help in phy project- detailed working of a fire alarm?

Answer 1

Based on closed vs. open circuit.

Without a fire/smoke, the alarm system is open...no current flows. It's like turning off your ceiling lights by opening a wall switch...no juice...no alarm in the fire alarm circuit. There are several ways to close the circuit, which sets off the alarm.

Smoke detectors use the physics that clear air is less conductive than dirty (smoky) air. So there is typically a gap in the smoke detector that, when filled with smoke, starts to conduct electricity and set off the alarm.

Heat detectors use infrared detectors that close the alram circuit when a sudden rise or excessive heat is detected. Note that the gradient, the rate of rise, and not the temperature from the heat is what usually sets these guys off. This prevents the alarm from going off as your warm body walks slowly by the detector. But excessive heat, above body heat, cooking, and such, is also used. So even if Fido goes running by the detector, his heat is too low to set it off.

Fire alarms can be internal only or sent to a monitoring service outside. Internal systems will make loud annoying sounds designed to be heard over loud TV's or while asleep. Monitored system will usually make the same kinds of sound and also send a signal by phone line to the monitoring site and, some, directly to a fire station. The monitors typically alert the fire station and then may call the source of the alarm to confirm the alarm and see if the occupants are OK.