Some Help.

Discussion in 'The Projects Forum' started by BBQ, Jun 1, 2014.

  1. BBQ

    Thread Starter New Member

    Jan 22, 2014
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    I'm looking for some assistance,I have a 12v low water level circuit based around the 741 op amp that remains on at all times what I'm looking to do is to have a basic circuit that will temporarily interrupt the supply going to it when another sensor is activated...The type of of circuit I'm looking for is when a transistor is switched on the other transistor switches off disconnecting the supply,I have seen one on google but cannot locate it, I have seen a couple of 2 transistor inverting circuits if that is correct name ? but need some guidance..


    Hope I explained it correctly lol

    Cheers
     
    Last edited: Jun 1, 2014
  2. crutschow

    Expert

    Mar 14, 2008
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    Need to know the voltage and current levels, both for the signal and the voltage you want to switch.
     
  3. BBQ

    Thread Starter New Member

    Jan 22, 2014
    23
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    Hi . Yes sorry forgot to mention 12 v and the circuit 125 mA... the circuit I can't find it now used 2 or 3 2n2222 if I remember correctly....

    Hope it helps thank you
     
  4. #12

    Expert

    Nov 30, 2010
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    Some unknown sensor turns a transistor on and you want the water sensor circuit to lose power when that happens? So many ways to do this. Let me think about it for a minute...

    OK, here's a first stab at it.
     
  5. BBQ

    Thread Starter New Member

    Jan 22, 2014
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    I will try this later on today and I will get back to you this evening #12 thanks again for the circuit
     
  6. #12

    Expert

    Nov 30, 2010
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    I hope you know the output voltage of the water sensor circuit will float to +12V when it gets shut off. If you want it the other way, reverse the polarity of the transistors and use the ideas in the drawing to get it the way you want it.
     
  7. BBQ

    Thread Starter New Member

    Jan 22, 2014
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    #12 When you mentioned reverse the polarity of the transistors what do you mean....I'm a newbie lol.

    Thanks again
     
  8. #12

    Expert

    Nov 30, 2010
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    The opposite polarity of an NPN transistor is a PNP transistor.
     
  9. BBQ

    Thread Starter New Member

    Jan 22, 2014
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    It's almost working the way I want it to but there is 3.6 v which is still going through to the sensor and still operational....I believe these can operate from as little as 3.3v how can I get it so It's 0v #12.

    Thanks Appreciated
     
  10. #12

    Expert

    Nov 30, 2010
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    The 9.1K resistor is the one that tells the transistor on the left to shut down the transistor on the right. If your sensor isn't hitting it with 12 volts, less resistance is needed in the 9.1K position. Try dropping it to 4.7k or 3.3k. Even 1k won't burn it up.
     
  11. BBQ

    Thread Starter New Member

    Jan 22, 2014
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    Great thanks I will give it a shot tomorrow after work ...what type of transistor configuration is this called for future reference.
     
  12. #12

    Expert

    Nov 30, 2010
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    I don't think it has a name, but then I was designing Sziklai pairs 40 years before I heard they have a name. To me, it's just a, "normally on" transistor and a "dump" transistor. Talk to anybody with a college degree and they'll think I'm nuts. :D
     
  13. BBQ

    Thread Starter New Member

    Jan 22, 2014
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    I tried the suggestions and went down to 1k the sensor is still on.... is there any other way of doing this #12

    Thanks
     
  14. #12

    Expert

    Nov 30, 2010
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    It seems to me there must be a mistake. Referring to this drawing with R1, R2, Q1, and Q2 I ask, How does Q2 allow current flow when it's base is grounded? It can't. Therefore, the unknown sensor is not shutting off all the way during the time it's not on.

    I say this not because I'm lazy or stubborn, I say it because guessing at another circuit for you without knowing the real properties of what you're working with is just another mistake waiting to happen. I COULD guess again, now that I know the unknown sensor doesn't go to ground when it's not on, but I'd just be guessing again.
     
  15. BBQ

    Thread Starter New Member

    Jan 22, 2014
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  16. BBQ

    Thread Starter New Member

    Jan 22, 2014
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    I disconnected the breadboard supply and used a 9v battery working perfect
    disconnected battery then reconnected the bench supply at 9v working fine up adjusted up to 9.5v sensor comes on....What do you think #12
     
  17. #12

    Expert

    Nov 30, 2010
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    The circuit I gave you cuts the ground, not the supply.
    I still don't know what the unknown sensor sends to the circuit I designed.
    I could design it to cut off the supply, but I still need to know what voltage and/or current the unknown sensor sends to my circuit.
     
  18. BBQ

    Thread Starter New Member

    Jan 22, 2014
    23
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    The sensor sends 12v #12 sorry I should have explained it all better
     
  19. #12

    Expert

    Nov 30, 2010
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    If the unknown sensor sends 12 volts, the original design should work.
    Use a wire to short the base of Q2 to the emitter of Q2 and see if this shuts off the current flow. When 12 volts arrives at R1, it should turn Q1 on, and Q1 shorts the base of Q2 to ground.

    You're going to have to do some experiments to find what has gone wrong.
     
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