 I guess the mid-'70s, smoke detectors had been hooked in with the house current, with the electrical system in the house. I think that started in 74, 75, something like that. If that's what you have in your house, don't even think about replacing it with a battery only. A lot of people do that, and it's kind of cheating. The code requires you to replace like for like. So if you have a wired detector now hooked into the house,