line to bring the voltage up to 5v but im not sure. thank you for your help.

    Attached is a pic of the unit i am working with
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
  3. rotccapt

    Thread Starter New Member

    Jul 30, 2016
    6
    1
    So if I understand the article you provided, I need to add a resister from the v+ line to the signal line and that should make it work for me? After looking at the board it is using a pic4f628a best I can tell
     
  4. rotccapt

    Thread Starter New Member

    Jul 30, 2016
    6
    1
    Maybe it's a 16f628a
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    The 16f628a is still a popular pic for starter.
    It almost looks as though the output is a transistor and the collector just exits via the
    Red wire, if so sure, looks like open collector.
    And Yes, if NPN open collector a 10k or so from output to +ve shoould provide an output swing.
    Max.
     
  6. rotccapt

    Thread Starter New Member

    Jul 30, 2016
    6
    1
    Thank you for your help, sure enough I added a 10k between signal and v+ and it works like a charm. Do you think a 1/4 watt resister wound be ok in this situation
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    Are you intending to use the sensor on its own with a self designed circuit etc?
    If so the device or system you are using would have to be known, but generally 10k should work, it is not all that critical.
    max.
     
  8. rotccapt

    Thread Starter New Member

    Jul 30, 2016
    6
    1
    This will be used on it's own, I will be getting the +5 volts from the battery eliminating circuit from a rc brushless motor speed control to power the tach. i will be using the tach to monitor the rpm of the spindle so i can set the proper spindle speed for the material i am cutting. I will probably make a small circuit board to pull the +5 and ground from the esc to power everything and give a place for the pull up resistor and all the connections but that is it.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    How do you intend monitoring the output of the sensor? What is it feeding?
    You usually need some kind of indicator.
    Max.
     
  10. rotccapt

    Thread Starter New Member

    Jul 30, 2016
    6
    1
    If you look at the first pic, I have a tachometer that was designed to plug in parallel with a hall effect sensor used for the ignition timing for a rc gas engine that uses a electronic capacitive discharge ignition unit. I wanted to use it on it's own to monitor the rpm of my spindle, it has a 4 character LCD on it that reads rpm x10. The problem I was having was trying to figure out how to make it work without the ignition unit. With your help I found that adding the 10k resistor between the v+ and signal wires on the hall effect sensor fixed the problem I was having.
     
    MaxHeadRoom likes this.
Loading...