3D Printer Safety Shutdown - Smoke Detector

by henryarnold Mar 2, 2017
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I built one of these this weekend and noticed that even though its only 3.3v PS to the smoke alarm it still outputs over 9v on the alarm trigger wire. I hope this doesn't cook the chips on the relay boards but at least in short test bursts everything works.
I tried 2 relays from amazon that claimed to be latching but they just toggled with a flipflop.

Question.... If you had to guess do you think the output on these smoke alarms would be capable to power the coil of a relay itself?
For someone with a setup like mine just a simple double pole relay mounted inside the smoke alarm would be perfect.

I'm not sure what your circuit is but the IOT power strip trigger input can handle 3-60 VDC or 12-120 VAC as an input. I can't comment on what other modules/relays can handle because I don't know what you used. On the original design, the latching is provided by the two diodes and the second 5V wall transformer. Once the error is detected, the second wall transformer keeps the power strip in the off state.

Oh, my question was not really geared towards my module. I was wondering if you were also getting like 3x the input voltages out of the smoke detectors signal wire? Second, wondering your thoughts about using the output lead itself to power the coil of a 3 or 5v mechanical relay.

I didn't check the output voltage of the smoke detector because I knew it was in the range of 3-60 VDC. I don't know if the output can drive a relay.

Thank you for the inspiration. If i need to dig into the system later I will check to see if it will drive a relay, but for now it works :)

Comments deleted.

looking for any suggestions. I currently use two of these IOT relays to run on my printer power supplies based off a 3.3v power signal from the 3d printers IO. I toggle this IO from octoprint.
So in my case I need the IOT trigger to be disconnected / open when the smoke alarm is sounding. it sounds like your using it in an opposite state.
2 questions:

  1. how many volts does the smoke alarm trigger output and is it outputting while the alarm is sounding?
  2. Any suggestion for a way to disconnect / interrupt the signal to the IOT relays trigger?

You can use 3-5 volts to operate a small relay that can be used to apply the signal to the IOT. For your example, 3V is the normal on condition. When the relay sees 3V go to 0V, it can apply 3V through the relay contacts to the IOT. You can also do the same with a small NPN signal transistor. You are creating a inversion of the signal. 3V = normal on, 0V = alarm off

Thank you for the inspiration. I ordered a couple tiny 5v to 3.3v regulators, 3v relay boards, smoke alarm from amazon. will try to assemble and design a mounting bracket to contain the electronics this weekend. I need to figure out a way to latch it on when its triggered. I think I have some diodes laying around. Are the diodes subtracting 0.7v? I noticed petg outgasses a little bit leaving residue on the glass bed. I hope this wont trigger the alarm. If so I guess I can switch to the cheaper less sensitive $12 model alarm.

Is there any risk of damage to the printer controller output circuit on the pin that turns on the power supplies (the trigger wire that runs to the relays that switch on the supply) Basically I want to interrupt the trigger wire that runs to 2 IOT relays by using an additional relay. Should I put some kind of large resister value or diode some place so it doesn't just become an open circuit? maybe a dumb question, I just don't know enough about protecting the output on the 3d printer. I assume the IOT side is built well enough that I can just disconnect its trigger wire without damage but the printer controller im not sure.

The extra 5V supply and diode are what latches the shutoff. That will still work after you add your circuit on the Octoprint side.

Any gas released by PETG will not trigger the smoke alarm.

Thank you. Very nice design. Hope to make one this week.

I'm not sure I understand why you recommend wiring the smoke detector to the battery connections with a 3.3V transformer (or 5V with three diodes) rather than with a 9V transformer, inasmuch as the smoke detector can use a 9V battery, and apparently several people have used this shutdown system with a 9V battery. Just wondering if I’m missing something.

The smoke detector I used runs on two AA batteries which is 3V. Some people are using smoke detectors that run on a 9V battery.

How about use the somke detector to control the power socket via RF wirless,like this one

That video shows a really great product. I did a check on Amazon and could not find it there. The video appears to be from Southeast Asia based on the power plug and the fan. I suspect that the socket controlled by a smoke detector is not available in the USA due to product liability issues. Using Bluetooth Smoke Detector to turn off a socket due to fire may not work reliably and could result in a lawsuit in the USA. If I missed the link to the product, let me know.

