I was going to suggest using a PIC with ADC, but I thought that the ADC's needed a fairly low source impedance to drive them...
From the 16F727 data, 9.3 A/D Acquisition Requirements,
"The maximum recommended impedance for analog sources is 10 kΩ."

Is this not true for most of the PICs?
That's why I put the 10K Resistor across the PICs input! The 500k feeds it... you end up having a 50:1 potential divider.