PIN photodiode collects, counts, and discriminates alpha energies to determine residences’ radon gas-caused cancer danger degrees.
Back in late July, when I was working on last month’s published piece on radon gas analysis, I longed to see what was inside the battery-operated sensing device I’d recently started using.
But I resisted the temptation to fire up the spudger and screwdriver set, because:
- I’d bought it myself, for $100+, and
- I intended to continue using it long-term, since radon gas concentrations vary over time.
Even if I trusted my ability to disassemble it and then put it back together again in still-full-functional form (which I don’t), I didn’t want to interrupt the data-logging cadence even briefly.
But Providence seemingly intervened on my behalf, as counterbalance to my reticence. Less than a week after my post appeared, a PR contact from Chinese company X-Sense, who swore she hadn’t even seen my coverage yet, reached out to see if I was interested in reviewing its latest consumer-tailored smart radon alarm, the just-introduced XR0A-iR. The device is so new that it only showed up on the company website a few days ago, as I write this on September 10, 2026.
Spooky, eh?
I accepted the invitation with the as-usual qualifiers in such situations: that the company would not see my coverage until it appeared in publicly published form on EDN’s website, and that X-Sense would have no ability to influence the content either in-advance or post-publication. The PR contact agreed.
I also asked for two units; one for hands-on testing (for which I planned to recruit my next-door neighbor, both because he didn’t have one yet and because I wanted to get a non-techie’s take on activation and ongoing use) and the other to satisfy my own teardown curiosity.
Smart equals pervasive data access
X-Sense agreed again. And late last month, two units showed up at my front door. My next-door neighbor successfully activated his yesterday; stay tuned for his hands-on observations to come in a future post. And that same (yester)day, I took the other one apart.
X-Sense is a smart home device manufacturer of which I wasn’t previously aware. The company’s products are analogous to those (for example) of TP-Link, whose Kasa and Tapo temperature, humidity and fluid leak sensors I’ve dissected in recent months. X-Sense already had two radon sensors in its product portfolio, the XR0A-SR and higher-end XR0B-SR, but they’re standalone-use units. The XR0A-iR is its first “connected” radon-related product.
I’ll as-usual start with some outer box shots, accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes. Top.
Front.
Left side.
Back.
Right side.
And bottom.
Packet secrets
The first thing you’ll see after removing the box lid is a comprehensive and consumer-comprehensible (those adjectives rarely get used together, in a positive sense, at least!) user guide.
Underneath it is our patient.
Surrounded by two mysterious baggies.
I jest; they’re not mysterious, because they’re labeled. One, containing silica gel, I commonly encounter to absorb humidity, thereby keeping the electronics dry. The other, marked as indicating that inside it was activated charcoal, was a bit more of an initial head-scratcher.
But then I recalled the mail-in radon lab test (mentioned in my prior writeup) that my wife and I ordered prior to purchasing our home a decade-plus ago, and which looked something like this.
I rattle sound-suspected at the time, and subsequent research has confirmed, that inside it was activated charcoal, used both for short-duration testing and (interestingly, at least to me) for filtering radon out of well water. That said, it’s not recommended for ongoing residential-air radon gas cleansing purposes. Here, I’m guessing, it absorbs ambient radon gas at warehouses and retailer shelves that would otherwise end up on the device’s sensor, adversely affecting subsequent initial-powerup measurement results in the process.
Onward. Here’s our patient, now freed from its previous cardboard captivity. Top (note the grille; hold that thought).
Front.
Left side (note the vents).
Back.
Right (ditto).
And bottom.
Before beginning the dissection, here’s an obligatory company-supplied conceptual image of the supposed “guts”, that I know you all love so much (me too).
The battery compartment often effectively does double-duty as the pathway inside the device. Let’s see if it pans out again.
Those four screw heads, one at each corner, look promising. The unit specs a 2-year (and user-replaceable) battery life claim, by the way. Not too shabby!
Before proceeding, though, let’s zoom in on the always-informative FCC ID (2AU4DDDM).
And now for those pesky screws.
(Non-)adhesive (non-)impediments
Ladies and gentlemen, we’ve hit the jackpot!
Those white glue deposits at both flex cable-to-connector junctions initially gave me pause. Was I not going to be able to keep this teardown non-destructive, always my preference?
Not so “gluey” after all, it turned out. Never mind. Keep calm and carry on.
Let’s focus our attention first on the top panel and interior of the upper enclosure.
Four more visible screw heads suggest a way to detach the former from the latter. Let’s see.
Bingo!
And now for another perspective, after flipping the enclosure over.
Setting the top panel aside for the moment, we now see that two more screws hold the display assembly in place.
You know what comes next.
And there she goes.
Removing four more screws, I’m betting, will give us a glimpse of the display backside.