BTW, my suggested solution may also fail to work resulting in someone suing the creator which is why I offer it as a build it yourself and take the risk yourself solution. I still think its better than nothing but you need to decide whether you trust the solution for turning off a flaming printer. :)

Will this relay work with a UPS that produces a stepped sine wave?

Yes. The power strip uses a mechanical relay which won't care about the type of AC power being switched.

When I was looking at sourcing materials, it occurred to me that a budget version could be assembled from just a hardwired detector like the BRK9120b6CP (about $12 at Amazon) and the matching relay (BRK RM4 Smart Relay for First Alert) which is about $17 at Amazon. The only other materials needed would be a bit of small gauge wire for connection to the detector, an enclosure (printed?) & a sacrificial 3 wire extension cord for the plug & outlet.

The smart relay has both N/C & N/O contacts rated at 15A. Wire things so the plug from the extension cord provides supply for both the detector and smart relay (via 3 small gauge wires per the user manual diagram). The hot wire from the plug is also connected to the relay common terminal. The extension cord outlet hot wire is connected to the relay N/C output & is used to power the printer. Neutral & ground wires are wired straight through to the outlet. When the detector goes off the N/C contacts open & power to the printer is shut off until the detector resets.

This approach does require working with things connecting to 120v, so if you are not comfortable or competent with household wiring projects you should stick with a low voltage approach. The advantages are a slightly lower cost & fewer parts.

thanks, I need an NC relay for my purpose.

Yes that is a great solution which has the same functionality. I did not suggest it because of the wiring of 120 VAC power. The solution I suggested only uses 5V to switch a commercially available power strip.

The cleanest solution would be a custom power strip where the smoke detector plugs into the power strip including the smoke signal output and turns the power strip off when smoke is detected. I suspect that no one offers such as product because if it failed and a house burned down, the company that produced the product could be liable for the failure of the device to shutdown the printer. That's the way thing are in the USA. Someone else posted an off-the-shelf solution that turns appliances off when smoke is detected. I can't find the product available in the USA. Let me know if you find it.

I'm setting this up with two smoke detectors :)
I'd like to have one in my enclosure.
Any idea if printing ABS would set that off in that confined environment?
I think ABS puts the most particles in the air.

ABS will not set the smoke detector off. I've printed lots of ABS with no issue. Only smoke sets it off.

Hey, thanks alot for the instructions, I installed this tonight! Happy to be printing with a little more piece of mind :)

Hello, Is there a way to keep the alarm going? (as in the beeping sound)


Yes, the newer IOT power strip has only one switched outlet so the smoke detector will continue sounding until the smoke clears. There is not a simple way to keep the smoke detector sounding beyond the smoke clearing.

Henry, Thank you for the excellent post. I built it as suggested but without the smoke detector power supply (I just use its batteries). The system works great! I feel much more comfortable leaving my Creality CR-10S running when I am not at home.

Incredible idea and work. Thank you so much for posting and sharing. Will add this to the print and create list.

Thanks for the fast reply. I will order the detector, power strip and diodes and assemble. This is really a good design and a must have. Thanks for putting this together and making it available.

The wall transformers that you refer to in your instructions are no longer available on Amazon. Can you provide the specifications off of your units concerning voltages and amperage's. I have a 120 VAC to 3 VDC 700mA transformer that I have and also numerous 120 VAC to 5 VDC 2.4 A transformers. Base on your write up I assume that these will work. Can you verify for me?

Yes, your wall transformers will work. Any wall transformer that puts out 5V or 3V for the smoke detector will work fine.

Loved finding this for my own printer. Haven't finished construction, but I'm expanding on this by making my own 3D printer box that this integrates with and includes a fire suppression system. It also has a RasberryPI that monitors the system over time and sends me a text for if it ever goes off.

I was annoyed to find that the fire alarm sends a range of 5-10 V AC, but I set up a circuit so that my RasberryPI could take it without dying.

Hmm, are you wiring up the hot to power the detector? My brk/first alert smokie puts out 10-11VDC when just run off of batteries. I just had to make a volt divider to use with my octopi gpio (3v3). I also am just using a cheap relay module I got off amazon (red pcb, has opto coupler with transistor coil drive), which needed no mods or external circuitry to work with the pi. It is pretty simple and works well with the enclosure plugin (so no python required)... powers off my printer, enclosure heater, and dehydrator then sends a notification to my phone if triggered... may add suppression later if I feel the need.

