The light inside never shines on the sensor. So how does the alarm see smoke?

smoke in room

0.00 %/ft

smoke in chamber

0.00 %/ft

light at sensor

4% of trip level

trip level

1.5 %/ft

chip

all clear

horn

off

light

always on

Press the steps in order, or press Auto. It runs in real time. The front cover is cut away so you can see inside.

How it works: Chatham County NC Fire Marshal, How Smoke Alarms Work and Smoke Alarm Facts & Questions (four parts, light angled away from the sensor, smoke scatters light onto it, a preset level, bug screens). NFPA, Smoke Alarm Information (three loud beeps, test monthly, ceiling or high on a wall). NIST Technical Note 1455-1 (smoke measured as %/ft of light blocked; alarms tripped at 1.5 %/ft on average; scattered light rises with the smoke) and NIST Supplementary Q&A (trip level 0.5 to 4.0 %/ft). CPSC, Sound Effectiveness of Residential Smoke Alarms (piezo horn, 3,500 to 4,000 Hz, at least 85 dBA at 10 ft, three-pulse pattern). Sound: OpenStax Physics 14.1 (v = f λ) and LibreTexts College Physics 17.2 (343 m/s). Example values: the smoke timing, the 5 s chamber delay, 4% stray light, the 3,700 Hz horn, the 9 V battery and every size. Smoke specks are drawn far bigger than real. Drawn to scale in millimeters with the Anatomy kit (MIT license).