By Sam Holland, co-founder of informal

Why this crossed our desks: This is part one of a SimpliSafe accessories teardown series that we’ll be publishing over the coming months. Their leak-detecting Water Sensor is interesting because it performs a very simple function, but the design criteria makes it complex. The device needs to be waterproof and detect water, but also needs to have a customer replaceable battery and pairing button.

Waterproof products are significantly more complicated to design than even “splash-proof” products. It’s always interesting to dig into the design decisions the engineers and designers made to hit all of their requirements. Here, I was impressed by how they solved the issue of sealing up a battery compartment while still detecting water on the underside of the product. The use of metal pins and a second PCB was also clever but added extra costs and complexity.

Redesigning the device to place the battery on the top side of the product would’ve drastically simplified the construction of the sensor and reduced costs!

Table of Contents

Exploded view & parts

1. Bottom cap lid
2. Battery holder
3. Bottom cap
4. Sensor PCBA
5. Button cover
6. Main PCBA
7. Top housing
8. Battery
9. Button gasket
10. Button
11. Housing gasket
12. Housing screws
13. Bottom cap screws

Water sensor overview

<aside>

Let’s check out the leak sensor first. This device is pretty simple: When it detects water, it alerts the SimpliSafe Hub.

• The top of the sensor is pretty boring with just a pairing button in the center (1).
• The bottom side is far more exciting! There are some embossed logos and regulatory text on the housing, which is a great way to discreetly include this required information. We can see a CE logo (2), RCM (3), and disposal instructions (4).
• Around the perimeter of the sensor are three pairs of metal pins (5) to detect water. Three feet (6) keep the pins elevated off the ground by a few millimeters, letting water pool up and make contact with the pins. </aside>

Water sensor battery door

<aside>

Twisting the sensor 120 degrees allows you to open up the battery compartment.

• Inside, there are two metal contacts (1) on the bottom housing and a few screws to hold the bottom assembly together (2).
• The sensor housing has two metal pins (3) to contact the contacts on the bottom housing. The housing is held together with three screws (4).
• The battery (5) is held in place with a nice snap hook and lip-molded into the housing.
• A molded-in arrow (6) on both the housings help for alignment when closing the device back up. </aside>

Cool feature #1: Twist to lock

<aside>

The bottom housing and sensor housing are connected using a twist-lock mechanism. This is a pretty common feature used for parts that are user-accessible without needing fasteners.

• On the sensor housing, three identical upside-down “L” shapes are molded into the housing 120 degrees apart from each other (1).
• Similarly, the bottom housing has three “bumps” on the perimeter (2).
• The bumps in the rear housing fit into a channel on the sensor housing (3). When the user twists the rear housing clockwise, the bumps rotate (4) and get caught under a ledge (5).
• A “speed bump” molded into the sensor housing (6) provides a tactile click when the two parts are attached. </aside>

Sensor housing teardown

A white perimeter O ring (5) sits inside a groove on the sensor housing. It is compressed when the sensor and bottom housing are connected.

<aside>

Three thread-forming screws (1) are holding the battery holder to the sensor housing.

These types of screws are awesome for reducing part count and cost, as they create threads in plastic while being installed into a “screw boss.” When properly designed, they provide a strong holding force and can be used a few times before stripping out.

Prying the battery cover away, you can see the contact for the bottom (2) and side (3) of the coin cell. A white piece of tape (4) hides the PCBA details from the customer and provides guidance on the orientation when replacing the battery.

</aside>

Cool feature #2: Button design

<aside>

One curious feature on the sensor housing is the pairing button (1).

• The button is waterproof and had some springiness to it — indicating a rubber gasket being used. I couldn’t figure out a good way to remove this button nondestructively and, in my excitement, forgot to take pictures during the process. Instead, I CAD modeled it up :)
• The button sits inside a hole in the sensor housing. A rubber gasket prevents water from entering the housing and provides some springiness to the button. A plastic cap is attached to the housing using a process called ultrasonic welding, providing a watertight seal. Neat! </aside>

Cool feature #3: Battery holder