I’m on a roll! It’s usually at about this time of inflated self-confidence, by the way, when I short out something, or cut myself and bleed all over everything. Consider yourself warned.
This is not the ventilation grill you’re looking for
Now for the top panel, and maybe the most baffling aspect of this design. Look back at the earlier top-side overview shot, and you’ll see what appears to be a grille. We already know from the box-bottom notations that this is a passive diffusion design; no (noisy and battery-draining) fans, only natural airflow through the device. Does it flow through the top?
The fact that the documentation only notates the earlier-seen enclosure side vents as “air intake areas” is head-scratching. And when you look at the grille from the inside, you see a black piece of plastic nearly completely covering it. So “nope” is apparently the answer to my question.
So, then, why is it here at all? The earlier-mentioned high-end standalone XR0B-SR model has a speaker up top and behind the grill, for audible-alert purposes.
But it’s also got buttons up top, versus in the front with this particular model, so it’s not like X-Sense can reuse any particular assemblage piece across multiple models.
My only other possible conclusion is that the company is striving to establish a common cosmetic “look” across all models. To wit, ironically, I suspect that the purpose of that black plastic piece is to prevent ambient airflow from going in and/or out the top, forcing it to instead route from one side to the other, presumably over the sensor in the process. Other reader ideas are welcomed in the comments!
Speaking of which, that varying black foam-and-shiny silver plastic circular region up top, held in place by an also-plastic brace, looks promising as a potential radon sensor location.
Let’s see what’s inside it, after first perusing other varying-location perspectives of the overall internal assemblage. Front.
Left side.
Back.
And right side.
Sensing insights
I’ve delayed entry, thereby building suspense, more than enough at this point. Here goes nothing.
This looks promising.
We’ve hit pay dirt (although a subsequent Google search on the “CD2026MAR6729” mark wasn’t, alas, even remotely fruitful)!
Upside-down temporary reinsertion into the cavity affords us a closer perspective.
Again, referencing the bottom packaging information, which described the unit’s measurement method as “continuous alpha spectrometry”, this is, I believe (readers?) a solid-state PIN photodiode, described along with other possible implementation approaches in a useful online reference that I came across during my research.
As the acronym suggests, the heavily doped p-type and n-type semiconductor regions, used (among other things) as ohmic contacts, are separated by an intermediary un-doped intrinsic semiconductor region.
The conceptual visual similarity with a multi-pixel image sensor is likely already obvious.
In this case, however, where (i.e. in a particular x:y pixel grid coordinate combination) an alpha particle has struck the semiconductor medium isn’t relevant; that one has struck is all that’s necessary to determine. The sensor normally points downward; the surrounding plastic is obviously no functional impediment. In the following photo, I put the assembly back together so you can see the routing of the seeming single-wire (surprisingly, at least to me) harness out of it.
PCB details
We still haven’t found the system’s intelligence and wireless connectivity, however. The prior internal assembly front view image suggests there’s a PCB underneath the sensor. Let’s see.
Here it is.
Let’s next see what’s underneath that shiny plastic cover.
A mess of passives (along with a few five-lead ICs, likely single-package dual-transistor devices) is admittedly not what I expected to find. Then again, as I’ve confessed plenty of times before, analog is admittedly not my area of particular expertise.
Two more screws to go.
And the comparatively boring PCB backside is now accessible for visual perusal.
Yawn…unless you’re into switches, test points and/or battery terminals, that is. Let’s give the PCB frontside one more now-standalone look.
The combo 2.4 GHz Wi-Fi plus Bluetooth LE module at left, with a common-frequency, combo-protocol antenna sticking out of it, is the Ai-WB2-32S kit from a previously-unknown-to-me company called AI-Thinker. Is there something explicitly AI-related to this particular product (and/or the manufacturer, more generally)? Or is it just one of those cases nowadays where anything sounds more important if you tack “AI” onto it? The under-Faraday-Cage photo published at the webpage is woefully low-res, but hey, there’s also a spec sheet.
And at center is the system’s processing nexus, the 32-bit Arm Cortex M4-based N32L406MBL7 microcontroller from another new-to-me supplier, Nations Technologies. It has onboard flash memory and SRAM, thereby explaining why I don’t see any discrete memories on the PCB.
Last, but not least, after carefully putting everything back together, the device fired right up.
It’s a miracle!
That’s a wrap for today, folks. As always, reader insights are welcomed in the comments!
—Brian Dipert is the associate editor, as well as a contributing editor, at EDN.
Related Content
- Radon: Level detection, risk determination, and as-needed mitigation
- Improve HVAC efficiency with accurate sensors
- My Geiger counter doesn’t count (sob sob)
- Design femtoampere circuits with low leakage, part 1 and part 2
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