Prevent House Fires With the Smoke Detector Disconnector




About: Hi I'm Michael! I love all things Science, Engineering, & 3D Printing. If you've enjoyed my work then I've love to hear from you!

Almost a decade ago I started a major house fire. Yep, accidents happen and sometimes one little mistake can have dire consequences. A young maker fooling around with electricity can be particularly vulnerable to those types of accidents.

Making mistakes is part of the learning process and fear of disaster shouldn’t deter you from experimenting in the garage. Suffering a house fire as the result of an experiment gone awry is a rare event, however, I felt that the maker community would greatly benefit from a cheap and effective form of preventative insurance.

The Smoke Detector Disconnector was designed specifically to prevent house fires. Think of it as a smart extension cord that reduces the chance of anything plugged into it harming a person or starting a fire.

This is actually a really simple tool made with 100% off the shelf parts. Nobody is selling anything that does what this does (the liability would be huge), so you'll have to make one yourself!

Read on to learn how and be sure to check out the video in the last step!


There are situations when typical electrical safety devices won't save you. A circuit breaker or a GFCI won't detect that your electric motor is jammed or if a blanket is covering your space heater.

When those things happen the fire triangle is complete and you end up with a dangerous situation. The ingredients needed to maintain fire are heat, fuel, and oxygen.Electrical fires start by providing the heat until a smolder eventually grows into an open flame.The key here is that there is a window of opportunity when the smolder is detectable but has not yet grown into a self sustaining fire.If you shut off the heat source soon enough, you can effectively prevent the fire. (Please refer to the video demonstration)

Even if the fire is beyond the point of no return, cutting the power is still useful. Electrical fires are particularly dangerous and have to be fought with a special type of fire extinguisher. Killing the power allows you to fight the fire with anything, including a bucket of water.

Note 1: Since smoke rises this device works best when mounted on a wall above the device in question!

Note 2: Your safety is my concern, but it is not my responsibility. I assume no liability for damage caused by the use or misuse of this design. Be aware that this device is intended as a last resort tool and using it does not guarantee your safety.

Note 3: Space heaters, whether portable or stationary, accounted for one-third (33%) of home heating fires and four out of five (81%) of home heating fire deaths. (source)


Recommended Tools:

*Dremel Tool: (For cutting holes into the box.)

*Wire Strippers & Crimping Tool: (For preparing safe electrical connections.)

*Digital Calipers: (for measuring the GFCI cut out.)

Bill of Materials:

(1) 5VDC Power Supply

(1) Smoke Alarm, Hardwire Photoelectric

(1) Plastic Project Box (8.75" (L) x 5.45" (W) x 3.6" (H))

(1) Solid State Relay, 3-32VDC Coil, 25A 250VAC Controlled

(1) GFCI Outlet, 120V 15A

(1) SPDT Mechanical Relay, 120VAC Coil, 15Amp Load

(1) 3 Wire Power Cord: (You can also easily to salvage this from old electronics)

(1) Rope Cleat: For wrapping up the power cord.

(1) Fire Extinguisher (16 oz, for class A,B,or C fires)

(1) Mount for Fire Extinguisher (you could also use the 3D printed model I've provided)

(1) Handle (for carrying & hanging)

(~) 14Ga Electrical Wire

**Note that these are all affiliate sales links, meaning that I will make money at the expense of Amazon if you purchase something though them. What little I earn through these links is used to support future projects like this one!


Measure the dimensions of the GFCI and mark up the box for cutting. The opposite side will need a 3/8" hole for the power wire to come in.

You will also need to cut a hole in the lid at least 1" OD so you can feed the wires into the smoke detector.


This really is a simple device, I feel I would be doing you a disservice to go into much more detail than 'mount all the parts'. The box size I have recommended is plenty big enough so you wont have trouble cramming things in there.

The point of this step is that the relays, GFCI, and power wire all have to be in place before you start attaching wires to them. The wall wart isn't shown because I left that loose inside my box.

