interference with infinite gain op amp

Discussion in 'The Projects Forum' started by aruna1, Sep 23, 2009.

  1. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    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 evan 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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  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    It's not in an "infinite gain mode". It's configured as an inverting amplifier. R5 is the feedback with C1 as a high frequency rolloff element.

    One big problem is that the emitter can hardly have a milliamp of current. It's drop is about 1.5 volts. The op amp isn't rail-to-rail, so it can only swing the output to perhaps 3.5 volts, leaving only 2 volts to pull current through a 3K resistor - that's only 2/3's of a milliamp. Not many photons at that rate.

    Nevertheless, if it works, your problem is probably in layout. Is this on a breadboard or a PCB? Check all your connections for quality.
     
  3. aruna1

    Thread Starter Active Member

    Dec 20, 2008
    46
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    my bad i forgot to emove 2M resistor.it shouldnt be there.and actually that 3K should be 100 ohms(forgot to change default value).and i built it on a vero board
     
  4. aruna1

    Thread Starter Active Member

    Dec 20, 2008
    46
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    and what is rail to rail?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Look at an OPA340PA. It's single supply, meaning it can use ground as the negative supply. It is also rail-to-rail, meaning it can swing the output to the power supply potentials - 0 volts and +5 in your case.

    That circuit running with no feedback on a prototyping board is going to be overly sensitive.
     
  6. aruna1

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    well actuall ts not a protobord (?) its a vero board (copper board with lotsof holes)
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    As I said in the other website's forum, almost everything is wrong with your amplifier.
    But now you changed it and removed its feedback resistor to make it even worse.

    The minimum supply voltage for the TL084 opamp is a lot higher than yours. Its inputs do not work properly if their voltage is less than +3V and yours is less.

    You should use the rail-to-rail cmos opamp circuit that was shown in the other forum.
     
  8. aruna1

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    hmm that op amp is not available
     
  9. aruna1

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    i think i found the soluton,i'll post new schematc.
     
  10. aruna1

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    here,this one seems to work.tested few times
     
  11. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    0
    here is a video.not good quality.if you watch carefully u will see as i move white-black paper(bottom) leds are begin to light one by one(above).

    (wish i have 3 hands so i can use first hand to hold sensor,second to move paper and third to hold the camera.its hard to do three hands's job with two hands[​IMG])

    use vlc or km player(or any other 3gp compatible player)

    @audioguru, practically it works,i dont know theories,he he:)
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Like i said on the other website, Texas instruments invented and manufactures your TL084. They also say that everything is wrong with your circuit.
     
  13. aruna1

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    how do you know who invented it?,there is ST version too
     
  14. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Audioguru and I have both been in this business long enough to have been around when this device was introduced.

    hgmjr
     
  15. aruna1

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    ok then.
    anyway why don't you guys give a try to following circuit and see it your self
     
  16. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I don't make circuits wrongly like you do.
    I would use the proper opamp in the proper circuit with a proper supply voltage.
     
  17. aruna1

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    sure no prob
    thanks anyway:)
     
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