Having problems building a 2-to-1 Multiplexer/Selector

Discussion in 'The Projects Forum' started by Aven Cherus, Oct 11, 2015.

  1. Aven Cherus

    Thread Starter New Member

    Oct 11, 2015
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    I'm electronics newbie, and I'm trying to learn little by little how to build certain components for digital logic circuits. So my design and troubleshooting skills are a quite undeveloped at the moment.

    Right now I'm a bit stuck trying to create a multiplexer out of transistors and such on a bread board with a 9V battery. I'm missing something when it comes to converting the abstract schematic into a more concrete version that I can use to actually build the device.

    I've attached the design I came up with. Around it are push buttons and then LEDs to show what's active.

    If anyone can offer some pointers or specific resources, that would be much appreciated.

    https://drive.google.com/open?id=0BxM4Z1C-377uVVFrTGNYbmpqQUk

    https://drive.google.com/open?id=0BxM4Z1C-377uLWUyME5Ra1JQems
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,653
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    Your circuit will probably work, ignoring the signal levels and offsets involved, provided that the emitters of the two output transistors are connected together. Be careful about reverse emitter-base breakdown causing problems.

    Note: One transistor's collector does not appear to be connected; I assume it is connected to the positive power supply.That transistor is in the second column, third from the top.
     
  3. JSCV

    New Member

    Oct 3, 2015
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    It looks like you are shorting positive and negative when you make ''SEL'' high.
     
  4. Aven Cherus

    Thread Starter New Member

    Oct 11, 2015
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    Oh, well, that's actually just me being awful at copying my diagram accurately. XD I think my eyes got crossed. I missed some other things too, let me update it.

    https://drive.google.com/open?id=0BxM4Z1C-377uc2I4LVN3VGY4MTQ

    Thanks, I'll go do some research on reverse emitter-base breakdown. It's a new concept for me.
     
  5. Aven Cherus

    Thread Starter New Member

    Oct 11, 2015
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    Thanks for the reply.

    How should I handle it? It's connected by a push button that is connected to the power rail. Does this potential feeding those transistor bases need to go somewhere in this kind of situation? I could have the totally wrong idea about how they work, but what I imagine is that the charge is attracted and blocked by insulation. Yet the charge is close enough that is attracts electrons to the edge of the material to complete the circuit. Is that accurate?

    One of the biggest things for me is trying to wrap my head around what do I need to consider since these are all using the same 9v battery.
     
  6. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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  7. Aven Cherus

    Thread Starter New Member

    Oct 11, 2015
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  8. djsfantasi

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    Apr 11, 2010
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  9. Aven Cherus

    Thread Starter New Member

    Oct 11, 2015
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    Well I was hoping to have it always enabling the bottom part of that AND operation, and when the selector opened the transistor it would divert to ground, shutting it off.

    But that wasn't how it worked out. XD

    I didn't label the resistance, because I'm so terribly new to all of this, I don't know what values are ideal. I've been mostly using 10k for transistor bases.

    If you don't mind explaining the short you see in this and what it would be doing, I would be grateful.
     
  10. Aven Cherus

    Thread Starter New Member

    Oct 11, 2015
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    I did solve this. My selector NOT needed another transistor and some diodes to do what I wanted. I did a lot of tinkering and through trial and error I discovered an arrangement that worked, but I can't fully explain the principles in it yet.

    I also ditched some of the transistors, because I really don't need any amplification or control over the power that leaves the output line.
     
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