Typically I wouldn't recommend putting a solid state relay inside a sealed box, but in this case the relay is not in use 99.9% of the time and even when it is it will be controlling fractional amperage. The mechanical relay that will be taking all the load will be fine because mechanical relays don't get very hot.

Step 5: WIRE IT UP

For this circuit to work it has to be wired up exactly as shown. I really don't think there's any way to make this work with fewer parts.

Under normal operation all current going to the GFCI goes through the normally closed mechanical relay. If the smoke detector is triggered the detector will put out a 5vdc signal that is typically used to trigger other smoke detectors. I used that signal to activate the normally open solid state relay, which then opens the mechanical relay to shut off the main power.

The solid state relay also activates the 5vdc power supply which in turn keeps the solid state relay triggered and keeps the alarm going. The end result is that once the alarm is triggered it will continue to stay triggered until you manually unplug the smoke detector disconnector. It works the same way as this other safety device I created.

Note: Be cognizant of the polarity of the relays and the GFCI when you are making the connections!


When its all done I recommend plugging it in for the first time in your bathroom GFCI, just to be safe.

Press the test button on your new device's GFCI to be sure that it works properly. Do not test this thing with a paper clip!!! Do not short the hot and neutral because the GFCI will not trip and you will get hurt!

Similarly, do not test your smoke detector with actual smoke! Real smoke can actually degrade the detectors functionality. It is not necessary anyway because the smoke detector has a test button on it.


Most smoke detectors work either by optical detection (photoelectric) or by physical process (ionization), while combination alarms use both detection methods to increase sensitivity.

Photoelectric smoke detection is much more responsive to fires that begin with a long period of smoldering while ionization alarms are more responsive to flaming fires.

Another common alarm system is the heat detector, which can trigger off of set temperatures or from the rate of temperature rise. Heat detectors are NOT recommended for use here.

All things considered, a photoelectric smoke detector is the best choice for our purposes. A combination alarm would be an even better detector but is also considerably more expensive.


A ground fault circuit interrupter is a device that shuts off an electric circuit when it detects that current is flowing along an unintended path, such as through water or a person. Having one on the smoke detector disconnector is not critical to the main function of the tool. However, the GFCI is an effective safety device that I thought made a solid addition to the usefulness of the smoke detector disconnector.

Be aware that GFCIs have difficulty controlling some electric loads. Motors (inductors) and capacitors store energy for a short period of time before releasing it. This can cause the circuitry in the GFI to falsely detect a trip condition.

NOTE: A GFCI is NOT the same thing as a surge protector. 'Surge protection' prevent damage to your devices in the event of a power surge, such as a lightening strike on a power line near your house. Protecting your electronics is not critical to the intended function of the smoke detector disconnector. Protecting humans is the point!.

Fuse: You may have noticed that I had a fuse on that wiring diagram. Your house already has circuit breakers (typically 15 or 20 amps) which are functionally equivalent to fuses, but you will want to add the fuse to your device if you build it with any components rated for less than 15 amps. I had that 10 amp power cord, so a fuse was necessary.


If you appreciate the work I've done here then let me know and please vote for this project in these contests!

The 'Home Automation Contest', because this tool is intended to function in the home without human intervention.

The 'Unusual Uses Challenge', because of the unusual use of the smoke detector.

The 'Before and After Contest', because of this tool's potential for preventing your home from changing into a disaster site.

Home Automation

First Prize in the
Home Automation

Before and After Contest

Participated in the
Before and After Contest

Unusual Uses Challenge

Participated in the
Unusual Uses Challenge



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    24 Discussions


    4 years ago

    This is a nifty little device. It continues to amaze me that someone can read what is in front of them and fail to understand it. To the people saying that this doesn't prevent fires: That's not the point. This device is designed to keep things from going past the point of something that can be handled quickly and easily by removing the cause of the fire. When the author says "last resort" he means "not a preventative measure but a containment one." Learn to process past your reaction and you might learn that your reaction has already been given an explanation.

