Transistor Logic Gate : NOT Gate Explanation?

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Discussion Overview

The discussion revolves around the functioning and design considerations of a NOT gate circuit using a light-dependent resistor (LDR) and transistors. Participants explore the operational principles, potential design flaws, and alternative components that could improve the circuit's efficiency.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes their understanding of the circuit, suggesting that when the LDR is off, it allows current to flow and turns the LED on, while when the LDR is on, it prevents current flow and turns the LED off.
  • Another participant notes that the circuit's behavior is understandable, although the mathematics may be complex, highlighting that the circuit drains the battery due to significant current flow even in the off state.
  • Concerns are raised about the design being odd and possibly intended for experimentation, with some participants agreeing on this point.
  • A participant points out a design flaw where Q5 could turn itself on through collector-base leakage, suggesting the need for a resistor from the base to ground.
  • There is a discussion about the purpose of diodes in the circuit, with one participant questioning which diode could be omitted for simplification.
  • Another participant suggests using integrated circuits like the 4069 or 74C04 as more efficient alternatives that contain multiple NOT gates and draw less current.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement regarding the circuit's design and functionality. While some affirm the original assumptions about the circuit's operation, others critique its design and propose alternative solutions, indicating that multiple competing views remain.

Contextual Notes

Participants mention potential design flaws and the need for additional components without resolving the implications of these suggestions. The discussion includes varying levels of understanding about the circuit's operation and design considerations.

Who May Find This Useful

This discussion may be useful for individuals interested in circuit design, particularly those exploring transistor logic gates and their applications in experimental setups.

cynthia0303
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upload_2016-4-5_1-7-51.png

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 :)
 
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cynthia0303 said:
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.
You still have the diode there with a voltage drop, but the other path has a diode as well (well, two including the LED).
 
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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?
 
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cynthia0303 said:
Is my assumption correct? Please tell me what you guys think?

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.
 
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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 :)
 

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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.)
 
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
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.
 
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Tom.G said:
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.)
Which diode should be omitted? Is it D3 or D4? BTW, Thank you so much for your explanation , you are so helpful!

meBigGuy said:
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
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.
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
 
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cynthia0303 said:
Which diode should be omitted? Is it D3 or D4? BTW,...

Move Q5 Emitter to D4 Anode. Replace D3 with a wire. You're welcome.
 
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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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