Having a special value resistor in the sensor seems a bit silly, a 50 cent pot can be any value resistor you want and a multi-turn trimmer can be set quite accurately.
Yes, but you need to know the value first!! ![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
Measure the voltage across the sensor? I don't know a lot about alarm systems but even if they used ordinary off the shelf resistors you'd still have to know the value.
Hi James.... I think you missed the original point, so I'll explain
![Cheesy :D](https://www.eevblog.com/forum/Smileys/default/cheesy.gif)
For those more elementary alarm systems, the equipment manufacturers tell you, and 'give' you, the custom
special value EOL resistors, and values. This simplistic 'method' was only a basic deterrent to stop people
from 'trying' off-the-shelf resistors, to bypass the circuit. We only had to make our own, if we didn't have one
handy, and we 'know' what the expected value is for that particular alarm panel. They had quite a small
margin for error!! Slightly too high resistance signals an alarm. Slightly too low causes a Tamper alarm.
(The circuits are normally-closed (with resistor in series) for a no-alarm condition).
Have a great day. Glenn