Smoke Detector

Overview

A smoke detector is a fixed sensing device that continuously monitors the air for the products of combustion and sounds a local alarm when they exceed a threshold. This unit is a combination alarm: it carries both a photoelectric smoke chamber and an electrochemical carbon-monoxide cell, so it responds to visible smoke from smoldering fires and to the colorless CO produced by incomplete combustion. Mounted on a ceiling, it protects up to about 900 square feet and links wirelessly to other units so any one detector triggers them all.

Air enters the Housing through vents into the Smoke Chamber, a light-trapping labyrinth holding an infrared LED and an offset photodiode. The CO Sensor is a three-electrode electrochemical cell behind a charcoal filter. Both feed the Main Board, where an MCU and a precision ADC evaluate the signals against alarm thresholds. When an alarm is declared the Sounder produces an 85 dB temporal tone with a spoken annunciation, while the Wireless Interconnect broadcasts the event over a sub-GHz RF link. The Power Module module runs the unit from mains with a sealed ten-year lithium cell for backup.

How it works

The photoelectric chamber works on light scattering. Its IR LED pulses into the labyrinth, where baffles block ambient light from reaching the photodiode. In clean air almost no light reaches the photodiode; when smoke particles enter, they scatter the beam and a fraction reaches the sensor, raising its output. The MCU watches this rise against a sensitivity window of roughly 1.5 to 3.5 percent per foot obscuration to reject dust while catching real smoke. The CO cell separately generates a current proportional to carbon-monoxide concentration, and the firmware integrates exposure over time, alarming faster at high concentrations. Either condition drives the piezo horn in its distinct temporal pattern, T-3 for fire and T-4 for CO, and asserts the RF interconnect so the whole network sounds together.