A smoke alarm or detection system may consist of 1 or more units mounted on the ceiling, as either stand-alone units or networked together in a system. Units are triggered when smoke and heat are present in proximity to the device. Temperature sensitive models react to heat; those with photoelectric sensors react to smoke particles in the air. Smoke detectors that use an ionization sensor rely upon chemical reaction, to detect smoke caused by flaming fires. They are the most common and are relatively inexpensive.

When a stand-alone unit is triggered, a fire alarm sound goes off. When one unit of a system is triggered the whole system responds with an audio alarm, which may include a visual alarm as well. Affordable, stand-alone single units can be purchased for under $20.00 and are battery operated. A network of detectors can be either 120V house current powered or battery powered; most units running on house current also have battery back-up in the event of power outage. Batteries for any type unit should be checked at least once monthly, and replaced when necessary.

A fire alarm system consists of:

• Detection devices:heat sensors, photoelectric sensors (for smoke) and ionization sensors (for chemical reaction).
• Alarms:including audio and visual features; such as buzzers, a fire alarm bell, or a fire alarm strobe.
• A fire alarm control panel:the main control unit of a system; connected to a central monitoring station at a home security company or local fire department.
• Manual signal boxes:the common "break glass, and pull switch" type boxes which are manually operated to signal a possible fire emergency.
• An emergency battery system:which kicks on in the event of power outage.
• A wireless push button fire alarm or distress alarm : such as those worn on a necklace or wristband by elderly or disabled people.
• Fire suppression components : such as a fire sprinkler system; engaged in the actual event of a fire.