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Is a transistor sensing circuit output analogue or digital?

  1. Apr 18, 2012 #1
    Is the output from a transistor sensor circuit analogue or digital? I know that you sometimes need a pull up or down resistor due to a very small current flowing through the transistor even when it should be off. However, in the circuit shown below for example, is the output signal analogue or digital. I think it should be analogue but the my notes on transistors are in the digital electronics section?

    http://www.kpsec.freeuk.com/images/tranldr2.gif [Broken]

    Thanks,
    A.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Apr 18, 2012 #2

    NascentOxygen

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    It appears that the intention is that the LED is either ON or OFF. So the light intensity (when present) should be sufficient to saturate the transistor. The 10k adjusts so with ambient lighting the transistor is securely off.
     
  4. Apr 18, 2012 #3
    If you where to graph the output of the transistor against increase light intensity would the graph show a distinct point where the transistor "turns on" like an op amp would. Or does it curve up and then level off?
     
  5. Apr 18, 2012 #4

    NascentOxygen

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    There is no positive feedback, so the transition would be relatively gradual over a narrow band of light intensities.
     
  6. Apr 18, 2012 #5
    End result looks pretty digital to me. The transistor has no degeneration resistor at the emitter. The voltage range at the base between the transistor being off or saturation is very small.

    Yes, you can balance the input so the transistor is on but not saturated, but it is going to be a very small window. All in all, the circuit is basically on and off to me.
     
  7. Apr 19, 2012 #6

    jim hardy

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    Analogue vs digital is more a description of the way we analyze a circuit than it is a description of the circuit itself.

    Take a look at Fairchild AN88. It describes using CMOS logic IC's as linear ampifiers.

    http://www.fairchildsemi.com/an/AN/AN-88.pdf

    My opinion - "Mother Nature built Her universe analog. Logic is an artifact of thought."


    my 2 cents
    old jim
     
  8. Apr 19, 2012 #7

    NascentOxygen

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    Yes, it's digital—except for that region where it's not. :smile:
     
  9. Apr 19, 2012 #8
    attachment.php?attachmentid=46399&stc=1&d=1334830137.png

    Then how does this alteration, adding the resistor between the base leg and the 0V rail reduce the region where its not digital?
     

    Attached Files:

  10. Apr 19, 2012 #9

    NascentOxygen

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    Lowering the B-E resistor will reduce the sensitivity of the circuit, necessitating a more intense light for the change of state. This might be useful in a brightly lit room, or in sunlight, so that the ambient light does not affect the OFF state.
     
  11. Apr 19, 2012 #10
    Like Jim Hardy said, if you really dig deep, everything is analog. It just depend on how you look at it. With your circuits, as long as there is no emitter resistor, the input that the circuit behave as analog in only a very small window, nobody use this as analog where you control the intensity of the LED. Any circuit that looks like this is only designed for on and off.

    If you try, you can make analog circuit like op-amp to become a digital inverter. AND some crazy people in music electronics designed a distortion pedal for guitar using CMOS logic inverter gate 4049 and slapped on feed back resistor and use it as a clipping op-amp!!!! It was so funny but it is on a selling product. The point is it can go either way if you try hard enough.

    Don't get hung on whether it is digital or analog. Look at the application. No engineer in the right mind using the circuits you draw as analog intensity control no matter you add in more adjustment pots or what. You want analog, you put a resistor between the emitter and ground, then you have a wide window where the LED can be control from dim to bright.
     
    Last edited: Apr 19, 2012
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