Help my OP Amp Amplification decreases over time

Thread Starter

NathanielZhu

Joined Dec 5, 2011
50
Hi,
So I'm using an OP amp Tl072 to amplify a blue LED output as a way to use a Blue LED as a color sensor.
I have the op amp itself wired up like this: https://blog.adafruit.com/wp-content/uploads/2012/05/Screen-Shot-2012-05-29-at-8.18.31-PM.png
Plus, a few other stuff so that I can read the voltage output with a multimeter....

Anyhow, the problem is this
Say at time 0, Voltage (dark) = 1V. Voltage (light)= 2V.
Wait 10 seconds, V(dark) = 1V, but voltage (light) = 1.9V.
Wait 60 seconds (dark) = 1V but voltage (light) = 1.5V.
Wait a long enough time, dark = 1V, light = 1.01V.

Somehow over time, the amplification is completely reduced to near 0.

And this decreasing voltage happens as long as the battery is still connected (18V input). Once I disconnect it and wait a while, it sort of resets back to time 0.

How do I keep the voltage constant as time 0?
 
Last edited:

JohnInTX

Joined Jun 26, 2012
4,111
All there is in your picture is a simple non-inverting amplifier with a gain of 2 that is not hooked to anything. Where's the LED, input circuitry etc? Do you have a resistor in series with the LED? Your post says TL072 but the picture says LM358 - which is it? You will get better help if you post a complete circuit, preferably a schematic with accurate part numbers.

Have you measured the battery voltage while all of this is going on?
 

GopherT

Joined Nov 23, 2012
8,012
Hi,
So I'm using an OP amp Tl072 to amplify a blue LED output as a way to use a Blue LED as a color sensor.
I have the op amp itself wired up like this: https://blog.adafruit.com/wp-content/uploads/2012/05/Screen-Shot-2012-05-29-at-8.18.31-PM.png
Plus, a few other stuff so that I can read the voltage output with a multimeter....

Anyhow, the problem is this
Say at time 0, Voltage (dark) = 1V. Voltage (light)= 2V.
Wait 10 seconds, V(dark) = 1V, but voltage (light) = 1.9V.
Wait 60 seconds (dark) = 1V but voltage (light) = 1.5V.
Wait a long enough time, dark = 1V, light = 1.01V.

Somehow over time, the amplification is completely reduced to near 0.

And this decreasing voltage happens as long as the battery is still connected (18V input). Once I disconnect it and wait a while, it sort of resets back to time 0.

How do I keep the voltage constant as time 0?
If your signal source is connected to the non-inverting input of the op amp with a capacitor, then you need a path to ground for the input signal. Where the capaciator is on an output of an electric guitar or directly on the circuit, your need some path to ground for the capacitor connected to the op amp.

Add a 100k or 1 M resistor from non-inverting input of the op amp to ground.
 
Top