The IOT Relay can handle AC or DC signals which makes it a good solution for being controlled by the smoke detector. Sounds like you came up with a unique spin on the safety shutoff.

thanks for the design. i just ordered all the parts from Amazon. How many printers can this handle? I have 3 3D printers at the moment.

Most 3D printers use around 300-400W. I came up with that number by allowing up to 240W for the heated bed and another 100W for the extruder and other electronics. The IOT relay can supply up to 12A at 110V which is 1300W. You should be able to run two printers from one IOT power strip. I think a third printer might be pushing things. You could try it and see if the IOT relay circuit breaker pops. It would be safer to build two setups.

Made one and it works great, thanks! I used batteries to power the smoke alarm and simplify it a bit

I'm happy to see a few people making this device. I hope others will try it. It is an inexpensive way to protect your home from a runaway 3D printer.

Thanks for this desgn. I plan to use it for my printer.

You're welcome. Some of the people making this design are skipping the wall transformer that powers the smoke detector and using the batteries instead. Another person skipped the diodes and the 5V wall transformer and just connected the smoke detector directly to the IOT Relay. This will work but has the disadvantage that once the smoke clears and the smoke detector goes off, the printer will be powered up again.

I do plan on powering it from a 5v supply reducing the voltage to 3.3 and solderng the: leads to the battery tabs

wish they had not changed the IOT design. I got one of each.

I also wish they had not changed the design. The new IOT still will work with this 3D Printer Safety Shutdown. The difference is that the smoke detector will not shut off but that could be considered a feature. The important thing is that the printer shuts down on detection of smoke.

But is there a way to use it in germany

This design will not work with power lines that are 220-240VAC.

Yes i know nur is there a alternentiv

The alternative is to use something like this:

Many of the Smoke Detector manufacturers make auxiliary relay modules. Check your local sources for "Smoke Detector Relay" You will need to create your own power strip. You may mount the module behind the smoke detector as designed or you be able to mount the relay module inside the power strip.

If you have electronic and Arduino design skills, you could do something custom like this:

I would skip the 555 and use an Arduino. There are examples of Arduino controlled smoke detector power off modules on the Internet.

Fire-Stop Smoke Alarm for 3D Printers

I'm slightly upset. Bought the same exact fire alarm but it doesn't have the option to power with dc supply, just battery or wiring plug.

Wait a second, you didn't have one either, did you? How DID you connect the 3.3vdc psu to the battery compartment?

That's correct. I soldered wires to the battery tabs. Batteries are also an option but you run the danger of them going bad if you don't monitor them.

Excellent. I've been toying with lots of ideas, like temp sensors on my OctoPrint R Pi controlling one of these rely devices.
This is probably better because it's an independent system.

Thanks. You will find that this design is simple and it is easy to put together. It also requires no software.

Thanks for posting the plans for this! I just ordered the power relay from Adafruit and the other stuff from Amazon.

One question though (apologies, but I'm thick) - I don't understand the implications of what you said:
"For safety reasons I powered the Smoke Detector with its own 3.3V wall transformer to avoid having 110 VAC in the circuit"

Could you explain a bit?

Yes. The smoke detector can be powered by 110V which means you would have to attach a power cord into the smoke detector and plug it in. The smoke detector can also be powered by two AA batteries. I felt it would be better and safer to power it through the battery connections rather than having it plugged in. I used a 3V wall transformer to do this. I connected the output of the wall transformer to the + and - batter tabs in the battery compartment. I suppose if you did a neat and insulated job of powering the smoke detector with 110V it would be OK. It will work the same.

Just FYI...when powering the smoke detector via 110VAC, the output when it detects smoke is only about 7VAC, which is not enough to switch the relay. Because I have it wired up already I'm scratching my head trying to think of a way I could still use 110VAC without starting over.

You may be correct. I never tried it with 110VAC. I used a wall transformer to power the smoke detector. Many people have gotten it to work so the design as presented should do the job.

Yeah, I'll probably just end up doing it the way you did it. I was trying to avoid soldering, but at least I know it will work. Thanks for the super simple design!

