Interfacing to End Of Line Resistor In Security System
use a voltage divider with comparators instead of op-amps,  eg LM339 is way cheap and has 4 comparator sections.  the outputs can be paralleled if you don't need to tell the difference between resistance too high and resistance too low.







simulate this circuit &ndash; Schematic created using CircuitLab

here is shown an alarm zone circuit with terminating resistor and a mixture of normally closed and normally open sensors  and the circuitry inside the alarm 

R1,R2,R3 make a voltage divider with two taps, one at 45% and the other at 55% this voltage divider can be shared with the other zones in the alarm. 

so long as the external alarm circuit presents about 2K ohms the divider it forms with R4 it will give a result between these two voltages and both comparitors will be in the high state in this state their output is undriven and R8 pulls the ~ERROR output high 

U1 takes power from the alarm battery (12V or 24V) this means the accetable window is a volt or more so a few hundered millivolts of inductive interferance on the alarm loop will not trigger the alarm.  

The pull-up R8 connects to the microcontroller power supply making the comparator output compatible with the microcontroller input voltage,  if the microcontroller input has an internal pull-up this part can be omitted.