Monitor 120VAC Smoke Detectors with Alarm Panel

In cases where you wish to monitor a line voltage smoke detector interconnected system as is required in all residential spaces, you can use a basic relay. Interconnected smoke detectors use a 3rd wire to interconnect all smoke detectors in the residential space. When any detector triggers, 9 VDC is applied to the interconnected wire. That is assuming the backup battery is 9V. If other than 9V you will need to make sure your relay can activate on that voltage. How the relay is interfaced into your alarm system or fire panel depends on the input method your system requires. The illustration below depicts a supervised fire circuit.