Understanding the Role of the Red Wire in Wiring Smoke Detectors

[fisico30]

wiring in smoke detector...?...

Hello Forum,

simple question about wiring in a house smoke detector. It operates at 120 V. Two wires, the black (hot) and white (neutral) are enough.
There is a 3rd wire, red, to interconnect multiple alarms: if one rings all the others ring too...
The connection between the different alarms is in parallel.

That said, I am not clear what the red wire does. Is it a hot or neutral wire?

I am trying to envision a circuit with, say 2, smoke alarms as two switches connected in parallel. When one switch is closed the other one closes too...but I am still wondering how the red wire comes into play.

thanks,
fisico30

 

[MATLABdude]

The signal is probably neither hot nor neutral (it's probably a low-voltage open-collector / drain setup). Think of it more or less as a signalling wire--the ones I recently retrofitted in my house (Kidde hard-wire CO/smoke alarms) will differentiate between carbon monoxide and smoke as detected at any alarm.

There's also an alarm (KBR / First Alert) that transmits the location of the fire or CO incident (when you have multiple alarm devices) and theoretically allows you to avoid the location of the fire or carbon monoxide incident.

Long story short, the third wire allows for a hard-wired communications between the various detectors, allowing the entire household to evacuate, no matter where a fire is located--i.e. all the alarms will go off when one does. I've also seen alarms from both Kidde and First Alert / KBR that feature wireless interlinking, meaning that you don't have to have a hard-wired connection between the various alarms.

 

[fisico30]

thanks MATLABdude,

so the red is a signaling wire. In my simple parallel circuit+ 120 voltage source, the red is just telling the other alarms to "close their switch" and start off...
thanks
fisico30

 

[MATLABdude]

That's somewhat abstracted, but yes: the red wire (and various adapters that allow detectors from different brands to work with one another) let the other detectors in the 'network' that they should go off.