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.

Hard to see here, sorry, but the small dark circle in the middle of the metal plate is an Am-241 button.