Lower voltage from 24V to 3.3V

Discussion in 'General Electronics Chat' started by Scalpel78, Jan 24, 2015.

  1. Scalpel78

    Thread Starter Member

    Aug 11, 2013
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    Hi guys,
    I've got some LJ12A3-4-Z/BX inductive proximity sensors that I want to hook up to an input on my board that only accepts 3.3V.

    The sensor is powered with 24V, and the output pin is 24V when no object is detected. When a metallic item comes within 4mm the output falls from 24V to 0V.

    I want to hook that output up to an input on my microcontroller card, but that input only accepts maximum 3.3V.

    I was thinking I'd create a circuitboard to put between the sensor and the mucrocontrollerboard, and that I could put a 3.3V voltage regulator on in, but the ones I have have maximum 20V input.

    What would be the simplest way of reducing the 24V to 3.3V? Add a 12V regulator in front of the 3.3V regulator?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Most proximity's have a open collector output, you could use a Opto isolator IC to transition from the 24v to the 3.3v
    BTW the app note shows 6v - 36v working for the prox. NPN N.O..
    Max.
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
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    Check whether your sensor has open collector or open drain output, it may be as simple as hooking the pull-up resistor to the 3.3V rail instead of 24V.

    If the output has active pull-up to 24V, you'll need an isolating diode so it can only pull the input down when its at 0V - you'll probably need a pull-up resistor to 3.3V for the input, and you'll probably need to use a Shottky-barrier diode for its low forward volt drop.
     
  4. Scalpel78

    Thread Starter Member

    Aug 11, 2013
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    0
    Thanks for the responses guys. How can I tell if the sensor is open collector or open drain?

    Max, I'm not sure what you mean at the end here? N.O..?
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Normally Open, output conducts on presence of metal target.
    You can get NPN or PNP N.O. or N.C.
    The one you have appears to have a 10k pull up resistor, I would tend to go with the Opto for the cleanest solution?
    [​IMG]

    Max.
     
  6. Scalpel78

    Thread Starter Member

    Aug 11, 2013
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    So, something like the HCPL-0900 optocoupler from Avago? Or are there cheaper/better ones?
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Nothing so exotic, the 4N35 is an old standby, 25c?
    Max.
     
  8. #12

    Expert

    Nov 30, 2010
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    Better at what? You haven't said whether you are detecting something once a week or a million times a second.

    and I think Max asked about the temperature.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I don't think there is any danger of that according the the frequency response of the unit shown!!.
    0.5Hz
    No, no temp mentioned?
    Max.
     
    #12 likes this.
  10. Scalpel78

    Thread Starter Member

    Aug 11, 2013
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    Thanks Max, I found the 4N35 at Farnell. I'll try to design a little board with it on :)

    This is for the endstop switches of a CNC machine I'm making. It's going to detect when the machine is at the extreme X, Y and Z positions.
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    Way too complicated. First, to find out if the sensor has an internal pull-up to 24V, power the sensor, and cause it to switch with a Voltmeter between its output pin and its own Gnd connection. If it pulls all the way to 24V, it has the internal pull-up. If you see a low voltage (few mV) with it switched either way, then it is an open-drain/open-collector (it doesn't matter which).

    Here are two ways to wire it depending if it has the internal pull-up or not:

    129.gif

    PS, I just went an reread the first post, and the question is answered. Use the lower circuit...
     
    Last edited: Jan 24, 2015
  12. crutschow

    Expert

    Mar 14, 2008
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    If you don't need ground isolation, you should be able to use a two resistor voltage divider to reduce the 24V to 3.3V.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You mention in your build that you are looking to mill aluminum, you need to really ensure rigidity of the gantry to do any kind of milling on a router type machine.
    Max.
     
  15. Scalpel78

    Thread Starter Member

    Aug 11, 2013
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    Yes, I'm hoping the setup is going to be rigid enough for aluminum. Do you have any CNC experience?

    I found an Opto Isolator Breakoutboard, which might be what I need.
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Only 35yrs doing industrial CNC retro-fits and P.C. based motion control design.;)

    A description of the I/O details would be nice?
    Max.
     
  17. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    That would be by far the simplest way to do it.

    If that is too simple. You can add a buffer to the output of the voltage divider. For buffer you can use op amp in buffer configuration.
     
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