Bill takes apart a smoke detector and shows how it uses a radioactive source to generate a tiny current which is disrupted when smoke flows through the sensor. He describes how a special transistor called a MOSFET can be used to detect the tiny current changes.
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Just for clarity: there are also cheaper smoke detectors out there, that rely on optics to detect smoke – but the presented ones are the better…
The cheapest smoke detectors I see are always battery only ionizing units. The optical units can be better depending on the type of fire. Combination units are available too combining optical with heat or ionizing. I think optical is uncommon for residential devices but more common in commercial installations. Optical beam detectors are used in large spaces like gymnasiums and warehouses where they can be located below the ceiling level for earlier detection.