Adding Smoke Detectors to a Security System

by Jeff Fisher

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Much has been written about stand-alone smoke detectors and all the lives they save. But if you're installing a hardwired security system, there are several good reasons why you should connect your smoke detectors to the system:

  1. One battery, in the security system, will power the security system and the smoke detectors. You'll never have to change a smoke detector battery again.
  2. The security system can call for help and alert the neighbors when a smoke detector goes off, even if you're not at home! (A short delay allows you to counter false alarms.)
  3. If any smoke detector goes off, the security system will alert people in all parts of the structure.

Hooking up smoke detectors to your security system is easy, provided your security system has the proper inputs, and you can program the security system.

Smoke Detector Types & Options

Photoelectric & Ionization

There are two main types of smoke detectors. Each operates on an entirely different principal.

Ionization Detectors use a very small piece of radioactive americium. Inside the smoke detector's ionization chamber, the americium releases alpha particles, these ionize the nearby air. Two metal plates have a low voltage applied to them and the ionized particles allow a small current to flow between the plates. Electronics in the detector monitor this current flow for changes. When smoke enters the chamber it disrupts the process because the ionized particles are attracted to the smoke more than they are to the plates.

Ionization type smoke detectors are very sensitive, as well as very cheap to make. But some folks don't like the idea of 'any radioactive materials in their home. Most ionizing detectors are stand-alone units with a built-in sounder and use a local battery for power.

You can get ionization type detectors practically everywhere for as little as $7 apiece. We don't carry them.

Photoelectric Detectors shine a light source through a chamber. A sensitive light pickup is positioned off to the side where it normally will not "see" any of the light from the emitter. When smoke enters the chamber, it scatters the light and the pickup senses it.

Photoelectric detectors are better at detecting smoky fires than they are at detecting "low smoke" fires, so they are often equipped with a heat sensor that triggers the smoke alarm when the temperature goes up very rapidly. Photoelectric detectors generally last longer and require less maintenance than ionization detectors. Photoelectric detectors are most often used in hard-wired situations where the detector is connected to external power and fire and/or security system.

We carry several types of photoelectric detectors.

Power Supply

Ionization type detectors are often stand-alone and run off of an internal 9-volt transistor radio battery.

Photoelectric detectors are often wired-in and run off of an external power source.

Wired-in smoke detectors that are not connected to a fire or security system are often powered by 120 volt AC from the house wiring. See our
Model 320CC smoke detector.

Wired-in smoke detectors that are connected to a fire or security system are usually powered by DC from the security panel. This gives the detectors a natural battery backup in the event of a power failure. Common voltages are 6, 12, and 24 VDC. With 12 VDC being the most common today. (The DEST6100 is a 12VDC system.) See our Model 429AT smoke detector.


Not all smoke detectors have a built-in noise-maker. Of course, all stand-alone units make noise.

But when smoke detectors are connected to security systems, a sounder is often not desired; The security system is responsible for making the noise, alerting the occupants, etc. The user should be able to override false alarms from the security system keypads, which should stop the noise-makers. If the sounder was in the detector, it wouldn't know when to stop sounding!

All stand-alone units have a sounder. Most AC powered wire-in units do have a sounder because they typically are not connected to a security system. Most DC powered units do not have a sounder because they typically are connected to a security system.

Auxiliary Relay

Some wire-in smoke detectors have additional contact closures available for use. These vary from model to model. On 120VAC models that are not connected to a security system, you can use these outputs to "slave" multiple detectors together so that when one goes off, they all go off.

Integral Heat Sensor

As was mentioned above in the description of how a photoelectric detector works, these units often compensate for their decreased sensitivity in "non-smoky" fires by adding a "rate-of-rise" temperature sensor. This is a simple thermostat that trips if the temperature goes up more than so many degrees within a certain amount of time. We highly recommend this option; it is very accurate and not prone to problems or false alarms.

Two- and Four-Wire Connections

Wire-in smoke detectors usually support either a two- or a four-wire bus.

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The four-wire bus is the "old" way of hooking up smoke detectors and is usable with virtually any alarm panel that has an available zone. The smoke detectors are wired in parallel, usually with one end-of-line resistor. This takes two wires. The other two wires carry DC voltage to the detectors. Thus each detector (except the last in the chain) will have eight wires connected to it. The last detector will have four wires and an end-of-line resistor attached. The detector chain is connected to an unused zone in the security system and the zone is set up as a normally open zone with one end-of-line resistor.

