A simple standalone smoke detector, or a device that could actually save your life.

About 17–18 years ago, I saw a similar sensor in the kitchen of one of the NRs. While the apartment owner was away, we even tried it out with the builders. But they were quite expensive back then, so I didn't buy one, and then I somehow forgot about it. A few days ago, a similar (if not identical) sensor was sent to me free of charge for review by the chinabuye store. Read the description, see the photos, and find out how it works below the fold.

A lot of time has passed since I first came across this kind of sensor. Since then, I've installed our SPD 3.2 units and their equivalents several times. But they all need an external power supply, and I wanted a standalone one.
Anyway, let's take it in order. The sensor arrived in a regular envelope. The model is SS-168.


Inside was the sensor in its packaging and a battery—but more on the battery later.


The sensor's box contained a manual in perfectly understandable Chinese and the mounting hardware in a little bag.


I mentioned above that the envelope also contained a battery. Don't be fooled by the text on the sensor's packaging saying that a battery is included. There is no battery in the box. The website does mention this, so I ordered the battery separately (fortunately, I could get it for free, since it was rather expensive and easier to buy locally). Still, I'll say a couple of words about it.
It's a standard non-alkaline battery. I don't know the manufacturer; it could be an OEM product, or perhaps a small company makes batteries under its own name.


The open-circuit voltage was about 10 V, and the expiration date was 2016, so it was relatively fresh.


But the battery isn't particularly interesting, so let's move on to the sensor.
The manual gave me almost no useful information, and I doubt there's anything useful in it anyway. Judging by the transmission frequency specifications, I even wondered if it was for a different sensor or a different version of this one.
I took the trouble to find a manual online in much clearer English.
www.firequip.com.au/files/Technical%20Data%20Sheets/Passive%20FirePro/SS-168%20Smoke%20Alarm.pdf
It turns out the sound level is 85 dB, and one battery lasts about a year.


Naturally, I wanted to open it up (how could I not?) :)
By the way, you can't put the top cover on the wrong way; it only fits in one position. Well, you could force it on however you like, but that's another story—some people even manage to plug power connectors into a computer the wrong way around.


But you can't get very far into the internals from the top, so I had to take it apart from underneath.
The underside also has a compartment for the battery (there's no cover included, but since the sensor is usually mounted on the ceiling, it probably isn't needed), as well as a mounting plate.


Inside is a circuit board with a smoke chamber (not to be confused with video cameras :) ) and a piezo buzzer.


A closer look at the board. You can see a chip with its markings erased (I doubt it's a controller or an expensive chip; otherwise they wouldn't have erased the markings), the chamber with its photoresistor and LED, the piezo buzzer, and a trimmer resistor.
The piezo buzzer is very loud—you can hear it clearly throughout the apartment, even with the doors closed. It can certainly wake even a sound sleeper.


The back of the board.
There are more components on the back of the board. The board is soldered fairly neatly, even cleaned quite well, and the components are labeled. There's a diode on the power input to protect against reverse battery polarity. You can also see contacts for installing an RF transmitter module (that's my guess), as well as pads for a relay. Apparently, the board is universal and made to work with all versions of the detector.


Compared with the SPD 3.2, their diameters are roughly the same, but the SPD is a little taller. The SPD also has a metal mesh covering its internals. The mesh also acts as a shield against electromagnetic interference and is connected to the board by a small metal tab. The detector being reviewed has a test button, with a note next to it saying it should be tested once a week (though I think once a month should be enough). Pressing it lights the LED and activates the buzzer for about 2 seconds.


This photo shows that the chamber in the SPD is noticeably larger, while its board is also noticeably smaller.


The chamber in the SPD is much more complex in design, and the SPD detector's electronics are noticeably more sophisticated too. But these detectors belong to very different classes. The SPD is certified to meet the relevant standards and can be used in fire alarm systems. The Chinese detector is intended for ordinary use, and it may be more prone to either failing to trigger or, conversely, false alarms—there are no standards governing it.


A few words about how smoke in the air is detected.
There are several detection methods: linear, through-beam (two sensors, a receiver and a transmitter), more compact models with a smoke chamber—two in one, so to speak—radioisotope sensors, and sensors with forced airflow. The color of the smoke can also be critical for the sensor.
I won't quote or rewrite Wikipedia; I'll keep it brief.
This one uses a smoke chamber. It's a point detector, meaning it detects smoke only in its immediate vicinity. The chamber's complex design is due to two completely opposing requirements. Its housing must allow air to circulate as freely as possible while blocking stray light as effectively as possible. The more complex the smoke chamber design, the better it suppresses unwanted external light and the fewer false alarms it produces. The shape of the baffles must also restrict airflow as little as possible. Naturally, a certified detector pays more attention to this than a simple home model.
The chamber contains an LED and a photosensor. The LED operates in pulsed mode to conserve energy. They cannot see each other because they are not on the same optical axis. When smoke enters the chamber, light from the LED reflects off the smoke particles and reaches the photosensor. The signal then goes to the control circuit (for example, it may trigger after several pulses or be synchronized with the LED). Once the alarm signal has been generated, the output device takes over: this could be a piezo sounder, a relay that opens the security system circuit, or a radio module.
For those who want to know more — ru.wikipedia.org/wiki/Пожарный_извещатель
This is how I understand it, so there may be some minor discrepancies.

