Education Science Technology

Most of our readers will know that one of the most common types of residential smoke detectors actually contains radioactive material—specifically, an isotope of americium—which is used to ionize air molecules in the detector itself. In this video, the always-engaging Bill Hammack, aka Engineer Guy, explains both the ionizing detector and the circuit that contains it, with particular focus on the MOSFET device which is the other critical component in that circuit (and is, incidentally, the namesake of Phil Torrone’s cat). Characteristic Bill quote: “To me, this is engineering at its best: Simple, reliable, and inexpensive. And saving countless lives.” Thanks, Bill, as always!


I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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