Is it possible to run a photodiode plus op amp on a 3V battery?
Just because the battery voltage is 3V doesn't mean there isn't some sort of switching regulator to boost the voltage. However, even if there isn't it is possible to achieve using just a single 3V rail, 2 op-amps, a couple of resistors, and a photodiode.

You just need an op-amp which can operate on low voltages. You will either need to bias the non-inverting reference point, or put an capacitor switch pump to generate a negative rail.

Here's an example with a dual op-amp package, one which is used to provide a virtual ground reference.

You'll want an op-amp which has very low noise, low input bias current, and low input offset current. Something like this TL032.







simulate this circuit &ndash; Schematic created using CircuitLab

The output is \$V=I_D R_G\$. Make \$R_G\$ small enough to avoid saturating the op-amp, but large enough to give you the full resolution. This is designed to operate with a differential voltage sensor, you can in theory measure Vout+ single-endedly, but it won't necessarily work as well because you'll miss out on fluctuations on the 1.5V reference.

Oh, and it probably goes without saying that this circuit doesn't include any of the logic/adc stuff. That is all extra, but simple microcontroller with a differential ADC should be sufficient to get a reading (these can operate in the 3V range as well).