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  1. 13-02-2020 #19
    cropwell's Avatar
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    cropwell
    Last Activity: 28-05-2023 Has been a member for 9-10 years. Has a total post count of 1,646. Received thanks 170 times, giving thanks to others 30 times.
    Quote Originally Posted by Kitwn View Post
    m_c,
    Rob,
    I think the point you are missing is that the 24v line in the diagram in post #1 is NOT a supply but is an input to a controller which has a pull-up resistor to 24v. When the probe contacts close, the transistor pulls both the controller input and the LED cathode to ground. When the contacts are open the diode action of the LED prevents the controller input being connected to the 5v supply.
    No Kit, I understood, that is why I was wondering what the impedance was (i.e. the value of the pull up resistor). I would presume it is in excess of 10K to keep the current down.

    I have just done a quick emulation :-
    Click image for larger version.  Name: Off.jpg  Views: 2025  Size: 14.7 KB  ID: 27370
    Click image for larger version.  Name: On.jpg  Views: 2066  Size: 15.7 KB  ID: 27371

    So it would seem to be OK apart from the reverse voltage on the LED.
    Last edited by cropwell; 13-02-2020 at 01:18 AM.
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