TP-Link’s Tapo T300 sensor detects water and other liquid-leak dangers

Unintended fluids dripping from above? Accumulating from below? The T300 alerts you to them all. And mysteriously threaded contacts suggest other uses, too.

Back in March, I covered the activation and ongoing usage impressions of three interrelated TP-Link smart home devices: the Tapo H100 smart hub:

display-inclusive Tapo T315 hygrometer:

and Tapo T300 smart water leak sensor:

Toward the end of that March piece, and reiterating a quote I’d initially included in a mid-May follow-up post, I wrote:

I’ve also got a redundant Tapo H100 smart hub and T300 smart water leak sensor, both sitting on the shelf, queued up for teardown, along with a display-less sibling of the T315 hygrometer, the Tapo T310 Smart Temperature and Humidity Sensor:

The Tapo T310 was tore down and analyzed within that same mid-May writeup, with the teardown of the Tapo H100 predating it in late April. And now, in early July, we’re completing the dissection triumvirate with the spare Tapo T300, which I as-always aspire to return to fully functional form post-disassembly for ongoing leak-monitoring use somewhere in the residence.

Revisiting past history

You already saw a set of box and other real-life shots for the sibling Tapo T300 in the initial mid-March entry in this series (assuming you read it, that is); that particular unit now resides at the base of my downstairs water heater. The “dumb” leak sensor previously at that location now sits below the also-downstairs whole-home water filter; another is at the back of my icemaker-augmented combo refrigerator/freezer in the kitchen.

As usual, I’ll start out with some outer box shots, also as usual accompanied by a 0.75″/19.1 mm diameter U.S. penny for size comparison purposes.

This last image of the bottom of the package reveals (among other things) the hardware version (v1.6, succeeding the original v1, as well as with its own v1.8 successor) and serial number:

The hardware version matches that of the Tapo T300 currently in use, although serial numbers differ (of course). Here’s a revisit of the associated box-bottom shot you saw in March:

Open sesame

Let’s see what’s inside:

starting with a sliver of quick-start literature (PDF…here are the accompanying full user guide and datasheet) and some protective foam:

Here’s our patient, still swathed in a translucent protective sleeve:

And now unclothed, once again echoing sibling-device images you saw back in March:

As before (referencing the packaging photos), with the exception of this bottom shot:

versus this differing-serial-number vantage point of the in-use sibling device:

in both cases (and in contrast to the bottom-perspective packaging precursors) now including the always-informative common FCC ID (2AXJ4T300).

The Tapo T300 comes already battery-equipped, as you’ve probably already ascertained from the translucent strip of plastic that begs for removal prior to first-time use, but a power-source swap will sooner-or-later be necessary (“up to three years” before replacement is the claim). The removal of two screws should gain us access to the battery compartment:

Toldja so (there’s two AAAs/LR03s inside):

Next up, four screws, one in each corner, this time with hex heads:

We have liftoff

And with them removed, the two sections of the case separate straightaway, with no further implements of destruction or elbow grease required:

The inside of the bottom portion is largely unmemorable. Nice gasket, though, for likely-already-obvious liquid-intrusion-prevention purposes (IP67). Speaking of liquids, note the four metal pass-throughs, one on each corner, originating with the bottom-side contacts you saw earlier:

The other, larger portion is much more interesting (IMHO, at least):

Leak warning-sound transducer aka “buzzer” (claimed 90 dB!) on the side:

Let’s get that PCB outta there. Removing two more screws should do the trick:

That’s what I’m talkin’ about:

Toward the right are a pair of additional feed-through contacts from the top, intended to catch drips coming from above (vs. already-pooled fluids from below in the prior four-contact case). In the middle is a visible-light pass-through originating at the multi-color multi-function status LED, which I’m betting we’ll see shortly. And at left is the mechanical button portion of the topside control switch. The buzzer on the side, fed by the red-and-black two-color wiring harness, you’ve already met, right?

Simply simple

Now for the PCB itself, beginning with the bottom side, you’ve already glimpsed in past shots.

The proximity contacts for the previously pointed out bottom-side contacts are in the corners, labeled P11-P14. Two of the four battery terminals are here; you might have already noticed that the other two are attached to the case itself. And although at first glance, I’d thought the sizeable cylinder on the left edge was an electrolytic capacitor, the “L323” PCB marking next to it suggests otherwise (analog experts: is this what’s known as a “radial inductor”?). Note, too, that the D6 diode site below and to its right is unpopulated, seemingly, unless my eyes are playing tricks on me.

Now for the more interesting (IMHO) topside (which, bafflingly, is screenprinted “BOTTOM”):

Dominating the landscape at left is the PCB mounted portion of the aforementioned control switch. Below and to its right is a sixteen-lead square IC labeled as follows (I “think”…the “S” and “5” symbols aren’t distinctly different):

300A
S906
S15

Readers’ suggestions as to its identity and function(s) are welcomed. My bet is that, as with the Tapo T310 Smart Temperature and Humidity Sensor, it’s another obscured-marking CC1 series-variant of Texas Instruments’ MSP430 embedded controller family, for (among other things) “Sub-1 GHz dual-band” wireless connectivity. More on that connectivity bit in a moment.

Above and to its right, and at the PCB center, is the status LED. To its right is another, larger IC, this one more easily identifiable; it’s the same Cmsemicon BAT32G135GE application processor that I’d found in the earlier Tapo T310 Smart Temperature and Humidity Sensor teardown. To its right are two more landing pads, labeled T9 and T10 and this time corresponding to the earlier noted topside-located drip-sensing contacts.

And above the entire circuitry assemblage is ANT5, the embedded antenna for the company’s proprietary ultra-low power wireless protocol. Since this application’s data rate (as with hygrometry) is low, unlike with a smart camera (for example), additional Wi-Fi connectivity isn’t necessary in this case.

Speaking of sides, I’ll wrap up for today with four more PCB perspectives related to its backside, since that’s where the bulk of the “vertical” parts are located.

Along with one other tidbit that I came across during my research. You might have already noticed that two of the four contacts on the bottom of the device aren’t solid; instead, they seemed to have unfilled (not to mention M2 screw-threaded) centers. You’d be spot-on with that observation, although nothing I’ve found in the product documentation explains why.

Well, this guy (or gal; dunno) used them to transform the Tapo T300 into a door open/close sensor. If it wasn’t already obvious, the Tapo T300 doesn’t directly leverage a moisture sensor, as a hygrometer does (for example). Instead, it detects normally absent current flow between any of the three paired sets of two contacts, interpreting that conductivity as evidence of fluid presence. The switch used in this creative design derivation, in its “closed” position, generates the same current flow. And this same concept can also be employed for other purposes. Nifty!

Over to you for your thoughts in the comments!

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

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