Please School Me On Proximity Switches


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    Default Please School Me On Proximity Switches

    Hi All,

    Another thread has got me thinking about proximity sensors for my current build.

    I already have a control box with everything wired from my first build, Gecko 201X, and the top of the line breakout board from CNCRP (well it was the top of the line several years ago when I bought it, I honestly don't remember the model number, I'd have to do some digging)

    So I will be using the same control box on this build. Previously I used the little square mechanical limit switches. Now I am considering proximity sensors. I have bulkhead connectors, nice and neat, on my control box, for audio cable jacks. In the past I have found these work well for 2 wire mechanical limit switches. I have a bunch of good quality audio cables. I really don't want to redo the bulkhead connections for 3 wire proximity sensors or figure out all of that......

    Also, my limit switches are Normally Open.

    So I found these on EBay:

    DC 2 Wire 6-36V 300mA NO 8mm Inductive Proximity Sensor Switch LJ18A3-8-Z/EX | eBay

    2Pcs 2-wire DC6-36V NO Approach Sensor Inductive Proximity Switch LJ8A3-2-Z/EX | eBay

    Looks like these are two wire proximity sensors. Why do some senors need three wires if these can use only two? I just want to find something with two wires that works and is inexpensive, and not think about it too much. If the simple answer is that they will work for me, really, that's all I need to know.

    They list 6 to 36 VDC as an operating range. I don't recall what the limit switch voltage is on my BOB, but I think 5V rings a bell. I have watched some vids of people with 3D printers using 5V on the 6 to 36V sensors and it worked fine for them, but also, in the vids I saw, they were using 3 wire sensors as far as I could tell.

    Has anyone out there used some of these 2 wire prox sensors? How do they work for you? What voltage did you run them at?

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    Community Moderator Al_The_Man's Avatar
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    Default Re: Please School Me On Proximity Switches

    The difference between 2 wire and 3 wire is the 3 wire are capable of switching higher current, for e.g. they have a NPN or PNP transistor output and they can pick up a relay, if needed.
    The 3 wire are either sink (NPN) or source (PNP).
    The two wire is used for low current required inputs, such as a transistor or opto input device, and can be used in source or sink mode.
    The voltage range is shown on the device or the manuf. literature.

    Al.

    Attached Thumbnails Attached Thumbnails Please School Me On Proximity Switches-sinksource-pdf  
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    Default Re: Please School Me On Proximity Switches

    Quote Originally Posted by Al_The_Man View Post
    The difference between 2 wire and 3 wire is the 3 wire are capable of switching higher current, for e.g. they have a NPN or PNP transistor output and they can pick up a relay, if needed.
    The 3 wire are either sink (NPN) or source (PNP).
    The two wire is used for low current required inputs, such as a transistor or opto input device, and can be used in source or sink mode.
    The voltage range is shown on the device or the manuf. literature.

    Al.
    I assume the current requirement for a limit switch / proximity sensor is pretty low? It says in the specs current output 200 to 300 mA.

    I used to be able to make a 555 timer out of JK flip flops. Now, I have no idea what you're saying. LOL.

    Will a 2 wire proximity switch work for my purposes assuming my breakout board supplies enough voltage? What's your best guess on that? Please keep it simple for me. I'm slow.



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    Default Re: Please School Me On Proximity Switches

    What type of breakout board do you have? Is it a PMDX 126?

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    Default Re: Please School Me On Proximity Switches

    Quote Originally Posted by ger21 View Post
    What type of breakout board do you have? Is it a PMDX 126?
    It was several years ago that I bought it, but yes, that does sound very familiar, I believe that is what I have. I'm looking through a manual I found online just now after reading your post. I don't know what revision I have.



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    Default Re: Please School Me On Proximity Switches

    Most 2 wire will run from 10vdc up and are good for around 200ma.
    That board shows a 12v supply.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

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    Default Re: Please School Me On Proximity Switches

    Thanks ger21 and Al_The_Man for your comments so far!

    So it looks like with my board that I can use the 12V unregulated power for those two wire proximity sensors. Whether the 6V to 36V will work with 5V doesn't seem to be an issue as far as I understand it.

    What I don't yet understand, is the current requirement and the current output listed in the sensor specs.

    For the one sensor I posted a link to from EBay, it says in the title 300mA, but then lower in the description, it says "Current Output 200mA".

    For the second sensor I posted a link to from EBay, it says "Current Output 300mA"

    So I don't know what this means. Does it mean that it needs to draw 300mA to function properly? Or that this is the maximum current that it can handle?

