interference with infinite gain op amp

Thread Starter

aruna1

Joined Dec 20, 2008
46
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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beenthere

Joined Apr 20, 2004
15,819
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.
 

Thread Starter

aruna1

Joined Dec 20, 2008
46
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.
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
 

beenthere

Joined Apr 20, 2004
15,819
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.
 

Thread Starter

aruna1

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

Audioguru

Joined Dec 20, 2007
11,248
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.
 

Thread Starter

aruna1

Joined Dec 20, 2008
46
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
)

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

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

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Audioguru

Joined Dec 20, 2007
11,248
Like i said on the other website, Texas instruments invented and manufactures your TL084. They also say that everything is wrong with your circuit.
 

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