As an added safety precaution, the last detector may get an end-of-line power supervision unit attached. This is a little relay that will set the whole thing off if power doesn't make it all the way down the chain. Since the smoke detectors in a 4-wire system are normally open switches, it is theoretically possible for power to be lost somewhere in the chain, while the alarm circuit is still intact. If the contacts in the un-powered detectors remained open, those detectors would not be operational and the security system would not be able to tell.

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The two-wire bus is the "new" way of hooking up smoke detectors. Most security panels now have two special terminals for attaching two-wire smoke detectors. In this arrangement, all detectors are wired in parallel to the two-wire bus. The last detector in the chain gets an end-of-line resistor. The security panel supplies power down the two wires while, at the same time, sensing if any of the detectors have tripped. This is a simple and fail-safe arrangement.

Placement of Smoke Detectors

At the minimum, you should have one detector for each level in your home. A detector needs to be placed within ten feet of all sleeping areas, since most fire deaths occur at night while people are sleeping. The unit should be mounted high on the wall, or, at best, on the ceiling. It should never be placed near the juncture of the wall and ceiling, as there is a "dead air" space there.

Avoid installing a detector near bathrooms with showers. Steam can sometimes cause false alarms and the moisture can rust metal components of the detector.

Avoid installing detectors in garages, attics or other places where the environment may have spiders (and webs), high levels of particulate matter in the air (dust, auto exhaust, etc.) and large temperature ranges from very hot to very cold.

Connecting Two-Wire Smoke Detectors To Your Security System

Smoke Detectors must be installed by qualified and licensed personnel.<br/>Installations must comply with all applicable federal, state, county, and city codes.<br/>When wiring smoke detectors in commercial applications or to "fire" panels, special wire and installation instructions must be used.
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Determine where each detector will be located. Detectors can be mounted on a standard single-gang box, a 4" octal box, a 4" square box, or they can be mounted directly to walls or ceilings (local codes permitting.)

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Run at least two-conductor wire from the alarm panel and daisy-chain to each smoke detector location. Leave about a 12" service loop at each detector location.

I use Belden 1242A-1 quad wire for this and just about all security system wiring (local codes permitting.)

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Cut the wire at the loop, strip the cladding back a few inches, and cut off the green and yellow wires (you won't need them.)

Strip about 1/4" of each red and black wire.

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Insert the black wires into the positions marked minus (-) and the red wires into the positions marked plus (+). Not that both plus and both minus positions are connected internally, there are just two screw terminals on each to make it easy to connect.

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At the last detector in the loop, attach the wire to one set of terminals as usual, and attach an end-of-line resistor to the other set of terminals. (The end-of-line resistors should come with your security system. They are simple 4.7K 1/8W resistors.)

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Lastly, connect the black wire to the -SMK terminal on the security system, and the red wire to the +SMK terminal.


Here's where we have to get security system specific. These are instructions for an Apex Destiny 6100 security panel. Consult the instruction manual that came with your system.

Using the PC software, set up an new zone (I used 33 for an example). Set the zone type to "24 Hour Fire". I set the spoken description to "CEILING" "SMOKE" "DETECTOR". There are various other options, like whether you want the system to "report" this zone (I should hope so.) There is a place to fill in what actions should occur. I think this is a good place to have the panel use X-10 to turn on all inside and outside lights. This will facilitate evacuation. Lastly, and most importantly, you specify that this zone is "connected to" the 2-Wire Smoke Input on the main panel.

Download the updated settings, and you're ready to test.

Note that this can all be done via the keypad, but it's a royal pain.

To test the system, just place your system in user test mode (user code + 0). Then hold a magnet up to the specified location on the smoke detector to test it. Count the number of blinks. 4 - 7 blinks is OK. Continue to hold the magnet in place and the LED will come on solid. The alarm keypads should say "ceiling smoke detector active". The smoke detector must now be manually reset. Press user code + 6 on the keypad. This will reset all smoke detectors. Exit user test mode either by letting it time out or pressing user code + 0 again.


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