“Confession” of A Photoelectric Smoke Alarm

CJ Lim
8 min readJul 14, 2020

Most would know the responsibility of me as a smoke alarm is, well, to detect smoke, which is a precursor to fire. When smoke density in the immediate area reaches a “critical” level, I’ll ring off to notify my occupants of a possible fire risk.

This is important as statistics have shown:

  • one may have less than 3 minutes to escape from homes on fire
  • most fatalities are from smoke inhalation rather than the fire itself

The early notification from me gives people precious minutes

  • to investigate if there’s indeed a fire
  • if there is, to try putting out the fire, eg with a fire extinguisher
  • if unable to put out the fire, to escape with family

Broadly speaking, there are 2 types of “me”:

  • Photoelectric smoke alarm
  • Ionisation smoke alarm

I’m the former one — the photoelectric smoke alarm, more precisely, the OSD-01 smoke alarm from Blaze Guard.

Cool so far? I have a “confession” to make. A confession of how I detect smoke.

Do note that most smoke alarms will have their warranty void if any attempts of tampering, including opening up of smoke alarm, are made. So do not try this at home with your smoke alarm.

OSD-01 Smoke Alarm from Blaze Guard

Below is how I look like with my outer housing removed. Let’s go through them part by part.

From Top to Bottom, Clockwise: Metal Shield, Bottom Cover, Housing of OSD-01

Let’s have a close-up look at my metal shield (below). This metal shield is the one that gives me my anti-dust feature to keep maintenance easy. You can see that the diameter of the holes on the metal shield measure in millimetres. This allows smoke particles to pass through me easily, while preventing dust build up within myself. Instead, dust will be trapped at the outer surface of my shield and my owner can easily wipe the dust away.

Here’s my bottom cover in greater detail.

Bottom Cover of OSD-01

These “pillars” give strength to my overall integrity as my metal shield is quite thin.

I’m designed with an irreplaceable lithium battery. You can see that both the positive terminal and negative terminal of the battery are soldered onto my circuit board. This makes me, the alarm, tamper-proof.

Do you know that it is recommended to use smoke alarms with irreplaceable batteries?

One may think a smoke alarm with an irreplaceable battery is less convenient since it doesn’t allow one to change battery when battery is low and/or one has to replace the entire alarm.

While the above may sound correct on first impression, the REAL question is will you remember to change battery when battery is low? Or will you replace the entire alarm after 10 years of use, or will you simply take the easy option of replacing the battery?

It’s likely to be No to both questions. Hence, using tamper-proof smoke alarms with long-life lithium batteries provides your family with 10 years of continuous smoke detection and no need to worry about changing batteries.

Now you may be wondering why 10 years? As with any electronic device, the hardware will deteriorate over time. Studies have shown that the electronics within the detectors fail at a rate of 3% per year. 25% is chosen as the acceptable cumulative fail rate. Hence, the 10 year mark was chosen since by the 10th year, it’ll be approximately 9 * 3% = 27%.

If you are now wondering “10 years isn’t short, does it mean I have to throw away the entire alarm and purchase a new one should the battery dies before 10 years?”, this is a valid question. And this is the reason why you should buy from reputable brands whose smoke alarms have underwent stringent certification, such as VdS 3131.

I (Blaze Guard’s OSD-01 smoke alarms) am VdS 3131 certified.

Source: VdS

In addition, Blaze Guard has a 10 years battery warranty on me, where a 1–1 exchange will be done if my low battery light appears within 10 years of my usage.

For avoidance of doubt, it does not mean one should skip conducting monthly testing of the smoke alarm that comes with irreplaceable batteries. Monthly testing is still necessary as one will never know when the electronics will fail.

Let’s move on to my housing. The black structure is my photoelectric (hence the name of this type of smoke alarm) chamber which does the magic of smoke detection. My chamber is mounted on a circuit board.

Photoelectric Chamber

Beneath the circuit board is my speaker. This is the part of me that will “scream” at high pitch when I detect smoke.

Alarm Speaker

My circuit board consists of various organs such as multiple resistors ( R ), capacitors ( C ), transistors ( Qn) and an integrated circuit ( U ). All these work in conjunction to help me detect smoke and sound off the alarm if required.

