Switching transistor question

Discussion in 'General Electronics Chat' started by bisctboy, Jun 28, 2009.

  1. bisctboy

    Thread Starter Member

    Nov 3, 2008
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    The attached picture shows how a switching transistor can use the 1v from a solar cell to turn on a light. Is it possible to reverse the transistor so that the switch turns on when no light is present? In other words, do transistors come in "normally closed" and "normally open" versions? I need to put both types in my circuit. If the sun is shining, I need current to flow in one part of my circuit while blocking the flow of current in another part of my circuit. Conversely, when the sun is not shining, I need the opposite to be true.

    Is the above the difference between npn and pnp?

    Thanks,
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Well Said.


    You always need a base resistor, a 1K ohm is agood value to start, and remember that the load current is less than the Ic of the transistor, and also heat dissipation is one thing to keep in mind.

    Rifaa
     
  4. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Here is a quick example of a inverter
    When solar cell has no light, than all current flows through LED, turning it on.
    When solar cell has light on it than transistor begins to conduct drawing current away from the LED, causing the LED to shut off.

    Click on the top left hand corner to enlarge it.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Transistor theory revisionists have proved unpopular around here; however a review of accepted theory is always a good thing. ;)

    It's easier to use LDRs (light-dependent resistors) to make light/dark detectors. Here are a matched pair:
    [​IMG]
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Sgt had said it all

    Rifaa
     
    Last edited: Jun 28, 2009
  7. Audioguru

    New Member

    Dec 20, 2007
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    A small solar cell won't produce enough base current to damage a transistor.
     
  8. bisctboy

    Thread Starter Member

    Nov 3, 2008
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    That's what I thought as well since I am using a 2v (.5v 400mA X 4) solar cell. I am having a hard time "writing" what I want to do so I have attached a hand-sketched picture of what I'm trying to accomplish. Obviously, my sketch lacks the resisitors, capacitors, etc that go with the IC's. It's just a rough draft. How would I implement transistors or Sgt Wookies "Light Detectors" into my circuit? I know they have to be connected to my solar cell to detect it's voltage.

    Thanks,
     
  9. ELECTRONERD

    Senior Member

    May 26, 2009
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    Instead of a solar cell, I know of a circuit that includes a CdS cell to simply detect whether light is off.
     
  10. steveb

    Senior Member

    Jul 3, 2008
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    The closest thing to a "normally-closed transistor" is a depletion mode FET. JFETs are usually depletion mode and you can get MOSFETs in depletion mode, although they are more rare than enhancement mode. JFETs will typically have higher resistance than MOSFETs.

    Ab enhancment mode MOSFETs or a bipolar junction transistor (like you've shown in your figure) would be like a "normally-open transistor".

    I'm not necessarily suggesting you use FETs in this applications. I'm just answering your question.
     
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