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A look inside a photoelectric smoke detector that uses infrared light to detect an increasing smoke level in a room.
I strongly recommend buying smoke detectors from local outlets like supermarkets, as that will guarantee a device that is compliant with your countries regulations.
There are very poor quality smoke detectors on eBay that use a cheap microcontroller to give basic functionality (if they even work at all) but for just a little more you can get real branded units.
I think this one either came from Poundland or the local Shoprite (Isle of Man) where they cost around £5.
This one uses a dedicated chip that is designed to self test in several ways, and also reduce the risk of false alarms by differentiating between specs of dust passing through the sensor and actual smoke.
The sounder is driven by a dedicated section of the chip with proper feedback to ensure it is loud by resonating at its maximum efficiency.
The shielded infrared smoke detection chamber has an emitter and sensor positioned to avoid direct light coupling. But the circuit does test the emitter and detector by looking for the low level coupling that occurs by reflection.
I recommend replacing smoke detectors at least every ten years. They can accumulate dust and reduce in sensitivity over time. They cost so little that it can be considered preventative maintenance. If you buy the same type it may fit on the existing base for an easy swap.
If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:-
This also keeps the channel independent of YouTube's advertising algorithms allowing it to be a bit more dangerous and naughty.

I've just bought a smoke detector from the local supermarket for us to take apart. I think this same one is sold by well the local supermarkets and also uh poundland. I think sells the same one so she'll just pop beside this off and slip the detector out and we can explore it internally. I will not set it off because that would just like swamp out the microphone big time, so here it is complete with screws.

I should put that out the way right: let's take a look at the construction, so this is a optical smoke detector and it should be the cap off. It does come with the battery, which is quite nice, super heavy duty. It's a standards of low-ish capacity but relatively long-life battery with the standard battery clip. It's also got this little plastic pin that's pushed down to the battery so that when you try putting it on without that battery in place the pin pops through this, and it stops.

You from locking it to the ceiling, so it keeps dropping off until you change that, let's unclip it. So i think this is just clipped together. One clip two clip it's just clipped back together. Well, that feels like it's more than clipped.

Oh, oh! There are pictures of screws under these labels. That's odd, why would they even put pictures of screws? There is it? Oh, there are screws under them, you'd think they'd just hide them. Is it just a seal or i don't know two screws anywhere under here nope? Oh there we go there, we go. What do we have? There's the piezoelectric cylinder um, it's a three contact version.

That means the middle one usually has a main connection onto the outside. The inner ring here on the crystal is the bit. That is the one that primarily drives the crystal, but then it looks as it's getting feedback pad as well. That gets a signal back to actually create a resonant frequency um.

It is glued, it's physically glued onto the plastic there, so it's not coming off easily. Here's the detection chamber and the three springy contacts going onto that test button. There's not a lot to see. Actually, i think i'm going to have to go a little bit deeper, there's a screw.

Let's go deeper, oh and there's a screen over a little chip. I wonder if it's a microcontroller, if it's a dedicated chip right, tell you what i'm going to take some pictures and then we can take a closer look at circuitry and reverse engineer it. One moment please, the exploration is complete. I've managed to locate this chip, although this is marked kd-5810, it tallies up with a chip.

Well, that's the first thing that comes up in google is a chip called mc145011 and they do appear to be pin compatible. They have slightly different applications. So if you want to have a go at reverse engineering, this yourself, here's the component side and here is the front of the smoke detection cavity. Now, before we start this, i think it's easier to look at the smoke detection cavity so i'll bring that in first here it is grabs a pen we have an infrared led which is pulsed at fairly high current and it fires into this chamber.

The chamber has these slotted louvers around it that are designed to block external light coming in, but they do allow ambient air smoke and indeed dust to go through. This is the thing that kills smoke detectors over time is probably dust build up inside these things. They do recommend you vacuum around them every so often, but really get you can't really get in this chamber. The other part of this, the lid of this it's worth mentioning, also contained had a little metal screen on it.

That uh is just for preventing rf noise from triggering this, and it has matching uh grooves to block light and also the light shield that continues around this, because this infrared led here is effectively firing a fairly narrow beam out and it's got loads of traps. Light traps to try and uh basically keep it to to an arrow beam, and it's also shielded from the sensor. The sensor is an infrared receiver in a black package. The reason it's black is because it's transparent, infrared light, but it blocks visible light.

