The one Smoke/CO Audio Sensor included with your system can be placed next to any one of the existing smoke and/or carbon monoxide alarms in your home. It detects the beeping sound from your alarm and sends your smartphone a push alert through the included gateway, which is connected to your home’s internet router. It is easy to setup yourself with the included double-sided tape or mounting clip, and the included battery lasts 5+ years for your convenience. If your existing smoke alarms are NOT interconnected then you could benefit from purchasing additional Smoke/CO Audio Sensors to make sure you are alerted no matter which smoke alarm is beeping at your home. The best way to know if your smoke alarms are interconnected is to push the test button on any one of them. If they all make a sound then they are interconnected.