Smoke detector third wire: RESOLVED. Thank you.

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,204
As you may have noted, in my drawing there's a PCB, but only for the sake of indicating there's going to be something there. At present I don't have a siren or driver circuit for it. Nor do I know any of the voltages I may eventually employ, as well as how I'm planning on interfacing the detector to the PCB. An Opto seems like a reasonable approach. As for a siren, should I go that route, it'll have to be something that has a long on pulse and a short pulse off so that it acts like a traditional siren. I should include a way to silence the siren should it go off by mistake.
 

Reloadron

Joined Jan 15, 2015
7,088
Tony, here is what I would try which is inline with a few post including Analog's. Power one up using line voltage and measure between the yellow wire and mains neutral. See what you have. Now feed it some smoke, you are on your own as to a smoke source. You can buy "canned smoke" for testing smoke detectors. We had to test our detectors at work using it but first place the system in the "Test" mode or expect the fire department. :) Anyway, the best way to know exactly what the yellow wire outputs is measure it under non-alarm and alarm conditions.

Ron
 

Hymie

Joined Mar 30, 2018
1,103
Once you work out what is going on, you might like to limit the alarm time should you have the siren mounted on the front of your garage. Imagine you take a 2 week vacation and the siren sounds (in the middle of the night) due to a fault – you won’t be very popular with your neighbours.

These days, rather than using a hardwired interconnect, smoke alarms are available with a wireless interconnect. Next time you upgrade/replace the alarms you could go for this option.
 
If it's a voltage, you can determine it's output Z, buy putting in a potentiometer as a load until you get 1/2 the initial voltage.
measure the pot resistance. That's the output Z.
 

Delta Prime

Joined Nov 15, 2019
1,311
[QUOTE="Tonyr1084, post: 1525979, member: 285953"]
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
[/QUOTE]
I don't wish to think about that.If it was me my water bottle would have contained 99.9% alcohol. ;) I believe you are more than capable of carrying out the testing, modifications by yourself. but why you have the entire AAC community at your disposal. It was very impressive and heartwarming to witness that
. good luck to you Sir.
 
Kidde does make a SM120X relay module (blob in heatshrink) which is driven by the Interconnect line and the relay contacts can switch alarm bells/alarm tie-in/lights etc. I haven't seen any teardown pics.
I would guess inside it has a couple transistors and small mains power supply which to power the relay coil. The aux 9VDC 5mA output I doubt is taken from the smoke alarm Interconnect line, it should be 0V at rest.
I'm not sure if NFPA-72 has a standard for the Interconnect wire voltage/current, to ensure compatibility between manufacturers.
I say it's a weak 6-9VDC signal between Interconnect and Neutral when alarming.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,204
Results:

No alarm:
AC volts = 0.003V
DC volts = 0.5mV

Alarm Test Button
AC volts = fluctuating readings
DC volts = 10.5V initially, then fluctuates

Later today I plan on lighting a candle then blowing it out and let the smoldering wick smoke waft into the alarm. See if I can get it to go into alarm for a continuous signal. If that doesn't work it will be an incense stick.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,204
OK, after smoke testing the voltage on the yellow line jumps from zero to 10 VDC and remains there during the entire smoke event. That's between Yellow and White. So there's a 10 volt presence during both the test and a detect event. During the test it goes to 10V momentarily, about a second, then returns to zero volts. During a detect it stays at 10V until the detector is cleared.

Next test will be to wire an LED and resistor between Yellow and White.

[edit] LED test: LED Vf = 2V. 470Ω resistor. 10VDC. 17mA. LED lights during test and during smoke. Should be easy from here. [end edit]
 
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AnalogKid

Joined Aug 1, 2013
10,177
I was going to suggest loading the yellow line with something to see how much current you can suck out of it while holding at 10 V, but it looks like you have an answer.

A hopefully redundant safety note: That 10 Vdc signal is riding on the AC Neutral. While this should be a safe wire to mess with, you never know exactly how someone else wired things ...

ak
 
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Reloadron

Joined Jan 15, 2015
7,088
A hopefully redundant safety note: That 10 Vdc signal is riding on the AC Neutral. While this should be a safe wire to mess with, you never know exactly how someone else wired things ...
With that in mind you may want to consider using a common OptoCoupler to drive anything you plan to drive with the 10 VDC when the alarm state is energized.

Ron
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,204
A hopefully redundant safety note: That 10 Vdc signal is riding on the AC Neutral. While this should be a safe wire to mess with, you never know exactly how someone else wired things ...
Am acutely aware of that. When I first bought this house, someone replaced all two prong outlets with three prong outlets. To make it look like it passed grounding inspection they wired together (inside the box) neutral and ground. Only I didn't check ALL the outlets. One had hot and neutral reversed. With the white wire tied to the ground terminal (and it was live) when I plugged my computer into it - no problem. I plugged my printer into a different outlet - again no problem. UNTIL I attempted to plug the cable from the printer into the back of the computer. Remember, this had a jumper between white and ground terminals, AND white was hot. That meant my computer cabinet was hot. So when I attempted to plug a grounded cable from the printer into the computer I got a good shocking AND a burned hole in the back of the computer.