That's going to help with things like lightning strikes and vicinity shining through windows or camera flashes or bright lights, in a room, there's stuff that might actually manage to get bounced through these things, particularly if they get little layer of dust on them and actually get in. So it won't theoretically see that level of light um and that's fundamentally it you've got the bema light. It can't directly see the led, but what it can see is uh smoke particulates. When you get smoke in this area, it will effectively glow and that will cast light onto that sensor.

It's also worth mentioning, though, that even uh, if this is absolutely clean, even the air, is absolutely pure enough. Light does bounce off the surfaces in here that some will get to the sensor, but nowhere near as much as when there's smoke there and that is used for a self-test routine. As i show show you in a moment, the circuitry has distinct modules. One of the oddities here is that it's got battery protection battery, reverse black protection, but the it's a diode.

But the diode is not in sears the battery, because, if they put in series the battery could drop something like half a volt and that could affect the battery life significantly in this instance. So what they do is it's directly across the battery. So if you're fumbling a battery on and you get the contacts on the wrong way round and they're not clipping in it's actually shorten the battery out when you do that, but uh when you put it around the right way, everything will return to normal use. I guess it's just relying on the impedance of the battery to limit that current, and it's only for those rare instances that someone might actually not look at the terminals and actually just shove them together without checking it has.

The sounder section is another significant section which is interesting. It's got three dedicated pins. A piezoelectric sounder is a back metal disc here, the main metal disc, what they call brass disc on this, and then it's got a disk of the crystal, and then it's got metal electrodes on it. The back one is the sort of common the main big circular donut shape.

One here is the main excitation electrode and the little one is indeed the feedback electrode that, because it's flexing it actually generates a feedback voltage, and what that means is that the suitable circuitry in here they can actually make this operate itself, resonant frequency, so it just. It's absolutely a maximum efficiency and then, when it's glued onto this little cavity in here, everything will be engineered to produce the max amount of noise with the minimum. Circuitry. That's very different from the cheapy chinese ones that, because they're using a cheap microcontroller with no feedback, they tend to drive the crystal the piezoelectric sounder with just a fixed frequency square wave and they try to make up for the lack of efficiency by actually boosting the Voltage and i wonder if this is doing a push pull arrangement on it, it's hard to tell from the the data, but they boost the whole chapter and doctor to try and get the volume up.

But i have come across. Uh smoke detectors from china that uh, you can actually sit with a bench and it's just going: beep beep beep beep, that's literally the volume level, it's terrible. It's almost like a fake smoke detector. Can i point out this time you might be tempted to save just a few quid and it literally is just a few pounds dollars or euros and get a cheap one off ebay, don't get something that's a from a local shop, because then you know you're getting Something proper, the ones from china were pretty much, some of them didn't work, they were fake smoke detectors and the cost difference is negligible.

So this thing has the three pads going to that crystal via the electrodes. On the other side, let me just show you the electrodes. On the other side, that's the three spring electrodes here and the only other components involved in that feedback circuit externally are a capacitor across the common. The that is the actual output pin and the sense pin and also the uh they've got a resistive divider here.

Just to scale that down the signal back to the correct level for the uh chip and these resistors here, this little divider will actually depend on the crystal the actual, the size, the crit, the ceramic disc. Here, there's a test button. The test button is quite interesting. It's just a very simple contact here: it is it's this springy metal strip, pressing against that wire, so that's just a loop of wire through there and the springy metal strip, and it goes down to just a common pin for that.

For that input, it's really just going from positive to the input and when you push the test button, it's not it's doing a lot more than just basically showing the battery is still working testing the sound during the led. It actually executes a proper self test and i'll explain that in a moment, so it does it. It does quite an expensive test on itself to prove that the infrared led and sensor are working. It's clever um right, the infrared led its section of circuitry is over here.

It's got this pin coming out here and it switches a transistor jsj3y. There is a resistor charging, a capacitor here to keep it sort of to give it a good local supply of high current there's the led itself and then the transistor switches. But it's got a resistor on its emitter. I'll show you that in the schematic in a moment, but it acts like self-current limiting, so whenever this pin goes high, it's going to basically make that led pulse at very high intensity inside for detecting the smoke.

The receiver is based around this circuitry. Here are these two yellow pads and it's basically a number of capacitors and resistors. It's all the circuitry is built in, and you know maybe i'll just cut to the chase here, because uh, i'm better, actually showing you what's inside this. That is the best bet right.