The detector's quiescent current is about 5 µA. Every 5 seconds or so, it jumps to around 20–30 µA, and every 40 seconds the indicator LED flashes, with the current jumping to 100–150 µA.
The dynamic current is certainly higher. A digital multimeter doesn't show it accurately, and the duration of the current pulse matters too. I didn't investigate this in detail; I'll just say that the internal 100 µF capacitor is enough for 5 LED flashes, or about 3 minutes of operation. As I mentioned above, the manufacturer claims a battery life of about a year from a single battery. The LED on the SPD flashes at 1 Hz, but noticeably dimmer.


Testing.
Since I smoke, I didn't have to start a fire at home.
I tried to position both detectors identically for the test.
Then I directed smoke toward each detector, both by exhaling toward it and by holding a cigarette at the same distance (as you can imagine, I don't have any special equipment).
Subjectively, the SPD triggers a little sooner; the difference was insignificant—for example, 3 versus 4 seconds. The only difference is that after triggering, the SPD locks out, and can only be reset by disconnecting the power (so you can identify which detector triggered). The one under review, on the other hand, silenced its alarm and returned to standby about 30–50 seconds after the smoke was gone. When it triggered, the LED started flashing at about 2 Hz.

The detector weighs 115 grams with the battery.


For now, I've installed it on the balcony. About 4 years ago, I clad the balcony with varnished paneling. Since we're almost halfway up the building and have neighbors nearby, there's a risk that something could catch fire on the balcony. I tried setting something alight, and it's not that easy, but I'm still a little worried. I also wanted to put the detector through some extreme conditions: bright sunlight, humidity, and temperature swings. It worked fine for a week, despite the humidity, bright sun, and heat. I still need to test it in the cold this winter.
Here's what the detector looks like in place. The ropes get in the way a little when taking photos.


One more close-up.


No complaints so far. I’m planning to buy a couple more: one for the kitchen and another for the cabinet on the balcony where my mini server is located (after all, it runs around the clock unattended).

Yes, there are sensors that send alerts over GSM. I might buy one of those to try as well—I'm curious, but that’ll have to wait for another time.

The sensor is very basic, of course, but it could save someone’s life. I’ve already had a situation where the neighbor downstairs put some potatoes on to boil, then lay down to rest. I noticed smoke coming from her window and called the fire department. When they arrived and climbed into the apartment (it was on the second floor, so it wasn’t too difficult), they found a smoking pot and a girl fast asleep. They barely managed to wake her and bring her around. She might not have woken up at all. A sensor like this can sound the alarm before the smoke makes you drowsy, and could easily wake or alert someone in the apartment.
So these kinds of incidents are no joke to me.
I have a gas stove with flame-failure protection at home, but that doesn’t rule out the kind of situation described above.

Summary.
Pros
The sensor works as it should, and might save your life one day.
It runs on its own battery, so there’s no need to run wires, and installation is very simple.
The price is more than reasonable. The same SPD 3.2 costs about as much as the one reviewed here, but for similar use you’d also need to build a circuit with an emitter and a power supply. These sensors also cost more on AliExpress (unless you buy 100 of them).
The sensor uses very little power, but that doesn’t mean you can skip periodic checks.

Cons.
You have to buy the battery separately.
Because of the simple design of the smoke chamber, I wouldn’t recommend installing the sensor somewhere it could be exposed to very bright light (for example, direct sunlight).
The sensor is sensitive to RF interference. If you place it on a cable from a switching power supply (not all of them, but it reacts to some), the LED starts blinking much more often (about once a second), though it doesn’t sound the alarm. Apparently, the polling frequency changes. So it’s best to keep it away from power cables. Our SPD is completely immune to this.

Of course, this isn’t a tactical hole punch with a built-in knife, flashlight, and compass that will help an office worker find their way back to civilization after accidentally ending up in the desert. It’s just a device that could genuinely come in handy one day. And it doesn’t matter where you find one or how much it costs.
In my personal opinion, every home should have one of these, and quite possibly more than one.
If I’ve forgotten anything, let me know. I have the sensor here, so I can do more testing, take photos, or describe it in more detail.

The sensor was provided free of charge for testing and review by the chinabuye store.