You are posting to a thread that is a few years old. You should have started a new thread, but I will respond to your question. You cannot modify a DC (battery only) smoke or CO alarm to operate on AC (line voltage). If you are using DC units, consider using Lithium batteries instead of alkaline. They are more expensive but will last much longer. Some models on the market today are using non-replaceable Lithium batteries that claim a 10 year life, but then have to be thrown away at the end of their life. Many models today are able to connect wirelessly to each other.