Tell you what um, i shall show you what's inside. Oh note, there's a potentiometer for fine-tuning this, that's quite interesting. This is where it will definitely swam power down team it down like this. Here is the sucker trays.

This could be visible. It's not quite up to my usual inky black standards. So let's take a look at the circuitry. Let's make sure it's focused down on that, the there's, the infrared detector, that i was just looking at the circuitry, with all its support components, the capacitors and a resistor, and it's interesting that that potentiometer is in the middle of a potential divider and there's a strobe.

Here and when it wants to actually test this uh, it has a 5 volt reference and it seems to pulse that pin and that's the only point in time that activates the internal circuitry to receive the signal. This is all about current saving. I could zoom down this this. This would be better if i zoomed in like this, because then it would be bigger, so uh when it wants to actually detect it, not only lights the led here, but it turns on this uh input circuitry, and this potentiometer seems to be to fine-tune it just To the mid-range of sensitivity um, it's very hard to say anything specific about this, because the actual circuitry, the actual inside guts uh, all i've really got here - is a block diagram for, what's inside with the oscillator, that's got the main frequency there's a detect input with A couple of capacitor connections and possibly an output um for adjusting the sensitivity um, and it's got all the timing, control logic and it's you know it's just so blocky that it's kind of it's very hard to describe stuff individually, but i'll describe its function afterwards.

What actually happens in normal use? Let's look at the infrared emitter circuitry. The infrared led has to pulse at quite high brightness because they want a good, bright uh in intensity for a good, decent range of test. So there's the 1k resistor charging 100 mg fired capacitor. In this there's, the led there's the transistor that switches it to the negative rail, the zero volt rail, via this current limiting uh resistor, but the current limiting resistor actually acts as a current regulator.

So when this infrared uh output turns the transistor on, not only does it effectively discharge this capacitor through the led, but because this resistor will develop a voltage across according to the current as the voltage rises and gets close to the base drive voltage, it effectively, acts Like a little current regulator, so you get a control pulse, a very specific intensity from that there's a test button which is really just the positive rail to the input. Um. Here's the interesting bit uh. There is a circuitry for the sounder, which has the resistive divider, but notably, this capacitor is actually between here, instead of actually over there in this particular application.

But that shows the main what they call the brass connection, and then it's got the silver which is the outer donut ring and then the feedback, which is the middle thing. The visible led here. This part here is an oscillator, but the visible led that sets the frequency. The whole thing operates, but not the resonant frequency of the cylinder it would detect, determine the number of samples and when they got taken, also to determine this beep beep beep.

The actual speed of that, but not the frequency, which will be resonant to that thing. We have the visible led, which is a resistor in series, but that does more than one thing when it's enabled it brings the led, but it also that puts a slight load in the battery and then there's a voltage divider here that senses the voltage across the Battery, let me just nudge this down, so you can see the voltage divider. So when the led turns on uh it pulls the battery voltage down. At the same time, this detection circuit becomes active.

It scales the voltage down, so it can actually go to a specific threshold inside and if the battery is getting too low under the load of that led flashing, it knows that it's time for the battery to need to change and that's when it will make that Beep noise, presumably it just fires the beep straight off that because it happens every time the led flashes so right. So here is the interesting bit. This is what happens when the thing is in normal operation. Let me bring this exhibit back in again the sensor chamber.

So when you see the little red led on the front of the smoke detector flash it's that little red led there at the same time, it's flashing, this led inside the chamber, but it's also turning this sensor up to full sensitivity. It's actually doing a self-test because it doesn't, this led, will flash many more times than the the red led in the front. But every time you see the red led flash. It uh turns again this up and it looks to see with just bounced infrared, even without smoke.

If it can see the led, if it doesn't see the led, it knows as a fault and starts sounding the the alarm mode. To actually show you that there is a fault, probably just that peep noise, just to show that something is wrong and you change the battery and it still peeps. It means like it means it's knackered, getting your smoke detector. So it's using that to detect uh that there is that the infrared led and the sensor are working, but in normal operation it will just flash it every so often like that it will just do a sample in this cavity if it detects and it uses low Sensitivity if it detects uh contaminants, it uh increases the speed it actually samples and also changes the sensitivity of the circuitry.

