Interconnecting your Household Smoke Detectors with your Alarm System and Home Automation System

In the last two houses I've purchased, there has been two separate smoke detection systems - the bare minimum 120V wired smoke detectors - one per floor of the house, and a single low voltage smoke detector wired to the home alarm system. The 120V wired smoke detectors, when wired with 3 conductor (14/3) house wiring - use the red wire as an "interconnect" communication wire. When one detector detects smoke, all the interconnected alarms will sound.

These limitations:

  1. It doesn't make sense to have a wired smoke detector situated 15 feet away from an alarm system smoke detector - wall acne - when only one detector is required for that space;
  2. The smoke detectors on the 120V circuit, when detecting smoke - are not connected to your home alarm system, and therefore cannot send the signal to your alarm monitoring problem that there is a fire;
  3. For additional precaution, there should be a smoke detector in every sleeping room (and especially if there is decentralized heating in those sleeping rooms, like baseboard electric heat). 
Made me consider an upgrade:
  1. Eliminate the duplication between the alarm connected smoke detector, and the 120V wired interconnected system. Use only the 120V wired interconnected system for smoke detection, and connect the interconnect signal to the home alarm panel;
  2. Add interconnected 120V smoke detectors in every sleeping room, in addition to the standard smoke detectors in the hallways on every floor of the house. 
Since you should replace your smoke detectors regularly (usually every 10 years), the last two renovations that I carried out required the replacement of all the smoke detectors in the house. I decided to go with Kidde smoke detectors, they are commonly available at your local home improvement centers, and, Kidde sells as an option a relay module that permits the interconnection of the Kidde wired interconnected detectors with your home alarm system (2 wire contact interface). 

The Kidde smoke detector interface is the SM120X. 
The Kidde SM120X Alarm Interface Module
Installation of the SM120X is quite simple. Two wires connect to 120V line circuit (hot and neutral), one wire connects to the smoke detector "interconnect" wire - usually the red wire in a 14/3 cable interconnecting all your wired smoke detectors, and 2 wires connect to your home alarm system as a dry contact interface. Here are some photos of an installation in a 4" junction box:

Kidde SM120X Alarm Inteconnect Relay Installed in 4" Junction Box. Note 14/3 (wire to alarm daisy chain), 14/2 (line feed wire), and 4 conductor alarm wire entering the box). 

Kidde SM120X Alarm Interconnect Relay Installation - Cover Installed and Labelled using a PTouch Labeller
 In the most previous renovation, I also wanted smoke detector indication to be relayed to my ISY-99i / 994i. To do so, I used small automation relays with multipole contacts, driven by the Kiddes SM120X Alarm Interconnect Relay Module. In this installation, all connections are made on the phoenix contacts terminal blocks (eliminates the marettes used for wire connections). The automation relays drive the EZIO61 - to transfer the smoke detector status to Insteon signals for my ISY-99i, 994i, and also to my standard DCS alarm system panel. In this way, the signals are independent between the alarm dry contacts and the EZIO dry contacts, eliminating any issues with the alarm panel independence.
Kidde SM120X Alarm Inteconnect Relay - driving coil on double pole relay - one set of relay outputs go to alarm panel, one set of relay outputs go to Smartenit EZIO6I Insteon Dry Contact Interface

Kidde also sells a carbon monoxide detector interface - the CO120X. I have purchased one of these for installation in my current panel, I just haven't had the time to install it yet. I'll provide a short update to this page when I do. 

Kidde CO120X Carbon Monoxide Detector Relay Module (Color coded blue to differentiate from SM120X)

As always - if you have any questions just post them in the comments and I'll try to respond fairly quickly. 


  1. Do you have a wiring diagram. I understand the Red, Black and White into existing 120v alarms, but what is tied into panel. Yellow or Orange and ?

  2. You can download the wiring diagram for the SM120X here at this link - please review, and let me know if you have any other questions:

  3. Do you have any specs on interconnecting multiple Kidde detectors ? The information on-line just seems to fall short of what I need to make informed decisions.

    Any help is appreciated.

    1. The initiating detector applies 9 volts DC to the signal line (relative to the neutral line) to indicate an alarm condition. I assume this is to allow the interconnection signal to work even when 110V power is lost - the backup batteries provide the communication voltage.

  4. So I understand the wiring and plan to connect both a SM120X and CO120X to my home's interconnected smoke/CO detectors. But I did have one concern, the wire's leaving the electrical box and connected to somewhere else (in my case a MIMO2+). Is it up to electrical code to do this?

    I plan to have all the connections in my 2nd story attic and did not want to risk my insurance denying coverage if they found something not up to code.

  5. I know you are not supposed to mix low and high voltage wires, which is kinda what is happening here. But maybe I don't fully understand what is/isn't allowed.

    Basically, what I want to do is explained here.

    1. Good practise is to separate the line voltage from the low voltage with a metal plate. I'll check out your post, thanks for the link.

    2. Hi , I have a Sm120x relay installed above a detector, I was swapping out a expired detector I noticed the relay was warm.
      Is this normal ? Can these go bad ?

    3. I would say it depends how warm. Have you tested the functionality - e.g. when you test the detector, you get a signal downstream from your SM120X? If everything is working, I wouldn't worry about it. If it's hot - that would be abnormal.

    4. Thanks for the reply , functionality is good ! Probably as warm as an iPhone charger transformer , but not hot .
      Could not find any info about the unit ,hence the question .
      Many thanks