If your home was constructed in the past twenty years, it almost certainly has several code-required smoke alarms. You can see, at right, an example of this. The home that I discuss here has half a dozen of these First Alert model 3120B smoke alarms.
The point of this blog article is to help you make some actual meaningful use of your code-required smoke alarms, above and beyond their intended function which is to permit the building inspector to grant a “certificate of occupancy” for the newly constructed home.
The residential building codes in the US have, for at least the past twenty years, required the installation of smoke alarms. And not only any old smoke alarms, but smoke alarms that that are linked to each other through an “interconnect” wire (invariably red in color), so that if any one smoke alarm in the house were to detect smoke, it would send a signal to all of the other smoke alarms in the house, and every smoke alarm would sound its siren.
In recent years, the code has further required that each smoke alarm have a battery backup (generally a nine-volt battery) so that the smoke alarm will carry out its functions even in the event of a failure of the regular 120 VAC power.
The idea of these smoke alarms is that they use the interconnect wire to communicate from each smoke alarm to the other smoke alarms in the house. This red wire is how each alarm tells the other alarms that smoke has been sensed, so that all of the alarms can sound their sirens.
This is all fine and good but it is of no use in terms of letting you know of a fire during a time when nobody is in the house. Which is the main point of this blog article.
What you will find is that in any particular house, the builder will have installed some number of code-required smoke alarms. Invariably the smoke alarms are made by the same company and are the exact same model. It is apparently understood in the home construction industry that what needs to happen is that all of the smoke alarms in a particular house are the same model, made by the same company. Invariably the wiring is black and white and red. The black and white wires provide 120 VAC to the sensors. The red wire provides an “interconnect” between the sensors. This red wire is what makes it so that all of the sirens will sound even if only one sensor has detected smoke.
This now brings us to the smoke sensor relay module. This modular has two sets of wires. At one end, the module connects to the black and white and red wires. At the other end, the modular provides a two-wire normally-closed “dry relay contact”. The idea is that these two wires can be connected to a zone in a home security system. In the security system, this zone will be configured as an “instant” zone, with meta-data for “smoke”. (In a DSC system this means the zone is defined with option “08”.)
As a general matter, each company that makes code-compliant smoke alarms will also have, somewhere in its ecosystem, a compatible relay module. The smoke sensor shown in the photograph above (First Alert model 3120B) is within the First Alert ecosystem. The relay module that works with this ecosystem is the fetchingly named RM4 relay module (manual).
With this background, we can now turn to the main point of this blog article. If you have not already done so, you should obtain a relay module that matches the ecosystem of your code-required smoke sensors. You would then connect the black, white and red wires of the relay module in parallel with the corresponding black, white and red wires that are connected to all of the code-required smoke sensors in your house. Then what remains is to identify the normally-closed dry relay contacts. With the RM4 module these contacts are brown and gray. These wires should be connected to a spare zone in your home security system (with appropriate end-of-line resistor as needed). The zone should be configured as “instant” and “smoke”.
You will then be in the happy situation that your home security system will benefit from all of the smoke sensors in your home.