Looking at that and it'll monitor, we'll just set the alarm off straight away. It's looking for an increase that keeps increasing, because what you could get here is you could get a spec of dust could pass through this. Then fred could bounce off it and if it does that, but then the dust then passes out of view, it will not set the alarm off it's designed to just to discriminate between uh, false alarms and uh actual smoke. But if that continue, that uh haziness in this chamber keeps increasing, then it reaches the threshold and after a number of set tests, it will start sounding alarm.

That's fundamentally it. It was interesting how this also had the little shield plate over the chip, which is nice? One thing would be better with this: the circuitry that is the most sensitive in here. Let me just uh bring this up again that failed to oh, no, it didn't actually did it. The socrates, the most sensitive, is the the receiver circuitry.

I wonder if it would actually be a good idea to flood a bit of nail varnish or something over here, although once you do that you're kind of modifying something out with its normal specs, but that could actually make it less sensitive to little spiders crawling across Which is what can set these off? I've had uh my smoke alarm just suddenly go baby then shut up again, and then i saw a little uh bug crawl out the other side of it. I'd obviously gone across that i don't know if i'd gone into the chamber, but i think it just bridged with its little moist bug feet. It bridged the circuitry and just caused that false alarm but interesting stuff. There's a lot more sophistication to the real smoke detectors and there is to the fake ones the the cheapy chinese ones uh, and it's interesting how they go that one chip is doing everything.

It's the component count for what it's doing is actually fairly low. It's also notable here they've got led position right next to the button for when they want the button itself to be the indicator, but in this case they've chosen a separate led from the button and it's nice. I wonder how they calibrate in the factory i'd. Guess they put in a jig with the test points and they tune it to a very specific voltage uh on the input under sort of test conditions.

But there we go interesting well worth taking apart and remember. Only by real smoke detectors and every well five or ten years, just swap them out they're, cheap! That's! What's that that's one or two pounds a year per smoke detector to have that protection in the background just changing the battery after, like 10 years or whatever it is, it doesn't really solve the problem of dust build up in these you're going to have much more Sensitive and reliable detector, if you just replace them on a fairly regular basis.

16 thoughts on “Inside a proper optical smoke detector status”
  1. Avataaar/Circle Created with python_avatars Pulverrostmannen says:

    Nice demonstration you made here, so now we need to look inside one of these radioactive ones which charges the smoke particles when the radiation strikes them and generates a voltage on the sensor, nah I know you already know this and I value your safety. keep up the good work!

  2. Avataaar/Circle Created with python_avatars Mike Cummings says:

    Same company that make light bulbs an torches. I'm sure the older ones had a radioactive component inside, the ones made in the 80s, 90s, I could be wrong though but no mention of such a component here, maybe the technology of these things has changed, just like a lot of things nowadays.

  3. Avataaar/Circle Created with python_avatars zole says:

    Somehow I don't trust these. The proper optical detectors has chambers almost the size of outside housing. They are properly certified and some can even tell the particle size eg. differentiate between different smoke types and dust.

  4. Avataaar/Circle Created with python_avatars Matthew Gies says:

    A science teacher taught me long ago that smoke detectors contain Americium-241 and should thus not be dismantled. Curious after this video, I looked into it and Wikipedia tells me that that type of ionization smoke detector is banned in several jurisdictions–but because of reliability concerns, not radioactivity.

  5. Avataaar/Circle Created with python_avatars ss says:

    As always, an excellent and fascinating analysis, Clive. Perhaps proper conformal coating would be more suitable to coat the sensitive receiver electronics. Though, it is still altering it as you say. People must bear in mind, the real cost difference between a correctly engineered smoke detector and a fake, is your life. Don't skimp on it.

  6. Avataaar/Circle Created with python_avatars John Crooks says:

    A great explanation of the circuitry and how a smoke detector works. Plus, the added info about how important it is to keep the dust trap chamber clean. Also, to press the test button on your regular testing day, e.g. Monday… If it is too high for you to reach, then use a long brush shaft or what you can to press the tester button to check the alarm.

    Please remember to never cover up or remove a smoke alarm unless it is faulty. Contact your building fire officer and/or handyman. If in doubt about its working condition telephone your local fire station on their non urgent directory number to inspect the alarm.

