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Interfierence with infinite gain op amp

  1. Sep 23, 2009 #1
    guys I'm making this color sensor.i have used 2 RED LEDs as both sensor and emitter. I have used TL084 op amp in infinite gain mode because Sensor LED generates very small voltage.
    my problem is op amp is too sensitive so it reacts when i tough circuit or negative wire (some times even my fingers reach the circuit [without even touching])

    this made circuit kind a unstable.other than that circuit works fine.
    any solution for this?

    here is the schematic

    thanks
     

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  3. Sep 23, 2009 #2
    reduce its gain by changing 2M resistor to something like 100K or 10K.
     
  4. Sep 23, 2009 #3

    berkeman

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    That is not how you amplify a photocurrent (like from an LED or photodiode). Instead, you should use a current-to-voltage converter configuration for the first stage. Google current to voltage converter photodiode amp, and you should get some representative circuits. Or your textbook or any opamp cookbook will have the circuit.
     
  5. Sep 23, 2009 #4

    vk6kro

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    I put a red LED across the input of a digital multimeter.

    It was able to generate up to a volt when held near a 60 watt lamp bulb.
    It had no trouble generating hundreds of mV at greater distances from the lamp.

    So, these are not small voltages. Note that the cathode of the LED generated +ve voltage relative to the anode.

    You could verify that your LED was generating voltage by using a digital multimeter, yourself.

    I would try something like the attached diagram with a lot less gain than you have now. As a start, try making the resistor across the Opamp 100 K and the input resistor 4.7 K. This will give a gain of about 20.
     

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  6. Sep 23, 2009 #5
    well i use another LED to emitte light so voltage generated on sensor LED is small.

    anyway i think i solved it with this circuit
     

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  7. Sep 23, 2009 #6

    vk6kro

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    Looks like that would give a voltage comparator action.
    ie your output light would be either on or off.

    Is that what you want?
     
  8. Sep 23, 2009 #7
    yup.just to detect black or white surface
     
  9. Sep 24, 2009 #8

    berkeman

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    If I were interviewing you for an electronics job, and I asked you to draw me a circuit to detect light, and you drew me that "fixed" circuit, what do you think would happen next?
     
  10. Sep 24, 2009 #9

    vk6kro

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    I checked the TL084 data sheet and it shouldn't have a voltage less than 7 volts as a supply.

    Also it does not say you can operate inputs close to or at either supply voltage rail, so you probably can't.

    So, you are probably losing a lot of sensitivity by using this circuit.

    However it is your post, so if it seems to be working for you then go for it.

    An easy test would be to put a voltmeter across the LED and slowly bring a light close to the LED. Measure what voltage it takes to make the output change state.
     
  11. Sep 24, 2009 #10
    well,i'll increase voltage.anyway do you think this op amp is not suitable to amplify voltage generated in led?.if it can be done with this op amp,can u suggest a circuit?.and also can you suggest a suitable common op amp for this? We cant use 741 here right?.lmc6482 is not available.
     
  12. Sep 24, 2009 #11
    ACTUALLY WHAT I WANT DO IS THIS.
    I NEED TO MAKE A SENSOR TO DETECT WHITE SURFACE IN BLACK BACKGROUND.SO WHAT I THOUGT WAS BY EMMITING RED LIGHT TO BOTH SURFACES,WHITE WILL REFLECT WHILE BLACK WILL ABSORBE LIGHT.SO IN SENSOR I THOUGT IF LED GENERATES VOLTAGE WHEN RECIEVING LIGHT I CAN USE IT HERE COZ WHITE SURFACE WILL REFLECT LIGHT AND SENSOR LED WILL GENERATE FEW mv WHEN IT RECIEVED THIS REFLECTED LIGHT FROM WHITE SURFACE.SINCE BLACK SURFACE WILL ABSORB LIGHT SENSOR WILL NOT GENERATE ANY VOLTAGE.
    SINCE GENERATING VOLTAGE IS LOW AND HAVE ONLY TINY CURRENT I THOUGT FET BASED OP AMP WILL SUIT THIS.BECUSE FET DOES NOT REQUIRE INPUT CURRENT.AND SINCE I WANT DIGITAL OUT(WHITE SURFACE OR NOT) I PUT POTENTIOMETER TO POSITVE INPUT TO SET REFERENCE VOLTAGE.
    AND SET SENSOR LED IN REVERSE MODE(positive pin to negative supply and negative pin to inverting input pin).
    SO IF VOLTAGE GENERATE ON LED BY REFLECED LIGHT IS LARGER THAN REFERENCE VOLTAGE OP AMP OUTPUT WILL BECOME LOW.OTHERWISE IT WILLBE HIGH.

    ACCORDING TO THIS I DESIGN ABOVE CIRCUIT.
    SO TELL ME WHATS WRONG WITH IT.
    THANKS
     
  13. Sep 24, 2009 #12

    vk6kro

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    I attached a circuit above. Post #4.

