Fire alarm system works in the following sequence:

1. Event is recorded by an automatic detector or by an individual;

2. Signal is applied to the cable input on the control station;

3. The control system decides which alarm devices must be activated and outputs a signal to activate the device.

Most widely used means of detection of the event are smoke detectors. They are typically mounted in round plastic blocks with an approximate diameter of 150 mm, but can be another shape or size. Since it is important to avoid false alarms, the layout of the detectors must be evaluated carefully. They are usually mounted on the ceiling or near the ceiling and in the typical household - away from the kitchen. They use main grid for power supply or use their own battery and can work independently, to interact with each other or in conjunction with fire alarm / security system.

Most models are photovoltaic (with detection by optical means) or use ionization, but some models can also be combined for greater accuracy. Also, there are models with built-in sound or light signals although the effectiveness of such converged devices is often insufficient.

Photovoltaic detectors use light sensors. They include a light source, a lens with her focus in the beam and photodiode or similar device sensitive to light and placed under appropriate angle. When there is no smoke, the light reaches the sensor without distorting. In the presence of smoke, however, part of the light is diffused and photodiode feels the difference in the amount of received light and activates the alarm. There are models with slightly different components, but the principle at work in all cases is based on measuring the amount of light and respond to changing it.

Optical detectors are more expensive than ionization, but also less prone to false alarms.

Ionization detectors contain a small amount (~ 0.2 mg) of radioactive ameritsiy 241, which emits alpha radiation. This radiation passes through Ionization chamber - filled with air space between two electrodes. Course of radiation causes a constant flow of electricity between the electrodes. Even the smallest amount of smoke entered the chamber absorbs the alpha particles, leading to disruption of ionization, interrupting the flow of electricity and recording of alarm event.

The ameritsiy is used because alpha particles are ionized better and also - because it is slightly radioactive, and its spread can be stopped within the plastic housing of the detector without danger to humans. Moreover, its half-life period is over 430 years and has not to be replaced.