Hard to see here, sorry, but the small dark circle in the middle of the metal plate is an Am-241 button.
Unpacking an Old Smoke Detector
I have been meaning to open this for a while now. This PCB came off an old smoke detector I found in a pile of electronic trash. A lot of these old smoke detector have two major enclosures inside the main unit. The one on the right houses the loudspeaker that plays a ~10kHz sound (also goes off for low battery. The one on the left houses an Am-241 button. The Am-241 releases a torrent of alpha particles into a metal plate. The metal plate is thus partially charged at any given time. If smoke enters the black enclosure, it attenuates the alpha particles at a much greater rate than normal air. The lack of alpha particles charging the metal plate causes the attached chip to send an alarm signal to the loudspeaker.
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