Can AC mains induce a negative voltage in my photodetector circuit?
You've built a little radio receiver (Diode, high gain wideband amplifier), and are probably  receiving something with it. Your multimeter can't show you the high frequency signal, but you get the detected output.

The smart-meter may have a radio transmitter. It's usually part of some sort of mesh network at around 900 or 2400 MHz. The dumb-meter definitely creates magnetic fields at 50 or 60 Hz. Either of these could be picked up by your circuit, or the multimeter itself.

Try some basic EMC mitigation tricks:


Keep all leads short
Twist wires together, particularly the ones between the photodiode and the circuit
put small capacitors everywhere.  100 nF between the power supply rails, 100 pF from the diode to ground and op-amp output to ground, to shunt the RF to ground
Shield the cable going to the photodiode (use a length of coax instead of twisted wire)
Change the length of the wire between circuit and diode - it might be a good antenna at 900 MHz, and a small length change could spoil that.
Don't earth your circuit to the box, that just makes the antenna work better.


You should also consider using a phototransistor instead of a photodiode. Choose one that's sensitive to visible light (most are for IR). They are a hundred times more sensitive. In my experience you can easily connect them straight to the ADC pin of a microcontroller with a simple pull-up resistor; light to dark could be a few volts change.