Disassembling a low-tech liquid-leak detector

What good is a non-networked water sensor? Still pretty good, as long as its existence, and specifically its embedded battery’s inevitable impermanence, are things you remember.

Back in March, I detailed the initial setup and ongoing usage impressions for a wireless sensing setup for my sorta-“smart” home, comprising TP-Link’s Tapo H100 smart hub.

Tapo T315 hygrometer (also available in a display-less option).

And Tapo T300 water (and other fluids) leak sensor.

The latter replaced a Gentronics model BWD-HWA leak sensor that had long sat at the base of our downstairs water heater, accompanied in the following photos by a 0.75′′/19.1 mm diameter U.S. penny for size comparison purposes.

As to why I made the hardware swap, here’s an excerpt from that initial March coverage, specifically discussing the original non-networked sensor.

Note, however, this bit in the operating instructions:

Replacing the battery: Replace the battery if the alarm has operated for an extended period of time, or if the battery expiration date is approaching. You may want to mark the battery expiration date on a piece of tape and attach it to the alarm when you install the battery.

Let’s be real. I know myself well enough to realize that once I set it, I’m going to forget it. I was admittedly surprised to learn, after replacing it (more accurately, moving it; it now sits below the whole-house water filter enclosure in a different room) that unlike my carbon monoxide detectors at their end-of-life dates, it didn’t at least chirp when its battery was getting low. That said, we’d only hear the sound if we were there at the time, and assuming it was loud enough to capture our attention. And further to that point, more generally, if we were away when a leak started, we’d be blissfully ignorant of what was going on…at least at first, until we returned home, that is.

As mentioned above, I originally just moved the original device elsewhere, replacing it at the water heater with the Tapo T300. That said, I eventually replaced the original in its new location with a Tapo T300, too, since the whole-house water filter is in a storage area with a normally closed, sound-muting door, and since the Tapo T300 successor is inexpensive (sub-$20).

The leak detector that time forgot

Earlier this year, we got the upstairs hardwood floor resurfaced, and in preparation I rolled forward the kitchen refrigerator. Imagine my surprise to find another Gentronics BWD-HWA “Watchdog” Water Alarm behind it! The plumber had done a flurry of initial work when we moved into the residence, also including hooking up the water line to the new refrigerator for icemaker-feeding purposes, and apparently (and wisely) put a leak sensor there, too. While I could claim that I’d not been aware of its presence until now, more likely is that he’d told me about it and I’d promptly then forgotten not only about the need to periodically change its batteries but more broadly about its very existence.

Unsurprisingly, as exemplified by the barely audible “bark” (normally, quoting from the product page on the company website, a “piercing, 110 dB alarm”) when I used a moistened finger to span the “Watchdog’s” under-side electrical contacts, its 9V cell was nearly dead.

I promptly ordered a third Tapo T300 to succeed it. And since I now had two legacy leak sensors on my hands, I of course then decided to take one of them apart. Here’s where the now-drained battery used to be, behind a slide-away cover.

And here was a slick-feature surprise, at least to me. The panel containing the electrical contacts was removable, leading to its potential relocation up to 6 feet away from the base unit.

Had I seen the product web page in advance of the teardown, I might have realized this earlier (in my slight defense, I don’t remember getting any instructions; then again, I don’t remember getting the device itself, either, so…).

That said, I’m not sure what the manufacturer means on that same web page by this phrase (bolded emphasis is mine).

The alarm includes 6 feet of wire, which can be extended up to 100 feet, allowing the sensor to be used in hard-to-reach places.

Span extension mention = dissection invitation?

I don’t see anywhere the detachment of the wire harness from the contacts ends for length-augmentation purposes. Surely, they don’t recommend taking the base unit apart, do they?

Just in case they do, though, I’m taking it as an implied disassembly solicitation (or at least sanction). The three back panel screws you may have noticed in an earlier photo are first to go.

With them out of the picture, the back panel comes right off.

Let’s focus on the “meat” of the hardware.

The three clips that hold the PCB in place detach straightaway.

Flip the PCB over, and its frontside comes into clear view.

Maybe I’m just being dense, but I still don’t see how to extend the wire harness beyond its 6’ default span in a consumer masses-friendly fashion.

I’m sure you’ve already noticed the alarm buzzer dominating the landscape. Aside from that, a discrete transistor (see the above image) and a smattering of passives, the only other notable thing to “write home about” is this IC, a Texas Instruments CD4069UB hex inverter.

And that’s as far as I’m going to go today. Even though these legacy leak sensors aren’t particularly “smart”, they’re still perfectly usable. As such, they’re destined for donation, to serve someone else far into the future. I’ll close with my usual invitation for reader observations in the comments!

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

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