I never trust anyone else's wiring. Even when having a licensed electrician do a job - the inspector in me always wants to make sure it was done correctly. I know the shortcuts some like to take. Like jumping neutral and ground together in the box to make a plug look acceptable to an outlet tester. The kind with three lights on them. That's one reason why I tested the outlet on my control box. It even has a schematic printed on the face and clearly shows hot going to the larger slot, which is wrong. I have to fix that by turning the outlet over and wiring it correctly. Had I plugged my smoke detector into that I could have been exposing myself to 110VAC on what I would have though to be a neutral line. Sure, I could have wired my test to match the outlet; but what happens next time I plug something into it and don't take into consideration how its wired. Like maybe an oscilloscope. I think it could turn quickly into oscillosmoke.
 

MisterBill2

Joined Jan 23, 2018
13,817
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!!
"9 Volts relative to WHAT?? Relative to the white neutral wire? I knew that there was some sort of signaling but I have never heard any description of what the signal was.
 

AnalogKid

Joined Aug 1, 2013
10,177
"9 Volts relative to WHAT?? Relative to the white neutral wire?
That is how I interpret post #27. The 10 Vdc signal has to be referenced to *something* going to all other detectors, and Line and Neutral are the only options. My guess is that the yellow is referenced to Neutral, but it actually doesn't matter for the operation intended.

ak
 

Delta Prime

Joined Nov 15, 2019
1,311
9 Volts relative to WHAT?? Relative to the white neutral wire?
a valid question that holds Merritt. quite frankly I dropped the ball. Independent of a cherished
member of this community inquiring advice on safeguarding his property. I failed in providing pertinent information regarding my conclusions. Which I believe is the definition of incompetence. I am a mere Mortal among Titans yourself included. You good sir and your contemporaries must know there are individuals myself included we have gained valuable insight on electronics only because each and every one of you break it down to such an extent that anyone can comprehend I salute you. And stand before you humiliated. it's true when they say you never want to meet your Heroes.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,204
a valid question that holds Merritt. quite frankly I dropped the ball. Independent of a cherished
member of this community inquiring advice on safeguarding his property. I failed in providing pertinent information regarding my conclusions. Which I believe is the definition of incompetence. I am a mere Mortal among Titans yourself included. You good sir and your contemporaries must know there are individuals myself included we have gained valuable insight on electronics only because each and every one of you break it down to such an extent that anyone can comprehend I salute you. And stand before you humiliated. it's true when they say you never want to meet your Heroes.
In post #28 I said it was between yellow and white. You're not a fool for missing that. Just an oversight. Something we're all guilty of. Especially when we don't read and comprehend all replies. Then again, sometimes there's so much information a small kernel gets overlooked. Doesn't take a hero to point it out.

Now and then we encounter individuals who browbeat others for no good reason. If you feel you've been denigrated - stop. You're just as valuable here as everyone else. Though some have superior experience and knowledge, even those who know less or even very little still make valuable contributions. I definitely put myself in the lower category. I'm not an expert; don't have tons of experience; and don't diminish others just to make myself feel superior. We're all equals here. Like riding in a plane. Some have window seats, some in first class, and some of us stuck in the middle seats. But we're all going the same direction at the same time. We have a wonderful opportunity to learn something here. Sometimes it's by asking a question. Sometimes it's by making a mistake. But nobody should ever be made to feel like a lesser of a member here.

The 10V signal is measured between the yellow wire and household (white) neutral line. Whether one detector is connected to hot (Line1) while another is connected to the other hot (Line2), neutral is common throughout the house. A comment was made about novice home electricians making wiring mistakes - and even professionals can make them - you can't always trust the wiring to have been done correctly. Remember my computer and printer experience. Just one hot and neutral reversed and I got shocked, my computer got a burn hole in the back and my printer cable needed to be replaced. Caution is ALWAYS advised when working with potentially dangerous voltages.
 
I recommend using an opto-coupler for safety isolating mains voltages which is an issue if Hot and Neutral are reversed, and due to mains transients which can appear on Neutral due to lightning. If the smoke alarm Interconnect went to a home alarm panel, it would likely give nusiance alarms or get damaged being directly connected. The smoke alarm circuit common is supposed to be Neutral and may not even worked properly if reversed with Hot, because of the high level common mode noise it would false trigger.

If you don't want to use an opto, a transistor can be driven from the 9V signal. The Interconnect signal is weak, a few mA just enough to light an LED. The problem is where to find the energy to drive the auxilary alarm/lights. You still need another DC power supply.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,204
These smoke detectors are simply AC powered, battery backup detectors that are interconnected via a third wire. No panels. Just that if you're downstairs and an upstairs detector goes off - all detectors sound an alarm.
 

Delta Prime

Joined Nov 15, 2019
1,311
Is the 3rd wire also the one used to get a signal from a different location?
Yes
I think you have to start a new post or something, I mean if you have a question, you have to take my answer ( yes) in context to post here that we were discussing…if you pose a new question then everyone here will see it….
 
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