    Hope this helps with any confusion. A lot of thought went into this and for the purpose intended it seems to work quite well.

    2 replies

    The author has stated

    'The Smoke Detector Disconnector was designed specifically to prevent house fires. Think of it as a smart extension cord that reduces the chance of anything plugged into it harming a person or starting a fire.'

    My point was and remains the same this device will not function correctly because it will be placed too low in the room a lots of smoke would have to build up before triggering this device.

    Yes use it but have a ceiling mounted device as well.

    Agreed. Also, for those unaware, NEC does require ceiling mounted smoke detectors, the must be "interlinked" and should be on their own dedicated circuit


    3 months ago

    First of all, thanks for posting this. I wanted a device like this to protect my 3D printer, and I was shocked at how difficult it was to find information. This is a great idea!

    However, after reviewing the design, I had a couple issues with your circuit design.
    1. Too complicated: relies on 2 different relays and a power supply. All 3 have to work properly or the power comes back on. Safety devices should be wired to fail off whenever possible.
    2. interconnect signals are not 5VDC. They are 9VDC and, as far as I can tell, simulate that with a 20VAC signal that is biased 9V positive. This is likely to burn out the coil in your 5 VDC relay which will negate the whole device (see #1).

    EDIT: (I did some more testing after my initial post, and found that at least one relay is required for robust operation.) After digging around a bit more and trying a few things, I decided to build the attached circuit. Connect a 5000 Ohm resistor to the GFCI-protected hot terminal. This will limit current flow to about 20 mA. Use a SS relay to short the resistor output to the unprotected common. This will trip the GFCI whenever the smoke alarm triggers even if just for an instant. The resistor is important to limit the current flow to prevent damage to the GFCI on each test. If you use a different resistor, make sure that you will still get enough current to trip the GFCI (~6+ mA).

    GFCI's do not last forever, so that's the main downside, but at least it's easy to test. I think the simpler design will be more robust too. I welcome any feedback, and again, thanks so much for posting this guide.

    Here's the SS relay that I used. There's normally no current going through it, so the heat sink is not required.

    GFCI smoke detector.PNG
    2 replies

    Reply 3 months ago

    1. Agreed, mine is complicated but I didnt see how to reduce part count at the time.
    2. The 20v signal is just for keeping the relays triggered, it does not interact with the 9v signal.
    3. >>>> Your updated design is very clever, I only see advantages! <<<<
    4. Nowadays there is another type of product available to address the core problem. I know a lot of 3D printers who use these self activated fire extinguishers designed for stovetops.


    Reply 3 months ago

    Thanks for the heads up on the stovetop thing. I was not aware of those products. We had just been discussing how to create a backup device like that. This is on my list of upgrades now.


    11 months ago

    This needs to be on every 3D printer. The parts on the links aren’t available.

    1 reply

    Reply 10 months ago

    Hi, some time ago I produced a very similar product especially for 3D printers. It's solid state rather than using a mechanical relay but adds a convenient little power control and emergency stop button for your printer as well. My design also includes printable parts for the casing and controls. See here


    Question 1 year ago on Step 5


    I investigated this amazing project and am going to try it myself, too. The only issue here in EU (230 VAC area) is the availability of that kind of detectors.

    But: I have currently plenty of these directly at hand:

    - 9 VDC photoelectric smoke detector with only 2 DC power wires

    It's diffrent from your detector, and that's actually the reason I'm writing this.

    So here comes the main thing:
    I'd be very interested to do the same kind of disconnector of this 9 VDC model with 2 power wires only.

    What do you think, would it be hard to implement? I know the very basics of DC, AC, relays, etc., but I'm not a big hobbyist so I would really appreciate some advise. I'm just very passionate of the idea ot this world saving product! (And yes, I promise to test it first with safety-everything on :) )

    Of course, now when I don't have that luxurious extra 5 VDC red wire, I would need to use the power that is directed to the beeping alarm itself, and I'd have to connect that to a relay...

