My first thought is that a 1.5 second period is very very slow for an opamp.Given not much information I might guess it is 60 HZ pickup (or 50Hz) that is being sampled at nearly 60 Hz or 30 Hz. The result you see could be the difference frequency between the noise 60 Hz and the sampling frequency.Your multimeter will filter out such variations.Remember, the UNO A/D is like taking a flash photo of someone moving. The photo will show the subject for only that instant. Likewise the UNO A/D will take a measurement that is near instantaneous as far as your project is concerned.
Strangely, I get stable readings with a multimeter, which means the problem may lie with the UNO board. I've attached a reading from the serial plotter where you can see the oscillation waveform.
I'm using a phototransistor in a common emitter configuration to capture the brightness of an LED.
Hi,How do you know the period of the oscillation is 1.5s?You can use the Serial Plotter as the x axis is not a constant time base.Thanks.. Tom..