Simple, low power way to buffer a 500mV threshold signal to an open collector output?
Here is a simple circuit using only discrete transistors and a few resistors.. total BOM cost should be less than 1 cent in quantity.







simulate this circuit &ndash; Schematic created using CircuitLab

It's a bit trickier than it might be otherwise because your margin between on and off is rather small. Threshold voltages on inexpensive JFETs and MOSFETs are not that well controlled.  This circuit will pull OUT low (say with 10K load to +3.3) when the input is higher than about 550mV. Below that voltage, Q1 hogs the current flowing through R1 and Q2 is starved, thus Q3 does not get hardly any base current. 

Or just use an IC comparator of your choice  with R3/R4 on one input. 

If you want to reduce current further and your load will be no lower than 50K, consider increasing the resistor values by 5:1 (say 470K 5% for R1/R2 and 100K 1% / 20K 1% for the R3/R4 divider). Or use 499K 1% for R1/R2 but precision is not important there. Total current drain should be less than 35uA not counting the load, which is comparable to (or less than) self-discharge of many batteries.