NOR GATE Connection revisited

Discussion in 'The Projects Forum' started by JDR04, Aug 12, 2012.

  1. JDR04

    Thread Starter Active Member

    May 5, 2011
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    Help please, this is driving me nuts.

    In my previous post I mentioned how D1 and D2 which are connected to the comparators outputs glow dimly when I connect the TTL NOR GATE into the circuit as per my schematic and I did not understand why or how I could resolve this issue.

    Another forum member kindly posted a schematic which I have also attached and I tried it out. LED's D1 and D2 still glow so that issue has not been resolved yet.

    I also don't understand why it is neccassary to add a third transistor to the circuit when I already have an output from the NOR GATE?

    Could somebody please help me out on this one - Thanks a lot -JDR04
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,606
    D1 will glow when Q1 is high because it is delivering current thru R9 to Q3. Same with D2, R10 and Q4.

    You have sneak paths there.
     
  3. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Hi ErnieM, so how do I go about resolving this issue. My concern is that it will run the battery flat in no time at all.

    Thanks - JDR04
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Your circuit has multiple possible issues. For example, LM393 cannot sink the current required by an LED and 220Ω in series to 9V. The minimum sink current is only 6 mA.
    What are you really trying to do?
     
  5. JDR04

    Thread Starter Active Member

    May 5, 2011
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    Hi Ron H, I'm trying to connect an LED through the NOR GATE in such a way that it will only light up when both sensors detect darkness at the same time.

    I need some way of indicating which sensor is triggering hence the two LED's. Am I making any sense or am I in the realms of fanatasy??

    I'm also using the project as a learning circuit as I go along and recieve advice from the forum members. Thanks again JDR04
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You can do it like this. Let us know if it works. Ask questions if you don't understand how it works.
    I used the same pot values that you used. I don't know if they are the optimum value.
     
  7. JDR04

    Thread Starter Active Member

    May 5, 2011
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    Thanks Ron H, do appreciate your time on this one. I'll build the circuit on tuesday and let you know how it goes. Thats if the local supplier has the trannies!!!!

    Thanks a lot again- JDR04
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I should point out that it worked in simulation, assuming that the phototransistor/pot combination are suitable for the illumination.:)
     
  9. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Ron H, I built the circuit and it worked great so a big thanks for that. Appreciate the help.

    I was using my scope to take various measurements from the circuit and was measuring Q1 when it seems I somehow damaged the Transistor. I can't be sure exactly at which point but I was measuring at the emitter and the base. Any ideas on this??

    I've done diode tests on the tranny and get nothing compared to when I measure Q2. Done all out of circuit.

    Any ideas as to how I messed it up would be appreciated. Thanks again - JDR04
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    If you shorted the Q1 collector to ground, you could get around 100mA of collector current, That would cause it to dissipate about a watt. If that were sustained for a few seconds, the transistor would die.
    If you shorted the base to ground, even for an instant, the tranny would suffer a fatal myocardial infarction.
     
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