Parts of a Smoke Alarm – Horn, Sensing Chamber, Battery

All 12 parts on the diagram

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The alarm 8

Cover The plastic cover. Its openings let smoke reach the sensing chamber inside.
Test button Sounds the alarm if the power supply, electronics and horn all work. On many alarms the same button also hushes a nuisance alarm for a few minutes.
Indicator light It is a small LED. Its flashes show that the alarm has power, and on some alarms which unit set off an alarm.
Sensing chamber Smoke drifts into this chamber and is detected there. The second drawing cuts open its two kinds.
Horn A small electronic horn that makes the alarm sound.
Circuit board A printed circuit board with the electronic parts on it. It watches the chamber, sounds the horn and keeps a check on the battery.
Battery Some alarms run on batteries alone. Wired-in alarms may have a battery as back-up power.
Mounting bracket It is fixed to the ceiling or wall. The alarm locks onto it.

The two chambers 4

Plates (electrodes) There are two charged plates. The ions in the air between them carry a small, steady current, and smoke drifting in disrupts it.
Americium source A very small amount of americium-241. Its alpha particles ionize the air between the plates.
Light source A small lamp, often an infrared LED. Its beam points away from the light sensor.
Light sensor (photocell) In clean air no light from the beam reaches it. Smoke in the beam scatters light onto it, and the alarm sounds.

How this list was compiled: Every name checked on 2026-09-30, a name kept only where two independent sources use it: Wikipedia, text only, Smoke detector, https://en.wikipedia.org/wiki/Smoke_detector (cover, test button, horn, sensing chamber, ionization chamber, electrodes, americium-241 source, light source, photodiode, battery); How Products Are Made, volume 2, Smoke Detector, http://www.madehow.com/Volume-2/Smoke-Detector.html (housing, alarm horn, printed circuit board, sensing chamber, electrodes, americium source, test button); the US Environmental Protection Agency, Americium in Ionization Smoke Detectors, https://www.epa.gov/radtown/americium-ionization-smoke-detectors (americium source, two charged plates, flow of ions); the US Consumer Product Safety Commission, publication 559, https://www.cpsc.gov/s3fs-public/559.pdf (ionization chamber and its plates; light beam and photocell); the US Fire Administration, Smoke Alarms, https://www.usfa.fema.gov/prevention/home-fires/prepare-for-fire/smoke-alarms/ (the two types, hush button, interconnected alarms); and two makers' alarm documents on archive.org, https://archive.org/details/manualsonline-id-dc541b4c-30c4-401a-b5b4-d2fec64ceb45 and https://archive.org/details/manuallib-id-2409352 (cover, test button, indicator LED, horn, sensing chamber, battery, mounting bracket). Circuit board is also in a third maker's manual, https://archive.org/details/manualzilla-id-7285769. Hints and detail lines are in our own words. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no maker's alarm, nothing traced. Licence: own work

How it works

A smoke alarm is a small case holding a sensing chamber, a circuit board, a horn and a battery. The first drawing names them. The second cuts open the two kinds of chamber, ionization and photoelectric.

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How each chamber senses smoke

In an ionization chamber a tiny americium source gives off alpha particles. They charge the air between two plates, so a small current flows. Smoke drifting in breaks that flow of ions, and the alarm sounds. In a photoelectric chamber a light source shines away from the light sensor, so in clean air the sensor sees no light. Smoke in the beam scatters light onto the sensor, and the alarm sounds. Either way the circuit board drives the horn. The test button checks that the power supply, the electronics and the horn all work.

The US Fire Administration recommends having both kinds of alarm, or dual-sensor alarms, which hold both.
The EPA says never to tamper with an ionization smoke detector. This drawing is for learning the names.

Questions and answers

What are the parts of a smoke alarm?

A cover with openings, a test button and an indicator light; inside, a sensing chamber, a circuit board, a horn and a battery or a wired power supply; and a mounting bracket that fixes the alarm to the ceiling or wall.

What is the difference between ionization and photoelectric smoke alarms?

The sensing chamber. An ionization alarm passes a small current between two plates and sounds when smoke disrupts it. A photoelectric alarm has a light source and a light sensor, and sounds when smoke changes the light that reaches the sensor.

What is the radioactive part in a smoke alarm?

A small americium-241 source, found only in ionization alarms. It gives off alpha particles, which ionize the air in the chamber so that a current can flow between the two plates.

What does the button on a smoke alarm do?

It is the test button. Pressing it sounds the alarm if the power supply, the electronics and the horn are working. On many alarms the same button also hushes a nuisance alarm for a few minutes.