Oscillating output from op amp with analogRead

[MarkT]

unless you're in a field far from any power lines, you can pickup mains ac interference.  The higher
the circuit impedance, the more susceptible it is to pickup.

In a typical home the stray ac voltage fields around the place can easily be dozens of volts.

And if you have a light sensor near any mains powered lighting, the light itself is modulated
at twice the mains frequency.

[Southpark]

Yup i know the serial plotter axes arent that reliable, 1.5s period is from my own timing.

What method did you use for your own timing? And what scale is the x-axis in that plot?

[Wangmaster]

I ran several tests and found that most likely, it was the fluorescent tube in my lab causing aliasing issues in my readings. Running the test in the dark with just my LED seems to solve the problem, adding a capacitor was icing on the cake.

I discounted the fluorescent lighting as the root cause earlier since the un-amplified photo-transistor reading seemed stable enough in Fl light. The aliasing waveform probably became significant once amplified 45 times.

Resolved, thanks for the help all.