smoke alarm vs warning device

smoke alarm and warning device both are nouns.

Word	Noun	Adjective	Verb	Adverb
smoke alarm	Yes	No	No	No
warning device	Yes	No	No	No

As nouns, warning device is a hypernym of smoke alarm; that is, warning device is a word with a broader meaning than smoke alarm:

smoke alarm: an alarm that is tripped off by fire or smoke
warning device: a device that signals the occurrence of some undesirable event

Other hypernyms of smoke alarm include alarm, alarm system.

smoke alarm (noun)	warning device (noun)
an alarm that is tripped off by fire or smoke	a device that signals the occurrence of some undesirable event