doorbell vs night bell

doorbell and night bell both are nouns.

Word	Noun	Adjective	Verb	Adverb
doorbell	Yes	No	No	No
night bell	Yes	No	No	No

As nouns, night bell is a hyponym of doorbell; that is, night bell is a word with a more specific, narrower meaning than doorbell:

doorbell: a push button at an outer door that gives a ringing or buzzing signal when pushed
night bell: a doorbell to be used at night

doorbell (noun)	night bell (noun)
a push button at an outer door that gives a ringing or buzzing signal when pushed	a doorbell to be used at night