Stormin Mormon formulated the question :
Walmart has a discount smoke detector for #3.97.
I know. I bought one for myself today for the hall,
and spent 10.97 and got the one with the light, for
over the bed.

The four dollar one came with a junk battery, need
alkaline 9 volt cell, sold separately.

Knowing that most people put em on the shelf and
ignore them, please offer installation with the
present. I used two drywall screws and a cordless
drill to install mine.

I took all of mine down. They were the 117 volt a/c type with the 9
volt alkaline battery backup.
I now use My Poms to alarm us. New type alarms are here in the box,
waiting for someone to install them.