    In the meantime, if you smell smoke, see smoke or the alarm sounds, Get Out & Stay Out. Never go looking for the cause or tackle on your own. Use any anti fire Blankets, Sand Buckets, Fire Hoses or Extinguishers etc to aide your escape. They are best used for that purpose to save your life. Leave the Fire Fighting to the professionals who are qualified to tackle a blaze of any size, small or large.

  7. Avataaar/Circle Created with python_avatars Livius Cazan says:

    Great video, as always! Would be great if you would do one on the various bms boards available on Aliexpress, such as the HW-391 bms. I've seen them used in upgrading power tool batteries from NiMH to Li-ion, and I wonder how reliable they are. Thanks for all the work you put into your videos.

  8. Avataaar/Circle Created with python_avatars Mervyn Partin says:

    Fascinating take-down.
    I've never been a fan of optical smoke detectors, due to my experience of them in the cable tunnels in the power station at which I spent most of my career.
    There was a mixture of ionisation and optical detectors due to the different smoke particles that could be expected in a fire, but during every harvest season, the thripps (thunderbugs or corn flies) would leave the fields and head straight for the optical sensors putting them into alarm. The only remedy was a massive program of insecticide spray, carried out either by our staff or the fire alarm contractors.

  9. Avataaar/Circle Created with python_avatars Martijn Wismeijer says:

    All my smoke detectors are Nest Protect. Expensive but will send a push notification whenever fire, carbon monoxide or steam is detected (steam can be switched off) and batteries last for years. They also give a nice glow at night when you walk past, eliminating the need to switch on a light in the middle of the night when taking a pee. Saving money on smoke detectors is penny wise, pound foolish in my opinion.

  10. Avataaar/Circle Created with python_avatars Frank says:

    I invested in high quality ones and I had no problems at all for many years now and the normal batteries last for ages.
    It beeps at low volume (only during daylight) if the battery is empty.
    There is some fine mesh around to keep dust out.
    I will buy those again if they expire.

    My parents bought cheap ones and they are triggered for no reason in the middle of the night and need batteries every year. They beep at full power as well when the battery is empty. Flashing the led all day and night.
    Just not worth it.

  11. Avataaar/Circle Created with python_avatars Jeremy S says:

    Hey. Any smart people out there? I'm not. I need to create a "gum stick" servo thing for my papercraft models. The concept is, several layers of heavy cardstock, with 2 electromagnet paper strips, and a magnetized metallic paper strip in the middle, that moves up and down. The whole thing would be somewhat flexible and a fraction of an inch thick.
    Could that be done?

  12. Avataaar/Circle Created with python_avatars Stoney_Eagle says:

    I find it fascinating that hanging a few 10 pound devices on the ceiling here and there will safe your life one day.
    Just how it detects smoke is soo cheap just a piece of plastic and 2 diodes, yet it's soo reliable.

  13. Avataaar/Circle Created with python_avatars Arcana Octonus says:

    Fire department near me gives them away for free every year. So it is primarily carbon monoxide fatalities during power outages. Hell, one landlord put a generator in a renter's house last year after an ice storm. Bunch of kids lost in that tragedy, like 7. I've crawled out on my belly myself when the combustion air inlet had a starling nest in it. You don't even know what's happening.

  14. Avataaar/Circle Created with python_avatars Charles Fasano says:

    The thing I hate most about smoke detectors is that it is extremely difficult to find the one that needs a new battery as the beep is not frequent enough and our house echos so much you cannot tell where the beep is coming from. Every room has a detector so we have about a dozen or so detectors in the house.

  15. Avataaar/Circle Created with python_avatars Dan Lemke says:

    I recently took down fire detectors that had no electronics from an old house I bought to remodel. Very heavy and took me some time to figure out all the gizmos that made it work, when it turned out to be quite simple, yet effective if you were within earshot. I would love to send you one if interested. Ear plugs are a must. You might already be familiar with them. They are quite heavy.

  16. Avataaar/Circle Created with python_avatars Andrew Radford says:

    My old boss thought he had a gheko in his light fitting. Took about a week of cool winter nights to work out it was the occasional low battery chirp of the smoke alarm as the battery finally reached ambient temperatureand lowest voltage late at night.
    Next year all rentals in Queensland Australia require hard wired or wireless interconnected smoke alarms, so if one goes off the rest will. Thanks to lithium cells, the wireless ones have a 10 year battery life.
    That optical detector is just brilliant except for the occasional bug detection.

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