I think some more advanced ones ran an internal “UPS” on 9V. Effectively a relay...

		Scoundreller 5 months ago \| parent \| context \| favorite \| on: LM8560, the eternal chip from the 1980 years I think some more advanced ones ran an internal “UPS” on 9V. Effectively a relay that would be held by mains and fail to 9V if the power went out with some replacement 60hz signal. As I recall, it would run the sound alarm but not the display.