    Thinking about the most basic electronics equation, V = I x R, you could calculate a current based on the resistance, and operating at 12 volts instead of the max of 36, you would only have 1/3 of the amps flowing through the wire. Obviously, this is a bit more complex, and it doesn't give you a resistance, but a "current output" in the specification. No idea how these switches work internally, not do I really want to know all the specifics, because I probably won't understand them without a significant time investment, but it seems that I do need to understand how much current will flow through the wires to see if they are acceptable to use.

    Looking at the user manual for the breakout board, it specifies that the current from all 5V connectors on the regulated power supply can not exceed 350 mA, and from all the connectors on the unregulated 12V supply can not exceed 300mA.

    This looks good at first glance, but for example, if two proximity sensors were activated at the same time, and each draws 300mA, then this would be a problem for me based on the breakout board specs. So someone, please explain what this "current output" specification from the sensor means in practical terms, and if this is only applicable at 36 Volts, with 1/3 of that at 12 Volts?

    Thanks again for all the comments so far.

    Please School Me On Proximity Switches-user-manual-jpg



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    Default Re: Please School Me On Proximity Switches

    The rating on the prox switches should be the max current they can pass. The sensors themselves require very low current to operate so I would think the current rating listed in the ad is how much current they can flow. And if I read it right, I think your board is going to require them to handle 6 milliamps. Just an educated guess. I've never used a 2 wire proximity switch, always used powered hall effect sensors. I thought that prox sensors were active devices that required power to work and that they always had 3 wires; 2 for power and one for the output. But I get lost when terms like PNP and NPN and sinking and sourcing start getting thrown around.



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    Default Re: Please School Me On Proximity Switches

    Two wire pass the current required for the typical input, opto device etc, they retain enough power across them in order to operate the internal electronics, without the need for a +&- supply.
    Al.

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    Default Re: Please School Me On Proximity Switches

    Quote Originally Posted by 1Jumper10 View Post
    And if I read it right, I think your board is going to require them to handle 6 milliamps.
    I don't think so? The board can deliver a maximum current in either 5V or 12V, depending on the pins used, it says don't exceed that current. The amps delivered depends on the resistance of the load. Or, am I misunderstanding the way the board works, and this is current limited, like you can program a VFD to limit the max current regardless of what the load (resistance) demands?

    And thankyou for posting!

    Quote Originally Posted by Al_The_Man View Post
    Two wire pass the current required for the typical input, opto device etc, they retain enough power across them in order to operate the internal electronics, without the need for a +&- supply.
    Al.
    Sir, you are going to have to speak more clearly. I can not understand you.



    Are you saying that I can not use them because they will use too much current given the specs of my break out board?

    Or are you saying that they pass, as in, pass the test, go ahead and use them?

    Or are you referring only to 1Jumper10's comment about 2 wire vs 3 wire and ignoring me all together?

    I'm going to start calling you Boomhauer soon.

    Also, thank you for posting! All posts appreciated! Please post again please (to clarify)!



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    Default Re: Please School Me On Proximity Switches

    Unfortunately many suppliers of these BOB's do not show the nature of the actual input of the board, this often makes it difficult to make an accurate assessment, but the 2 wire consume less total current that the 3 wire, I don't foresee a problem however of using two wire, you may have to try either feeding the + side conductor to the 12v supply and the other to the input, it may also work by connecting the -ve conductor to GND (common) and the other to the input, they will often work either way, again depending on the nature of the input electronics.
    Al.

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    Default Re: Please School Me On Proximity Switches

    Quote Originally Posted by Al_The_Man View Post
    Unfortunately many suppliers of these BOB's do not show the nature of the actual input of the board, this often makes it difficult to make an accurate assessment, but the 2 wire consume less total current that the 3 wire, I don't foresee a problem however of using two wire, you may have to try either feeding the + side conductor to the 12v supply and the other to the input, it may also work by connecting the -ve conductor to GND (common) and the other to the input, they will often work either way, again depending on the nature of the input electronics.
    Al.
    Thanks for the Reply Al_The_Man!

    I think I will go ahead and order some 2 wire prox sensors. I can hook them up to a 12 volt battery perhaps and test the current draw before I use them. They aren't too expensive from EBay.

    I need to decide on a sensing distance. Any advice there is appreciated.

    Also, I was wondering if anyone knows how robust these sensors are? If the sensing distance is small, and I crash the gantry into them at a higher speed, well, I might smash the tip of the prox sensor.