Circuit Board

There are 2 switches ( SW ) in me (circled in red above). Depressing any will trigger my test function, or silence me if I’m screaming. It is these 2 switches that gave me the unique feature of a large test/silence button, essentially the entire surface area, as compared to most of my other counterparts in market with small test/silence buttons.

Test Button of OSD-01 Compared To Other Models In Market

By now I have covered all the components within me, except that of photoelectric chamber. Before we dive into this, let’s talk a little on how I as a photoelectric smoke alarm generally works.

How Photoelectric Smoke Alarm Works?

A photoelectric smoke alarm contains a source of infrared, visible, or ultraviolet light, a lens, and a photoelectric receiver/photodiode. In light-obscuration type, the light emitted by the light source passes through the air being tested and reaches the photosensor. The received light intensity will be reduced due to scattering from particles of smoke, air-borne dust, or other substances; the circuitry detects the light intensity and generates an alarm if it is below a specified threshold, potentially due to smoke.

Source: ElectronicsForU.com

In light-scattering type, the light is not directed at the sensor, which is not illuminated in the absence of particles. If the air in the chamber contains particles (smoke or dust), the light is scattered and some of it reaches the sensor, triggering the alarm. This is the type that is deployed in the OSD-01 smoke alarm that we will be examining next.

Source: ElectronicsForU.com

Below is the top cover of my photoelectric chamber. Other than the housing for the photodiode detector and infra-red (IR) emitting diode, you can see that my surface isn’t smooth. Instead I have plenty of reticulated structures. This is to reduce the possibility of reflection of light from the top cover into my photodiode detector, which otherwise, may result in false alarms.

Similarly, my entire chamber is constructed in black plastic to prevent reflection of light into my photodiode detector.

Top Cover of Photoelectric Chamber

A close-up view of my infrared light emitting diode (IR LED) sitting in its housing. An IR LED is a solid state lighting (SSL) device that emits light in the infrared range of the electromagnetic radiation spectrum.

Infrared Light Emitting Diode (IR LED)

A close-up view of my photodiode in its housing. A photodiode is a semiconductor device that converts light into an electrical current.

Photodiode Detector

Light from the infra-red emitting diode is transmitted across my chamber (orange arrow). Under normal conditions, the light will simply hit the wall at the opposite end of the chamber, with no light hitting my photodiode detector. Without any light on my photodiode detector, no electrical current is generated and hence my alarm don’t sound off.

In the event of smoke particles within my chamber, infrared light from the IR LED is scattered (yellow arrows) upon hitting these smoke particles (red dots). Some of these scattered light will fall on my photodiode detector, thereby generating an electrical current and making me scream.

Within my photoelectric chamber, I have a vane sidewall to allow easy access of smoke into my chamber, yet at the same time offering adequate obstruction of light into my chamber. Recall external light should not be in my chamber or it will be picked up by my photodiode detector and eventually I will scream, a false alarm in this case.

The sidewall of my chamber further away from my battery has an easier access for smoke particles than the side nearer to my battery since my battery is in the way of smoke particles into my chamber.

To reduce false alarms, the arrangement of the vane sidewall is such that smoke particles will circulate in a clockwise manner within my chamber.

If smoke particles enter my chamber via the sidewall further away from the battery, it will flow towards my IR LED, instead of my photodiode detector. Light from my IR LED will be scattered upon hitting the smoke particles, however this scattering is unlikely to reach my photodiode detector since the scattering happens nearer to my IR LED. You will notice there’s a centre obstruction (circled in yellow) to prevent the scattering nearer to IR LED to reach my photodiode detector.

There needs to be significant smoke particles in the atmosphere to fill up my chamber. At this point, through scattering of light in between smoke particles in my chamber, some of these light will reach my photodiode detector. An electrical current is generated as a result, activating my speaker and thus the making me scream (smoke alarm is triggered).

This concludes my entire “confession” as a photoelectric smoke alarm. Hope this has been useful to all of you.

Before ending, a gentle reminder to install me (smoke alarms) in your homes if you haven’t. The early warning from me can make a difference between life and death.

It is not just an early warning. It may be your only warning. Yes, I am that important :)

For full disclosure, I have vested interest in Blaze Guard. You can visit this link to learn more about Blaze Guard’s OSD-01 photoelectric smoke alarm.



CJ Lim

Editor of my own life, father of 2 and husband to 1.