A smoke detector detects smoke
A smoke detector is sometimes called a fire detector, and is a device that makes a sound when it detects smoke particles. A smoke detector can be one standalone device, or several connected to a fire alarm system. The two most commonly used smoke detectors are optical smoke detectors and ionization smoke detector. The latter species are the cheapest (and most sold). It works with a radioactive source that emits and recaptures particles. The flow between the smoke takes off and the alarm can be activated. The optical detectors transmit a beam to a light-sensitive cell, the smoke will scatter or absorb this beam, causing the alarm to go off.
How does a smoke detector work? [YouTube] 94zx-VketqM [/youtube]