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Transistor Logic Gate : NOT Gate Explanation?

  1. Apr 4, 2016 #1
    Hi everyone. I would like to ask about this light sensor using not gate circuit. I use LDR as an input and a LED as an output. I have tried this circuit in Proteus, and it works. When the LDR 'off', LED turns on, and vice versa.
    But i don't really understand about the working principle of this circuit.
    This is my assumption :
    if LDR is off, Q5 will be 'open' cause there is no current flow. So , current will flow from R5 to R9 to Q6 , Q6 is 'short' because basis get the current flow from battery , hence LED is ON.
    if LDR is on, Q5 is short, so the current from the battery will not flow through 1K because it will choose a resistance free way. Hence, the LED is OFF.
    Is my assumption correct? Please tell me what you guys think? I will appreciate that so much :)
    *sorry for my bad english , i'm still learning :)
  2. jcsd
  3. Apr 4, 2016 #2


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    You still have the diode there with a voltage drop, but the other path has a diode as well (well, two including the LED).
  4. Apr 4, 2016 #3
    It seems like an odd circuit to me.

    But the way it works is easy to understand though the math is likely a bit harder.

    Basically there's enough current to forward bias one transistor. (This from the selection of part values, not solely the topology.) So when Q5 is on, Q6 is off. When Q5 is off, Q6 is on.

    This is an odd arrangement in a battery powered circuit because there is significant current flow even in the off state. This drains the battery.

    I wonder what design considerations prompted the choice?
  5. Apr 5, 2016 #4


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    Yes. And you have the right explanation.

    I also agree with Jeff that it is 'an odd circuit', probably designed just for experimenting and learning.
  6. Apr 5, 2016 #5
    Thank you so much guys for your answers. I am using this NOT gate circuit which i found on web. I don't really understand about the design considerations though. Maybe this is only for experimenting :)

    Attached Files:

  7. Apr 5, 2016 #6


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    Looking at your most recent you post, it looks even more like 'only experimenting.' The only purpose I can see for the diodes is to protect the transistors and LED when someone connects the battery backwards. (But it could have been done with just one diode.)
  8. Apr 5, 2016 #7


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    One design flaw in this weird circuit is that Q5 can turn itself on through collector-base leakage. Needs a resistor from base to ground.
    Other than that, it looks like it was designed by one of those genetic evolution circuit generator programs.
    http://www.eecs.harvard.edu/~rad/courses/cs266/papers/koza-sciam03.pdf [Broken]
    Someone is either really smart, and is doing stuff I don't get, or just kept sticking parts together until it finally sort of worked.
    Last edited by a moderator: May 7, 2017
  9. Apr 6, 2016 #8
    Which diode should be omitted? Is it D3 or D4? BTW, Thank you so much for your explanation , you are so helpful!!

    Does the resistor act as a pull-down? My teacher told me once but i don't really understand why.. I never heard of genetic evolution circuit generator programs, it sounds cool but creepy :D
    Last edited by a moderator: May 7, 2017
  10. Apr 6, 2016 #9


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    Move Q5 Emitter to D4 Anode. Replace D3 with a wire. You're welcome.
  11. Apr 7, 2016 #10


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    Much easier: Use a 4069 / 74C04. It contains 6 "not" gates and draws almost no current. Even better: Use a 74C14 - it will perform better around the turn-on/turn-off points.
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