    That opamp is fine but it can't operate with the inputs close to the supply rails. Most opamps can't do this.

    The LED generates a voltage but this will vanish quite quickly if you try to draw current from it. It may require a large resistor to work properly but check that after construction.

    If you want on/off output, make the ratio of the feedback resistor and the input resistor to the -ve input as high as possible.

    Can you use room lighting instead of the other LED? It would make the job easier.
     
  14. Sep 24, 2009 #13
    no room light.its for line follwer robot.no background light is accepted

    and can you explain how your circuit works
     
  15. Sep 24, 2009 #14

    vk6kro

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    Have a look at this link:
    http://upload.wikimedia.org/wikiped.../300px-Op-Amp_Non-Inverting_Amplifier.svg.png

    It is a standard circuit.
    You can read the full Wikipedia article here:
    http://en.wikipedia.org/wiki/Operational_amplifier_applications

    With a single power supply, you use a resistive divider to form an artificial ground. So this is the "earth" symbol in the diagram. This is normally bypassed with a capacitor to one of the supply rails if you were using it for audio. In this case, it probably doesn't matter.
    This is the wiper contact on the pot in the diagram above.

    The LED is a voltage generator with a high internal resistance, so it looks odd in this circuit, but as a voltage generator it is normal to put it there.

    Just a typo from above:
    "It may require a large resistor to work properly"
    This would be a large value resistor across the LED. Maybe 2.2 M.
     
  16. Sep 24, 2009 #15
    ok thanks,I'll check
     
  17. Sep 24, 2009 #16
    and what is the minimum input voltage for this opamp?
     
  18. Sep 24, 2009 #17

    uart

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    A couple of quick comments on your circuit.

    1. As a passive detector you're usually better off using the LED in reverse biased "photo-current" mode as berkeman suggested.

    2. If you do want to use it in the active "photovoltaic mode" as per your circuit then you have the LED polarity wrong. As a photovoltaic device it will generate a positive voltage at the anode in response to incident light. That is, forward biased voltage direction but reverse biased current direction (quadrant IV) in that mode.

    3. You should use an op-amp termed "single power supply" which means that it will allow input voltages referenced (typically) to the negative supply. Try LM324 or similar.

    4. You should use a very small amount of positive feedback to give the circuit a little bit of hysteresis (not too much though or it will latch up). You'll need to know what you're doing to get this right so you're likely to need further help or research to do it.
     
  19. Sep 24, 2009 #18

    vk6kro

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    A couple of quick comments on your circuit.

    1. As a passive detector you're usually better off using the LED in reverse biased "photo-current" mode as berkeman suggested.

    Why is that? He wants to use it in photovoltaic mode.

    2. If you do want to use it in the active "photovoltaic mode" as per your circuit then you have the LED polarity wrong. As a photovoltaic device it will generate a positive voltage at the anode in response to incident light. That is, forward biased voltage direction but reverse biased current direction (quadrant IV) in that mode.

    No, he wants the output to drop as light level increases.

    3. You should use an op-amp termed "single power supply" which means that it will allow input voltages referenced (typically) to the negative supply. Try LM324 or similar.

    Good idea. Forgot about the 324.

    4. You should use a very small amount of positive feedback to give the circuit a little bit of hysteresis (not too much though or it will latch up). You'll need to know what you're doing to get this right so you're likely to need further help or research to do it.

    He hasn't got much input signal. I was reluctant to mention hysteresis.
     
  20. Sep 24, 2009 #19
    thanks uart and vk6kro.
     
  21. Sep 24, 2009 #20

    uart

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    I didn't say there is anything wrong with using photovoltaic mode (if done correctly) just pointing out that for a simple detector that reverse biased photocurrent mode typically gives a more robust solution with more output swing and better noise immunity. Were these factors not a significant part of auna’s original problems here?

    You've completely missed the point, I'm not referring to the op-amp output. A photo-diode develops a positive voltage at the anode, but auna1 has the anode connected to the negative supply and cathode connected to the op-amp input terminal. When illuminated the cathode will be driven to more negative than that negative supply rail. Do you really still think that is what he wants!

    Presumably aruna actually has this correct in his circuit (or it couldn't work) and has just drawn it incorrectly in the schematic (I expect he just tried it either way until he got it to work). I corrected him because I think it's important to get things like that right. Many people (apparently yourself included) think that photovoltaics work the opposite polarity to what they actually do. I hate repeating myself but you're making me do it. In photovoltaic mode the diode develops a "forward biased diode polarity" voltage (that is, anode positive and cathode negative) and any current that it supplies flows in the "reverse bias diode" direction (that is, out of the anode). Can you not see that aruna has this the wrong way around in every schematic he has posted?

    That's why I said "a very small amount" :) And note that this wouldnt even be an issue if he was set up properly in reverse bias photocurrent mode.
     
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