@vuninho . I've experimented using a basic sound detector to identify the frequency (3.1 kHZ) of smoke alarms beeps. The approach is non invasive, not requiring any mods to smoke alarm, and seams to work with most types. It uses a software algorithm called Fast Fourier Transform that you can find libraries for Arduino's, and only requires a few components. Testing shows it was able to identify the beeping of a smoke alarm over 35 feet away, with average background noise in my house. Here's the link to my post that describes the technique if you're interested. http://learntohackit.com/project/fire-alarm-beep-detector .