Introduction: Prevent House Fires With the Smoke Detector Disconnector
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!
Step 1: THEORY OF OPERATION:
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)
Step 2: BILL OF MATERIALS & TOOLING:
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) 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) 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)
**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!
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Step 3: CUT THE BOX
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.
Step 4: MOUNT ALL THE PARTS
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!
Step 6: FINAL ASSEMBLY AND TESTING
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.
Step 7: SMOKE DETECTOR NOTES:
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.
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Step 8: GFCI NOTES
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.
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Step 9: VIDEO & FINAL NOTES
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.