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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!

Sean Michael Ragan

Sean Michael Ragan

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 makezine.com. My work has also appeared in ReadyMade, c’t – Magazin für Computertechnik, and The Wall Street Journal.


  • Anonymous

    Lol, Mosfet the cat :)

    • Anonymous

      Because he’s not only high maintenance, but high impedance as well?

  • http://pulse.yahoo.com/_NZK5IVRFCW4XXHL7RGKG4JGGLU Chort

    This is only the “ionization” type of smoke detector. Most commercial applications (office buildings, large public facilities) use optical type sensors. An optical emitter and detector are facing each other in a small chamber. Smoke in the chamber changes the light transmission signature and triggers an alarm. This allows for a more variable state of detection, not just an on or off state.

    • http://twitter.com/engineerguytwit Bill Hammack

      Good point. We had a section in the script that alas had to end up on the cutting room floor. I noted that these are only used for homes (domestic) and not for industrial and other uses. Here is the section of the script that didn’t make it in: “As with all engineered objects this is the right device for only the right situation. In Japan for example, early versions of this alarm could not be used because the high humidity mimiced smoke and triggered the alarm. And it is really appropriate only for domestic or home use. Smoke detection is considered more important than the detection of fire by temperature change, because the asphyxiating effect of
      smoke from foam plastics is the prime cause of death in domestic fires. Industrial fires are more complex, and often involve very fast rates of temperature rise, sometimes with very little smoke.”