Did you ever assemble your 3D Printer Safety Shutdown - Smoke Detector? I would love it if you would post it as a thing made. If it didn't work, let me know and I'll help you.

Yes I did... or, at least I did a more simplified version. I've never done any soldering and after I got everything and started to dig in to the project, I felt like soldering the power supply to the battery compartment was more than I could handle, so for now I've wired up the orange and white wires to the IOT relay, and I'm powering the smoke detector with batteries.

Thanks for posting the first make of this thing!

Great application of this idea! I made my own version using relay logic and an off the shelf smoke detector + 2 relays if your interested

Thanks. I considered going the way you did but decided I wanted to create something with mostly off the shelf parts so others could build one easily. Unfortunately no one has tried this design. It is however protecting my home :)

Good idea.
Can you/someone recommend parts available in Europe (220 Volt) ?
Thanks Harry

I don't know of an IOT Relay that operates with 220V. EBay has relay modules which can be controlled with 5V. You will have to provide your own wiring with such a solution.

wish I had one, my printer has cost about 50,000 dollars of damage to my home. :(

I'm really sorry to hear that. What happened? Even though I use this safety device, I often wonder what could wrong if I didn't have my printer protected.

I had just come up with this exact same idea about a month ago! Kinda nice someone else did the work for me now lol

Thanks for the effort :)

Its good that you had the same idea. I actually got the idea from a crowd funding campaign. Their solution uses a custom smoke detector which I think is not as good as a factory produced smoke detector. The only problem with this solution is that the IOT Relay is out of stock. I'm hoping they get it back in stock soon. I would like to see some people try this idea out.

IOT Relay is back in stock!

Thanks for your work. I am very interesting in your work, because I want to build something similar.

The smoke detector you are useing cost in Germany 50€. Not so cheap.

But he power swith box is not available for German Plugs and is out of order at the moment.

Any smoke detector which has three wires will work. The one I mentioned is not the only one. Smoke detectors with three wires use the third wire to communicate an alarm to other smoke detectors so that all smoke detectors will go off when one detects smoke.

If you are comfortable with electronics, you can substitute the following for the IOT Power Strip.
You will have to supply your own power cord and plug for the printer or you can build the circuit inside the printer.

The IOT Relay is in stock but does not support 220V

Thanks for your answer. I think the idea with a relay is very good. I will try this.

The Signal that is comming out of the smoke detector (the third wire you mentiond) is this a an "on" signal when no smoke is detectet or an off signal?

It is an "on" signal when smoke is detected. I didn't check the voltage but I believe it is at least 5V. If it is more, you will need to shift the level or use a relay that operates on the voltage. Also plan to use my latching scheme otherwise when the smoke clears the power will come back on.

My plan is to use a relay that is open in normal mode so you need voltage to close it.
All will operate with an Arduino.
The reason is because it is easy to add something like a CO2 fire extinguisher or to generate a warning message over internet. It is more complicated I know.

Your plan sounds interesting. A cell phone alert would be cool. With an Arduino in the design, you won't need my external latching function. The Arduino can keep the printer off once an alarm occurs. I look forward to seeing your design.

I think the alarm message via phone is one of the last things I plan to realise.
More important is that the right detection and cut of power source.

Today I tried to programm the code for this.

There is a little video from my test exymple.
Unfortnely the speech is in German.
For better understanding:
Red + Sound = Alarm
Yellow = ready but relais is off
Green = in work, relais is on
Blue = stands for the relais
Left switch is reset, for stopping power or for reset the alarm
middle switch is start switch for turning the relais on
the right switch stands for the voltage from the smoke detector I only simulte a incomming voltage, thats what the smoke detector does if an alarm accure.


You can not turn with start after an alarm accures. First you have to reset.

I watched it and even understood a little. I took German in High School and have been to Germany 5 times. Your design looks good. I'll be interested to see how it goes when you add the relay and control the printer.

Excellent idea. I'm surprised that there have not been more reports of house fires, especially with these cheap Chinese power supplies. Thanks for the idea!

Thank you. I was prompted to do something after my heated bed connector overheated and burned. I realize that a printer catching fire is a rare occurrence but I would prefer to not be part of a rare statistic by having my house burn down.


If anyone has questions, I'll be glad to answer them. Just post them here and I'll post an answer.