    Perhaps I will have to incorporate some rubber bumpers into the design as well, in which case the proximity sensors will need a longer sensing distance so the sensor trips before the machine contacts the bumper and does not run the risk of smashing the tip of the sensor.

    If anyone has some further advice of the subject, I'm all ears.

    Thanks again to everyone who has replied so far!



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    Default Re: Please School Me On Proximity Switches

    Its better to mount them out of the line of travel of the axis if you can. If something goes wrong it won't cost you a crushed sensor. Plus I think, don't know, that you would get more consistent sensing if the sensor detected an edge as it passed by as opposed to sensing a surface as it approached.

    Sent from my Pixel XL using Tapatalk



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    Default Re: Please School Me On Proximity Switches

    Quote Originally Posted by 1Jumper10 View Post
    Its better to mount them out of the line of travel of the axis if you can. If something goes wrong it won't cost you a crushed sensor. Plus I think, don't know, that you would get more consistent sensing if the sensor detected an edge as it passed by as opposed to sensing a surface as it approached.

    Sent from my Pixel XL using Tapatalk
    That's a great idea. Thank You.



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    Default Re: Please School Me On Proximity Switches

    I'm still torn on what to buy.

    I'm also looking at the square type. They are smaller, looks like they may be easier to incorporate.

    1PC SN04-D Inductive Proximity Sensor Detection Switch Two Wire NO DC 6-36V | eBay

    Anyone have experience with these? The one I linked to is two wire normally open, which is want I want.

    I was also looking through the manual for my BOB.

    Low must be able to sink 6mA at less than 0.8V.

    These proximity sensors don't happen to list their specifications as to what the bleed through current is. I know there will be some, I just don't know if it will be less than 6mA at less than 0.8V. I'd prefer not to have to mess around with additional resistors if the bleed is too high.

    Actually wait a minute, not sure if I'm even reading the manual correctly. The page is still posted in post #7 of this thread if anyone wants to look.

    I also found out that for the proximity sensors we were looking at earlier, the 200 to 300 mA is a maximum and that these typically have a high ohm resistor build into them, so the actually current flow should be way way less than that.



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    Default Re: Please School Me On Proximity Switches

    Just because a device output is rated at a certain limit, it is not necessary to operate it at that current, like I said, it is unfortunate they do not show diagramatical details of the input. I work with industrial equipments of many kinds and it is normal practice to show the I/O details in order to make intelligent decisions on devices used.
    If I were to purchase any other kind of equipment, I would pester the heck out of the supplier if he did not supply this information.
    One of the advantages of 2 wire devices, is they operate as sink or source devices so on equipment that desires just one it does not matter.
    Al.

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    Default Re: Please School Me On Proximity Switches

    Quote Originally Posted by Al_The_Man View Post
    Just because a device output is rated at a certain limit, it is not necessary to operate it at that current, like I said, it is unfortunate they do not show diagramatical details of the input. I work with industrial equipments of many kinds and it is normal practice to show the I/O details in order to make intelligent decisions on devices used.
    If I were to purchase any other kind of equipment, I would pester the heck out of the supplier if he did not supply this information.
    One of the advantages of 2 wire devices, is they operate as sink or source devices so on equipment that desires just one it does not matter.
    Al.
    Thank you for the input. Yes, it appears the maximum current is not an issue.

    I am aware that the SNO4 is being used in the 3d printer community at 5V, just haven't seen the two wire being used there, which is the SNO4-D. I also have the 12V on my board. I just don't know if the bleed through current will be too much for it not to register as an "open" line. Of course if that happens, I guess I can always add a resister to lower the bleed?

    Probably I just just make up my mind and try it out, square or round. It's only around $25 for all my sensors. What's the worst that could happen?

    Al, do you have a preference between the links I posted, as to what you'd try if you were in my shoes?

    That's the thing about ordering from China on EBay, it takes a month or more to arrive, so you can't really buy one and try it.



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    Default Re: Please School Me On Proximity Switches

    Where bouts are you again?
    Ont?
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
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    Default Re: Please School Me On Proximity Switches

    Quote Originally Posted by Al_The_Man View Post
    Where bouts are you again?
    Ont?
    Al.
    I sent you a PM with the coordinates.



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    Default Re: Please School Me On Proximity Switches

    Al_The_Man has just given me a great idea.

    I will be using two pneumatic cylinders, counterbalancing my Z axis. I think the ones I bought are "S" type for sensor. They are stainless steel but I think they have a magnet? At any rate, perhaps I can use two of the two wire pneumatic cylinder sensors for the limits on my Z axis. Does anyone know how they attach to the cylinder or have any other advice or suggestions?



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