A fire alarm system consists of a number of interconnected devices that detect the presence of smoke, fire, carbon monoxide, or other emergencies and alert people by visual and audible devices. These alarms can be set on manually using devices like manual call points or pull stations, or they can be set on automatically by smoke detectors and heat detectors. Alarms come in the form of wall-mounted sounders, horns, or mechanised bells. They can also be speaker strobes that flash an alarm and then play an evacuation message warning occupants not to use the elevators or evacuate the place.