Problems while using op-amp as a comparator on a single supply
As you're using an AC input, a much less complicated configuration would be this. 

I'm not claiming that your signal levels and gains are necessarily ideal, but am just reproducing them, to illustrate the change in design for single rail operation.







simulate this circuit &ndash; Schematic created using CircuitLab

R1 and R2 establish a voltage of about 1.66v on OA1, 1/3rd of rail to deal with the 0-3.5v VCM range. In this inverting configuration, their impedance doesn't really matter as they go to the +ve input, they could be in the 10k or even 100k range. (However (and I later realised I'd done this more or less automatically) I've sized them so they have a parallel resistance of 1k. This is what you'd use if you had a non-inverting configuration, as then they'd form the divider bottom resistor with a 1k resistance down to the virtual ground.)

Given the relatively large +500mV signal you're looking for, you could simply put pot R5 from rail to rail. However, its adjustment range can be reduced like this with R6 and R7. You could choose R5, 6, 7 values to give 1.5v at the bottom of R5 and 2.5v at the top, using a 1k pot, 1.5k for R7 and 2.5k for R6 (or 2.4k or 2.7k, there's a wide adjustment range after all). Or you could choose a tighter, or looser, range around your trip point of 1.66v+500mV.

The resistor string R1,2 will track the resistor string R5,6,7, so the stability in the face of temperature and rail voltage fluctuations will be 
quite reasonable.