Why does this circuit work.

Discussion in 'General Electronics Chat' started by Nick Bacon, May 16, 2017.

  1. Nick Bacon

    Thread Starter Member

    Nov 7, 2016
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    As you all probably know, I am not an electronics engineer or had any training. I just try and understand why the things I do work or don't work.

    In an effort to take a signal and convert it into on and off states, I used a darlington array. I found that I could take a 12V signal feed into one channel on a darlington and turn on an led. I also wanted to activate a relay and have another led that was illuminated when the 12V signal was off. By applying a 1k resistor to the output of one channel and feeding that output to the input of another channel, the output of the other channel was inverted. It works brilliantly for switching a number of circuits from just one input.

    My question is why does it work. I have attached a quick schematic. if 12V is applied to Pin 1 then LED 1 is illuminated and LED 2 not. When 0V is applied to Pin 1, then LED 2 is illuminated and LED 1 not.

    I am just interested to know why is works.

    DAR.jpg
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Its a Darlington output chip, so if you put a high on the input, the output goes low, and vice-versa, the second gate just inverts the first gate. QED
     
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  3. Mike Mitchell

    New Member

    May 16, 2017
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    Note R3 is acting as a pull-up for IN2, but IN1 is floating when the switch is opened. This Darlington driver might not care because it uses BJTs that need current to operate, but most CMOS logic would act erratically because it would not know what to do when the switch is open, and pickup static charges.
     
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  4. Nick Bacon

    Thread Starter Member

    Nov 7, 2016
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    Thanks Dave, good to know I am not doing something that is bad practice.
     
  5. dl324

    AAC Fanatic!

    Mar 30, 2015
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    UDN2981 is a buffer, not an inverter:
    upload_2017-5-16_10-29-55.png
    upload_2017-5-16_10-30-26.png
     
  6. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Schematic isn't well drawn. Most would use the gates arranged to convey function vs using the IC footprint. If that's what your schematic editor does, you're stuck with it.
     
    Last edited: May 16, 2017
  7. dl324

    AAC Fanatic!

    Mar 30, 2015
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    If I didn't make any mistakes transcribing your drawing, it doesn't do anything useful:
    upload_2017-5-16_10-49-20.png
    I substituted CD4050 for the buffer because I didn't want to bother creating one.

    The buffers can't sink current so the LEDs can never turn on.
     
  8. Nick Bacon

    Thread Starter Member

    Nov 7, 2016
    130
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    As I said it was a quick drawing to show my point and not a full schematic.
     
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