Smoke detector third wire: RESOLVED. Thank you.

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,190
@Delta prime is correct, you should start your own thread if you need help.

Let me recap: The third wire remains at zero volts. When the alarm is triggered there is a 10 volt signal sent out along that line. Any and all other detectors on that line will see the 10 volts and will begin an alarm. It will continue as long as the originating detector continues to send the signal. Once the original detector stops sending the 10 volt signal the other alarms will silence as well.

Welcome to AAC. If this didn't answer your question then feel free to post one of your own. As far as this thread is concerned, I've gotten my answer and am no longer looking for input.
 

MisterBill2

Joined Jan 23, 2018
13,686
@Delta prime is correct, you should start your own thread if you need help.

Let me recap: The third wire remains at zero volts. When the alarm is triggered there is a 10 volt signal sent out along that line. Any and all other detectors on that line will see the 10 volts and will begin an alarm. It will continue as long as the originating detector continues to send the signal. Once the original detector stops sending the 10 volt signal the other alarms will silence as well.

Welcome to AAC. If this didn't answer your question then feel free to post one of your own. As far as this thread is concerned, I've gotten my answer and am no longer looking for input.
I think that there is a way to put a lid on a thread.
 

agarg

Joined Jan 14, 2023
1
The smoke alarm interconnect is a CMOS I/O pin from the IC, which typically runs off 9VDC from a battery and/or mains capacitive voltage divider.

As a daisy-chained wired-OR configuration, any smoke alarm can assert the interconnect line taking it high. It's pretty low current around 4mA with a weak battery, to 16mA typical. Up to ~40 can be tied together, although this is not so practical to find the fire in one of 40 rooms lol.
There is also some logic to mute the signal during power up, and if any one detector has a low battery chirp that is ignored. Also current limiting.

If you want to pick off the signal from a mains-powered smoke alarm, I would use an opto-coupler like H11G2 and 2k resistor from Interconnect (orange) and AC Neutral (white). Red is 120VAC Hot. The 9V signal is between Neutral and Interconnect. This is my schematic to connect to home alarm system, draws 1.5-2.5mA.

*Careful there is a danger that you can mess up/disable a smoke alarm with any mods or extra circuitry/wiring done wrong. This circuit is not UL/CSA tested or approved at all.
Also, old smoke alarms just wear our- either the chamber is full of dust or the capacitive-divider components age badly. They don't last forever, so for the $10 I would just buy a new one.

If you use this circuit to trigger the alarm through an opto-coupler (which is great as it gives you full isolation) but then how will it be silenced?
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,190
If you use this circuit to trigger the alarm through an opto-coupler (which is great as it gives you full isolation) but then how will it be silenced?
Welcome agarg to AAC.

Triggering the alarm through an opto does in fact isolate it from the main circuit. An opto will switch on when it is powered from the 10V source. When the 10V is gone the opto shuts off and the alarm ceases to signal.

If you have further questions on this subject you can start a thread of your own. My issue was resolved over two years ago.
 
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