LED in IDC Loop

Is it possible to have a low-voltage LED across the IDC, and have it light up in alarm? I am aware that my panel can support 2-wire smoke detectors, and when they activate, the panel sends a small amount of voltage across the lines to light up the smoke detector. I first thought of the idea when I hooked an LED to the annunciator terminals on my System Sensor 2451.

No. There is ALWAYS voltage across an IDC except when a switch type device (pull station, mechanical HD, sprinkler switch) is activated. In a 24 volt system there will be around 24 volts across the IDC. That is how there is power to make the 2-wire smoke detectors operate. The LED on the SD is turned on internally by the SD. When a 2-wire detector is in alarm the voltage across the IDC decreases due to the increased current draw by the detector in alarm. An LED placed across the IDC would light all the time and could interfere with the supervision by making a parallel path around the EOLR.

Most panels have relay contacts. You could connect the LED to the panels Aux. power and break a lead at the alarm contact. In the event of an alarm, the LED will light up. That “Click” you hear when the panel goes into alarm is the relay, it should be easy to find. I have a strobe connected to my trouble relay like this. When the panel goes into trouble, the relay opens which closes an external relay i have and activates the strobe.

Another option is to use a pull station with a Dual-Pole Dual-Throw (DPDT) switch, like the Fire-Lite BG-12LA. It has a second set of contacts which close when the station is activated. You can use the first 2 to short the IDC and place the panel in alarm, and connect the second 2 contacts to a separately-powered LED; then the LED would only activate with the pull station.