I am looking for more information on the mains powered fire alarms with "interconnect" wire.
There seems to be plenty of information available on using the interconnect wire to interface with a fire alarm to detect when a fire alarm triggers, i.e., "reading" the fire alarm. It seems that a triggering fire alarm puts 9 V DC on the interconnect wire, which is easilly detected using e.g. an optocoupler.
Q's:
How much current can be drawn from the interconnect wire? I assume a few mA's for the opto led is fine?
However, I am looking for information on using the interconnect wire to interface with a fire alarm to externally trigger the fire alarm, i.e., "writing" the fire alarm. One could then use the fire alarm for other alarm purposes (e.g. as a burglar alarm, panic alarm, ...)
Q's:
I assume that simply putting 9 V DC on the interconnect wire will trigger the fire alarm. Is that correct?
How much current needs to be delivererd in that case? Is the fire alarm in that case powered by the interconnect wire, or does it merely detect the interconnect signal and use its own power?
Are there any schematics available for both "reading" and "writing"? In any case, it seems it would be needed to galvanically seperate the 9 V DC part from the Arduino. I am thinking along these lines:
deduce 9 V DC directly from mains (R, C, D, zener) (using a transformer does not seem to make sense if the 9 V DC needs to be relative to the neutral, there is no galvanic separation)
use an optocoupler or relay controlled by the arduino to couple this 9 V DC to the interconnect wire (in which case there is galvanic separation between the arduino and the rest) => Arduino output to trigger the alarm
use a second optocoupler to detect 9 V DC on the interconnect => Arduino input to detect triggered alarm
Not sure if this makes sense, I am not familiar with the interconnect system. Is it documented somewhere?
Fire alarms are there for a reason - to save lives. You start messing with the manufacturers design and they fail at a critical moment you will be the one that will be held accountable for any damage/loss of lives etc
I understand your concern and I appreciate your warning. However, please note that it is not my intention to "mess with the design": The interconnect wire is the interface exposed by manufacturers to interface with the alarms, I am merely looking for information that will allow me to correctly interface. Please also note that my question is directly motivated by safety: In my region it is increasingly popular to directly wire fire alarms into the home automation system, I believe that interfacing with stand-alone fire alarms is safer. Finally, please note that I am also a qualified electrician.
areslagae:
Finally, please note that I am also a qualified electrician.
And so am I. I don't know what country you are based but in the UK you are advised you to follow the manufacturers specs and install procedures etc any deviation will hold you liable if anything goes wrong. You should also know this being qualified
Your objections are valid and noted. I am asking for educational purposes. It would be great if we could stay on topic and get some information on the interconnect interface in this thread
I think all the responses to your questions is: if you cannot supply the manufacturers specs for the interface, then you will not be able to use it without assuming the manufacturer's liability. Being an electrician has no bearing on the problem.
Surely a reputable alarm manufacturer that has a system that has a user interface has documentation for that interface. Are they still in business? Does a local firm install the system?
I cannot seem to find manufacturer specs. However, this is not a manufacturer-specific interface. It seems to be used universally and globally for all wired interconnected smoke detectors, especially in the states ("orange wire"), and it seems that fire alarms from different manufacturers aso interconnect. The interface is analogue (probably simply voltage or no voltage) so I assume it is very basic.
There are "output" modules from a lot of manufacturers that connect to a smoke detector and switch a relay when the interconnect wire signals an alarm:
This is used a.o. for adding a sirene. You could also attach it to an arduino.
However, I am looking to go the other way, i.e., signal the interconnect wire from the arduino.
I'll try to dig a bit deeper and maybe contact a manufacturer.
There may not be a fixed standard, found some info here
Which might help you . At the simple end of the market it may just be a wire that goes hi in alarm condition - you could check that , more complex units have some form of coms
However, there seems to be a large family of mainly identical IC's that powers most of these optical detectors. See below for one of the data sheets.
The interconnect seems to be a relatively dumb I/O pin:
This pin can be used to connect up to 40 units together in a wired-OR configuration for common signaling. VSS is used as the return. An on-chip current sink minimizes noise pick up during non-smoke conditions and eliminates the need for an external pull-down resistor to complete the wired-OR. Remote units at lower supply voltages do not draw excessive current from a sending unit at a higher supply voltage.
I/O can also be used to activate escape lights, auxiliary alarms, remote alarms, and/or auto-dialers.
As an input, this pin feeds a positive-edge-triggered flip-flop whose output is sampled nominally every 625 ms during standby (using the recommended component values). A local-smoke condition or the push button-test mode forces this current-limited output to source current. All input signals are ignored when I/O is sourcing current. I/O is disabled by the on-chip power-on reset to eliminate nuisance signaling during battery changes or system power-up. If unused, I/O must be left unconnected.
In your link, the above is reffered to as "the original "dumb" smoke detector interconnect".
Your link seems to decribe a much more advanced system of kiddle where they run a more advanced digital protocol over the interconnect wire to distinguish between alarm conditions.
I'll dig a bit deeper, and maybe try to get some smoke detector schematics. The interconnect system seems to be very simple at first sight. Deriving the 9 VDC voltage from mains is a slight complication however.
I would look here for some information Alarm to USB
Some (not too many) years ago as the product was being developed, there was a lot of forum discussion on the interface to some common alarm systems. I seem to remember the interface was RS-485 but the communication codes were not published and need to be reverse engineered.
I personally have not used one of the above devices, simply because hard wired devices gave no benefit over the keypads I could purchase (with the exception of initial programming). More functionality would require a wireless link which I am not comfortable with for entry / fire alarm. This is not a technical issue but one that has to do with how well I sleep at night.
Several manufacturers have "relay modules" that switch a relay when the smoke alarm sounds (e.g. for an aditionnal sirene), as I already stated.
Apparently, some manufacturers also have "switched input modules" that allow an external device to trigger the smoke detectors (this is apparently used to interface with sprinkler systems or other builing alarm systems).
So with one module of each one can interface both ways between an arduino and a set of smoke detectors. This also addresses MarkDerbyshire's concerns btw since these are pre-made modules so you only need to wire them up.
The PCB of the "switched input module" simply looks like a transformerless power supply combined with a battery power supply. So probabbly not much going on there except putting 9 VDC on the interconnect wire.
Another option is of course to open the fire detector and look up the datasheet of the smoke detector IC being used.
I guess this more or less answers my initial question.
The 9 V DC interconnect seems to be more or less a de facto standard although several manufacturers deviate from it (e.g. Kiddle running a digital protocol over the interconnect wire, and I have also seen 24 V DC fire alarms with an interconnect which might run 24 V DC rather than 9 V DC). Most manufacturers of course recommend to only interconnect with their own devices.
However, aditionnal info is of course still welcome.
