Most simple way to detect small voltage changes(~150mV)
You are wanting to detect Changes in Voltage ,not absolute voltage .This means that you can capacitively couple to remove the nominal 18VDC component .You can easily amplify the AC signal that represents changes in voltage .A simple transistor or opamp will do to give a gain of say 10 .This nominal 1.5 Volt peak signal could go into an analog input for processing .