    But unless I find poles in the circuit of the detector that give constant current (when the TEST button is pressed), I may have to use the speaker power poles that get only intermittent current (beep-beep-beep-... sound) and connect those to the relay.

    But even that woulnd't be a problem if I would find a proper latching relay that would switch the AC outlet current permanently OFF from the beginning of the first beep.

    What do you think, how could it be done? I know it wouldn't be as fancy as yours (since yours has 2 power sources instead of 1) but technically? And what kind of components would it require?

    If you have time to give some advise, I'd be very thankful! I'd like to cover my whole house with those if I just get to know the right relay type and how to connect it (in case it differs noticeably from the picture).

    Actually, if you want, I can buy all the stuff for you and send it to you, if you want to give a documented example (like a circuit picture) about it. How does that sound? :)




    4 years ago on Introduction

    While I can see in theory how this might work in practice it won't a) prevent a fire and b) alert you to one until it has got out of control.

    The reason I say it won't prevent a fire is because, as others have said, a smoke detector only alerts you to something that is already happening, hopefully while there is time to do something about it.

    Smoke detectors only effectively work if they are placed at the highest point in the space they protect away from the corners and edges, centrally placed.

    This device is likely to be mounted at a maximum height which will be half the length of the lead supplying it. There a maximum of 1m I would suggest.

    Smoke is hot and rises to the highest point, it would take until the smoke filled the room down to the detector which is going to be below head height by which point you would be choking and the would be out of control.

    Reliance on this device to detect and alert you to a fire as well as to shut of the device would be a little foolish.

    I showed this to a Fire Officer who stated that firstly tampering with a detector is not a good idea, they do contain a radiation source and this idea simply would not function as suggested.

    Its great to have ideas and I am not knocking that in any shape or form. However please don't trust your safety to this or anything similar. Fit a smoke and a heat detector in your workspace and work safely at all times

    2 replies

    Thank you for the thoughtful feedback, but if the explanation in Step 1 and the live demonstration doesn't convince you of the potential effectiveness of this device then we'll have to agree to disagree.

    I have noted that this is intended as a tool of last resort and that using it does not guarantee your safety.

    Also, photoelectric detectors do not contain radiation.


    Reply 3 years ago

    Even the ones that do have radiation are down on the lowest scale possible and shielded inside a metal container so handling them with or without the plastic cover is identical as far as risk goes .

    Good idea here . I expected to see it hacked into the speaker output and did not know of a 5 V signal output . I'll have to look more closely /if I can get past the radiation.


    3 years ago

    Lol. Looks like a camera.


    4 years ago

    I like your idea. Another ounce of prevention.
    I would like to point out an additional safety tip. Do not ever plug a GFCI device into a GFCI plug.
    If you make this with a GFCI outlet, and you it plug into another GFCI outlet the GFCI becomes much less sensitive and likely will not interrupt the circuit.
    I worked in construction and we had to take many OSHA classes and this double GFCI is very strongly expressed. Workers have been electrocuted due to thinking multiple GFCIs would be safe. Bun instead this practice prevented the safety device from functioning.

    1 reply

    Reply 4 years ago on Introduction

    so In other words, all those hairdryers with GFCI cords plugged into GFCI outlets are less safe!

    we've made everything so safe in our world, that we are making things less safe by making things more safe! LOL


    4 years ago on Step 6

    This gets a thumbs up from me. Safety is all important. Good work.

    wood doctor

    4 years ago on Introduction

    you sir are a genius! yes we must consider liability. apart from that this could save alot of lives.


    4 years ago

    it's definitely a good idea, although once a fire is started, cutting the electricity won't do much if the fire is allowed to spread. it may help in putting out the fire though. best to have an inline fuse or breaker circuit and always be present. always ensure you know what you are doing and have a good idea if what the outcome may be..