Newer fire alarm devices operate over 2 conductors rather than having separate power wires ran to each device. This configuration delivers power so that devices like smoke detectors can function, while detecting alarm condition over the same pair of conductors. In my circuit the devices are not supervised and alarms are not latching because that is not the purpose of this demo.
*added 10 ohm resistor to account for wire length and 1 ohm resistor to account for contact resistance*
-Credit : Hurz
