Not smart, but solar: Analyzing another thermo-plus-hygrometer

Connectivity is all well and good…well, sort of, as it invariably comes with a price, literally and/or figuratively. Simple’s sometimes best, all things considered, and ambient-light power’s also nice.

When you want to monitor and adjust the internal humidity (and temperature, while you’re at it) of your residence or other facility, a “smart” connected hygrometer such as the one I tore down last month is convenient, since you can check both the measurements-of-the-moment and longer-term legacy trends from anywhere (even when you’re away) using your mobile device. A “smart” hygrometer can even alert you when those measurements stray beyond predefined boundary conditions. And if it includes a built-in display, you can keep your smartphone stowed away and still see the data.

All that connectivity and integrated intelligence comes with a bill-of-materials cost adder, however. And there’s always also the latent (or not) potential for hackers to gain access to that same data stream. While you might not care if someone halfway around the world (or down the street, for that matter) knows your home’s humidity and temperature, you’ll undoubtedly care a lot more if that same “smart” hygrometer ends up being a penetration “vector” for a broader attack, revealing your location and Wi-Fi network login details, for example, along with providing strangers with access to more privacy-violating LAN devices such as security cameras.

Acceptable = respectable

As such, a non-connected sensor is a credible (and sometimes the preferable) alternative. At the beginning of April, I saw a two-pack of BaldrTherm 2.2” solar-powered digital thermometer and hygrometers marked down to $9.99 at Amazon and, curious to try out (and tear down) such a device myself, pressed “purchase”.

I’ve subsequently seen the same two-pack listed there for as low as $8.99, exemplifying a broader BaldrTherm promotion that I’m guessing is motivated by a product line transition combo of redesign and migration to larger, more visible data-rich, 3.2” display devices:

with in-progress awkward consequences:

And to be clear, the company offers plenty of “connected” product variants, too. But today we’ll dive inside a fully standalone-operation offering, complete with a solar cell power option that’s more broadly photon-source agnostic (albeit presumably still visible light spectrum-centric).

Since I know how much you all love conceptual teardown “stock” images, I’ll start with one of ‘em:

And now for our actual patient, as usual beginning with some outer box shots, also as-usual accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes:

Flip open either of the latter two flaps:

and inside you’ll find two slips o’literature (the “user manual”, such as it scantly is, can be accessed in PDF form here):

and two sleeve-swathed examples of today’s teardown victim:

Diminutive in size and price

Here’s the now-“naked” device from various perspectives. Note the transparent piece of plastic (which BaldrTherm refers to as an “insulation sheet”) sticking out one side, which keeps the battery inside from prematurely draining while sitting on store shelves pre-purchase, until removed by the buyer-now-owner (and whose very presence was initially confusing to me, as I’d assumed the energy storage cell in the interior was solar-rechargeable; keep reading).

In spite of the battery still being disconnected, and after a brief delay after initial exposure to my home office’s overhead lighting:

the display came on and the device started working:

I was initially surprised by this unexpected functional transition, until I pondered and realized the underlying reason why, which the user manual also spells out:

Time to get inside. You may have already noticed in one of the earlier overview shots the two coin edge-inviting slots (one of them doing double-duty for the “insulating sheet”) on one side.

Had I thought to grab the penny I had handy, they might have sufficed. As it was, the flexible tip of the “spunger” I was trying to use made it ineffective, so much so that I peeled off the backside sticker to see if I could find any screw heads underneath it. Nope:

Switching to a flat-head screwdriver eventually accomplished my objective, however:

Hot and (not) heavy

Here’s where things started getting interesting and, in retrospect, amusing. I happened to notice that, presumably during the initial disassembly process, the spring terminal at the anode (“negative”) end of the AAA battery inside had become dislodged.

Normally, such batteries’ cases have a thin plastic outer insulating layer that prevents short-circuits with the cathode directly below it:

Not in this case (bad pun intended), however, or maybe it got scratched during disassembly, too. Because when I grabbed the sides of the battery to remove it, my fingertips got scorched. I quickly grabbed the aforementioned flat-head screwdriver and flipped the battery out of the chassis that way instead.

While I waited for it to cool, I carefully rolled it around and learned that it was a non-rechargeable conventional alkaline cell, instead.

In retrospect, including not only a rechargeable battery but also the necessary recharging circuitry in the design would have ballooned the bill-of-materials cost, and I later noticed that the documentation made it clear that the battery was not to be replaced, apparently if for no other reason than to preclude owner burns and other potential mishaps.

If so, though, then why the tempting coin-shaped slots on one side? Inquiring minds want to know. Surprisingly, the cell still held a meaningful modicum of charge; I’d apparently been sufficiently speedy in noticing and rectifying the short-circuit circumstances:

And the device still worked, both with the battery removed:

and with it temporarily reinstalled once safe to touch again.

Internal details

Onward. The solar cell is tenuously held in place with a single piece of tape on one side and the case sides on the other.

The PCB to which it’s attached is conversely more firmly ensconced by two screws.

You know what comes next:

We have a liftoff:

Now for the other, more circuitry-meaningful front side:

Flipping the LCD over reveals its elastomeric connector on one end, which normally presses up against electrical contacts on the PCB itself:

This is one rugged little device; pressing the two halves back together with my fingers and exposing the solar cell to light reignites the display and broader sensing-and-reporting capabilities (albeit with the measured temperature presumably inflated by my body proximity).

Here’s a closeup of the PCB frontside:

showing the elastomer-mating contacts at bottom, a piece of insulating tape at upper left and normally between the LCD backside and a 220-µF capacitor first glimpsed in the assembly rear-view images I shared earlier:

and at upper right, and left-to-right, the humidity and temperature sensors. Underneath the identification-blocking black epoxy blob in the center is presumably the SoC.

Capacitor and missing-battery buffers

In closing, after putting everything back together, the device still worked, after a brief wakeup delay and initially for only a short and cyclical timeframe.

After which, functionality eventually stabilized as long as sufficient light remained available.

Specifically, I’m guessing, commensurate with the fact that there’s still no battery (re)installed. What’s the relationship here? It has to do, I think, with the core purpose of that previously noted capacitor. Remember my “backup batteries and supercaps” piece from last month? This is effectively the supercapacitor, intended to smooth out transient ambient illumination variability-induced impermanence in the solar cell’s output.

I’m guessing that the capacitor is taking a few system-reboot cycles to get to full stored charge capacity, particularly given that there’s (abnormally, versus the normal configuration) no battery installed to alternatively supply the system with the necessary electrons. Agree or disagree, readers? As always, please let me know your thoughts on this and/or anything else that caught your fancy in the comments!

—Brian Dipert is the associate editor, as well as a contributing editor, at EDN.

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