What's this thang?

[David Meiland]

Photo looking at the popcorn ceiling of a bedroom in a house with cedar-paneled walls, circa 1977. There was one of these in each bedroom, generally in the corner furthest from the door. None in hallways or other rooms. My first thought was 'wireless alarm device' and probably a smoke or CO detector, but the only alarm box installed was original 1977 equipment, the type of thing I would associate with older devices. I couldn't find any other alarm equipment anywhere. I'm sure you guys know what this is.

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[Jerry Simon]

https://www.alarmgrid.com/products/honeywell-5809

[John Kogel]

What Jerry linked is a heat detector.