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Need to use 2 proximity sensor on the same input.

Discussion in 'Sensors and Actuators' started by Simon Girouard, Feb 2, 2018.

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  1. Simon Girouard

    Simon Girouard

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    Nov 18, 2015
    Hello,

    I want to add limit detection to a CNC and for this, I got proximity sensors. Here are the specs:

    LJ12A3-4-Z/BY
    Inductive type proximity switch with PNP Normally Open transistor switch. Output upon detection.
    P/N: LJ12A3-4-Z/BY
    Power: 6-36VDC
    Output Current: 300mA
    Detection Range: 4mm
    Detected material: Iron/Steel alloys

    Note: I already have the sensors and would like to use them and not have to buy something else if at all possible.

    A way I saw to connect this was to apply a 12v source to the sensor and then have the sensor output signal go through a 7805 to the input pin of CNC board. But, this is good for 1 sensor.

    How would I go about if I want to connect 2 sensors to the same input pin on my CNC board?


    Thanks !
     
  2. Tha fios agaibh

    Tha fios agaibh

    2,088
    699
    Aug 11, 2014
    I would just add blocking diodes,
    one for each sensor.
     
  3. Minder

    Minder

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    Apr 24, 2015
    You can OR open collector outputs, What do you mean go through a 7805 (regulator)?
    Your CNC BOB will need to be sinking input due to the sensors being source (PNP) devices.
    M.
     
  4. Tha fios agaibh

    Tha fios agaibh

    2,088
    699
    Aug 11, 2014
    I think he means to use a 7805 to drop 12v to 5v after the sensor.
    I don't know how close to 5v it needs to be, but I was thinking of just using a 6v ps and use diode or two to drop the voltage a bit.

    By the way, what does BOB stand for?
     
  5. Minder

    Minder

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    Apr 24, 2015
    BOB is Break-out-Board in CNC parlance.
    The nature of the board or BOB input would clarify what is needed, if anything, to interface the Prox.
    M.
     
    Tha fios agaibh likes this.
  6. Simon Girouard

    Simon Girouard

    9
    0
    Nov 18, 2015
    Thank you all for your input.

    Yes, BOB means "Break-Out-Box". The pins in a BOB are essentially the pins of the parrallel port on a computer and requires 5v for an "On" state and 0v for an "Off" state. But, by experience, anything between 3.5v and 5v will be good to trigger an "On" state.

    Note: The CNC software allow for programming the input to be "active-high" or "active-low" according to what I need.

    I decided to use a 12v source to power the sensors because the specs shows they require at least 6v and I was afraid that it would be unreliable if I used 5v.

    At first, I was going to use a relay or an opto-isolator to drive the input of the BOB but when I saw someone using a 7805 to drop the voltage to 5v, I thought it may be the cheaper and less complicated way to do this.

    So, no matter if I use a 7805, a relay or other means, I still need to figure out how to hook up 2 sensors before that part of the circuit so a sensor's output voltage will not climb up to the other sensor.

    I understant the "OR"ing concept would do what I need but my knowledge in electronics is somewhat limited an I wouldn't know how to do this in practice. Is the diodes suggestion essentially doing that?
     
  7. Minder

    Minder

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    Apr 24, 2015
    I assume you are using Mach S/W, or similar, I don't quite understand if you have a BOB between the prox inputs and the P.Port, you should be able to use higher voltage than 5v as that would be rather restrictive for a BOB!
    If the inputs on the BOB are simple opto isolators, you just need a series resistor to be within the limit for the opto.
    What BOB is it? post a link.
    And if they are as above then you just parallel the prox sw's.
    M.
     
  8. Simon Girouard

    Simon Girouard

    9
    0
    Nov 18, 2015
    Minder,

    First, thank you for being patient with me. As stated before, my knowledge of electronic is limited and I may very well have misconceptions about certain things that leads me to not understand what may seem obvious to experienced electronicians.

    Yes, I use Mach software on one CNC machine with a Gecko G540. This type of sensor arrangement will also be used on another CNC machine that uses a Linux software with a board named HY-TB3DV-T.

    This is where I got the idea for the sensor hook-up using a 7805 (Note: this is not the board I have):
    [​IMG]
    My concern is not the voltage sent to the BOB or parallel port (or whatever is on the right side [pin 3] of the 7805), it is the voltage that one sensor will output to the other sensor if both sensors are connected in parallel (the black wire from both sensors connected together).
     
  9. Minder

    Minder

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    Apr 24, 2015
    Do you have a part No of the board you are using??
    Two open collectors can normally be paralleled or OR'd either one or both with trip the input.
    M.
     
  10. Kiwi

    Kiwi

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    74
    Jan 28, 2013
    upload_2018-2-4_9-23-17.jpeg
    Try this.
    The 1N4007 diodes will pass the 12v output from either, or both, sensors to the LM7805.
    The output from one sensor can't feed back into the other sensor.
    Think of them as one way valves for electricity. Current will only pass in the direction of the arrow.
     
  11. Simon Girouard

    Simon Girouard

    9
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    Nov 18, 2015

    This is what I was looking for. I will try that.


    Many thanks!
     
  12. Minder

    Minder

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    Apr 24, 2015
    I would still like to see the BOB input details, you may not need all that.
    I am a believer in KISS if possible.
    M.
     
  13. Simon Girouard

    Simon Girouard

    9
    0
    Nov 18, 2015
    One board is a Gecko G540. The other one is a HY-TB3DV-T.

    There are no other boards. The computer's parallel port connects to these.

    I too like to keep it as simple as possible.
     
  14. Bluejets

    Bluejets

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    Oct 5, 2014
    Just wondering on the accuracy of these proximity switches for cnc work.
    Normally any cnc position detection is within fractions of a thou".
    I use slot detectors and nothing special to boot ( removed from old printers) which give 0.3 thou" repeatedly which is good enough for the application I have.
    Open collector operation.
     
  15. Minder

    Minder

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    Apr 24, 2015
    Unfortunately a typical Chinese manual, no details on the input that I can see?
    You can often overcome these issues in the future if using a 2 wire prox.
    M.
     
  16. Minder

    Minder

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    Apr 24, 2015
    Incidentally where did you get the information on the voltage level for the Prox inputs? I see nothing in the manual to indicate it at all, just that they are sinking to D Ccom.
    M.
     
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