Fire - Smoke and CO Detectors
Cost about $60 each. Cheapest to buy direct from Kiddie. Check Kiddie site for the latest model. Want hardwired AC, Smoke and CO, replaceable (ideally rechargeable) batteries.
Need to use 14/03 wiring in a continuous dedicated circuit. Note that it needs a red wire which is used as the interconnect. Could use thinner than 14 gauge but it is probably cheaper and easier to use regular 14 household wire.
They have a timer inside that sounds an alarm after 7 or 10 years from first use. When it is end of life they need to be replaced.
Can buy in packs of three.
http://www.homedepot.com/catalog/pdfImages/92/928a4004-505f-4770-addd-3b95bb3f2c7a.pdf