Smoke detector third wire: What kind of signal does it send out?

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,737
I have a couple old smoke detectors that are AC powered with battery backup. I replace smoke detectors every 5 to 7 years. The ones I have in place now are rated for 10 years - that's not the concern. As I said, these are older units and may still work. Thinking about using them in the detached garage. However, I'd like an external signal that indicates when there's smoke detected in the garage. Nothing fancy, maybe a siren.

They have black and white wires (110 VAC) and a yellow wire that interconnects to other similar alarms. When one sounds they all sound. I'm wondering what kind of signal they are using to communicate. Anyone have any idea? Somehow this yellow line (have seen other color wires, but the intent is the same), this line signals the other alarms to sound their alert. I'd like to possibly use that signal to trigger an out-door alarm so that if there's smoke in the garage - an alarm will make someone react; possibly call the authorities and alert them.
 

Delta prime

Joined Nov 15, 2019
377
Hello there and welcome to AAC :p:p:p
The two wire cable should be the power source.
The 3- wire cable should be looping all the smoke detectors together.
The third wire of the 3-wire cable is used for the alarm signal.
When any alarm detects a fire, it sends a 9-volt signal on the 3rd wire. Any alarm that detects a 9-volt signal on the third wire will begin sounding its alarm immediately.
I was just kidding about the welcome to AAC in a good way!!
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,737
Thanks @Delta prime. That 9V - - - AC or DC? I'm thinking DC because of the battery involved as a backup.

Oh, and thanks for the warm welcome. I'd like to welcome you too.

")
 

Delta prime

Joined Nov 15, 2019
377
Both Photoelectric Sensor and Ionized Alarm with Battery Backup use 9VDC I have and recomend putting a depleated battery no more than 7.5V
to see if the unit churps if it does than your good to go!& thank you for the welcome:D


Moderators note : reduced font size as large fonts are like shouting
 
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Hymie

Joined Mar 30, 2018
837
AC powered smoke alarms normally use a transformer-less method to generate a 9Vdc circuit with reference to the supply neutral. The third wire interconnect signal is normally at 9Vdc (with reference to neutral) in the alarmed state.

With all the smoke alarm interconnects commoned, one alarm entering alarm mode causes all to enter alarm mode (sounding). Better designs will include an indication of which unit entered the alarm state first – aiding fault finding.

Because the interconnect is neutral referenced, it is important to observe the correct polarity wiring of the live and neutral at each connected AC alarm.
 

Delta prime

Joined Nov 15, 2019
377
Ok.so 7.5V is only for the battery backup the smoke detector will chirp if battery backup is below a certain voltage , it is just to check to see if it is working properly now just hook up the smoke detector start a little fire blow it out see if it detects smoke when the alarm goes off see how much voltage is on the third wire. from there I'm sure you could figure out what you want to use as a signaling device. I responded because I had done the same thing 3 years ago same scenario different unit so there you go & I did not mean to yell at you cuz I didn't know what the settings were & it's different every time I log on on my phone and I use different phones it's rather frustrating.:eek:
 
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.
 

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Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,737
Now I'm confused the TS is using one smoke detector but wishes to use the 3rd wire dc signal to activate an very loud horn or speaker instead of using the on-board transducer.:p
That's exactly what I want. A single unit and some detector/alarm to be sounded OUTSIDE the building. The space is only 565 sq. ft. Smoke detected inside the space will alarm me or my neighbors. Someone will send someone to investigate the alarm.

A few days ago I was sharpening a shovel. One of my grinder wheels is a stone. The other is a rubberized polishing disk. I sharpened the shovel and turned my back. Smelled smoke. Apparently a buildup of wood collected in the shield of the rubberized wheel, and the friction started it burning. I doused it with a water bottle, disassembled the shield and cleaned out the smoldering ashes. Imagine if I had not noticed it when I did. No smoke detector and that could have grown in size. So I want to incorporate the detector along with an external alarm so that if I'm in the house I'll hear the alarm and go investigate; and take appropriate action.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,737
So apparently what I'm hearing is that it's a line that is looking for a specific resistance. With that resistance I can probably create a voltage divider and use a comparator to trigger an alarm. I have yet to bench test the detector; and yet to experiment with how to manipulate its signal.
 

Hymie

Joined Mar 30, 2018
837
Prairiemystic’s solution above will do exactly what you want (even providing the circuit diagram). If the op-to is capable of driving the voltage/current of the siren, then no other parts are required. Otherwise you might need the op-to to switch a relay to drive a mains powered siren.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,737
Yeah, I've dragged a copy of that schematic to my desktop. I'm willing to test it but it involves mains power. I'm not afraid of mains but I highly respect the danger. Maybe later today I'll give it a go. There are other more important things to be done though.
 

AnalogKid

Joined Aug 1, 2013
8,493
Based on diagram #14, the reference for the alert signal must be one of the AC power lines, probably the neutral. It would be good to know the yellow wire voltage wrt neutral in the alarm condition.

Also, a photo of the pc board would save a lot of electrons.

ak
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,737
Photo's (too big to upload the full images)

First image is PC Solder side (a lot of wax covering - don't know why)
1593199364114.png

Second image: Three pins in J3 are L(B) = Line; I/O(R) = In/Out and N(W) = Neutral.
1593199419787.png
 

Hymie

Joined Mar 30, 2018
837
The whole point of the op-to isolator is to provide reinforced insulation between the diode (mains) and transistor (SELV) circuits within the package.

You should ensure that in your PCB layout you have at least 3mm between the circuit connected to the op-to diode and the op-to transistor circuit.
 
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