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interfierence with infinite gain op amp

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aruna1

Member
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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aruna1

Member
You can't just use an opamp with no feedback - if you want exceptionally high gains, use more than one amp - but anything with that degree of gain is going to be overly sensitive.

You also can't just leave the +ve input floating free like that.

Check out Robot Room: Color Sensor from a Reversed LED and Op Amp
i havent let + pin float. i have connected sensor led to + pin.I have already red tutorial in robot room. in it he has connected led to - pin while i connected it to + pin.i dot see how it affect outcome (otherthan inverting otput state)
thanks
 

Nigel Goodwin

Super Moderator
Most Helpful Member
i havent let + pin float. i have connected sensor led to + pin.I have already red tutorial in robot room. in it he has connected led to - pin while i connected it to + pin.i dot see how it affect outcome (otherthan inverting otput state)
thanks

It's floating, a LED doesn't stop that, why not do the same as the working example shown?.
 

aruna1

Member
It's floating, a LED doesn't stop that, why not do the same as the working example shown?.
well what i want to do is use op amp as a voltage comparator.compare voltage generated on LED with inverting input and change output.
but in his circuit he used two op amps for that. s there anyway to do it with one op amp?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
well what i want to do is use op amp as a voltage comparator.compare voltage generated on LED with inverting input and change output.
but in his circuit he used two op amps for that. s there anyway to do it with one op amp?

I think you'll struggle to do that, why not use a double opamp in an 8 pin DIP.
 

audioguru

Well-Known Member
Most Helpful Member
The minimum supply for a TL081 is 7V. Yours is only 5V so it might not work with only 5V.
Its inputs do not work properly when their voltage is within about 3V from its negative power supply voltage which is 0V in your circuit. One of your inputs is less than 1.7V.
The LED causes the voltage at the input of the opamp to go less than 0V.

The RobotRoom circuit uses a Cmos opamp that works when its input voltage is at 0V.
 

audioguru

Well-Known Member
Most Helpful Member
You need to learn about electronics and about electronic parts.
 

audioguru

Well-Known Member
Most Helpful Member
How can it work when the opamp is the wrong type and does not have a supply voltage and input voltages within its specs?
 

aruna1

Member
here is the 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:mad:)

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

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Last edited:

audioguru

Well-Known Member
Most Helpful Member
i use TL084 it works from 5v
The manufacturer says it probably will not work with a 5V supply and its minimum supply is 7V.

Its inputs do not work at the very low voltage that you have them.
 

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  • TL084.PNG
    TL084.PNG
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aruna1

Member
The manufacturer says it probably will not work with a 5V supply and its minimum supply is 7V.

Its inputs do not work at the very low voltage that you have them.
i think it depends on manufacturer.chk this. it has min of 6V
 

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  • TL084-2.pdf
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audioguru

Well-Known Member
Most Helpful Member
Your supply voltage is too low for a TL084 opamp.
Your DC input voltages are too low for a TL084 opamp.
Why don't you do it properly?
 

Dick Cappels

Active Member
aruna1, congratulations -your circuit works. If you even intend to make more than a few of these or offer it for sale, you would do well to heed the advice given earlier in this thread, as ignoring that advice would eventually lead to some amount of heartbreak.
 

aruna1

Member
@dick
sorry sir my english isnt good.did you try my circuit? Is it work or are you just kidding?
I'm open to any comment so you can say anything.
 

aruna1

Member
@audioguru
well,i'll increase supply voltage.will it work or do i have to do anything to sensor led too? 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.
 
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