By Sam Holland, co-founder of informal

Why this crossed our desks: After seeing the complicated design of the SimpliSafe Water Sensor, and the simplicity of the Temperature Sensor, we wanted to see how the Motion Sensor stacked up. This sensor is the most baffling of the ones we’ve torn down so far, and has the least in common with the other SimpliSafe accessories. It stands out from the Water Sensor and the Temperature Sensor, due to the internal architecture and electronics used. Our assumption is that an external contractor or manufacturing partner did the design work on this sensor — or that it was maybe the first product made by the team.

The button and battery wiring could’ve been eliminated with a simple redesign, creating cost savings from not having to hand-solder items together, and reducing complicated assembly steps. We found the fastener-free assembly to be very difficult to disassemble, but well designed.

We wonder if the first version of the design had more screws to hold everything together, and they shifted to using snaps later in the product development process. There are some circular molded plastic features that don’t need to be on the housing, which sometimes means they’re artifacts from modifying the tooling later. Anyway, this was a fascinating teardown for all of these reasons. Read on to understand how we arrived at this conclusion!

Table of Contents

Overview

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The Motion Sensor uses a technology call passive infrared (PIR) to detect motion within specific areas and then alerts the SimpliSafe hub.

• We can see a large, milky lens on the front of the device for the PIR sensor (1).
• Like the other SimpliSafe sensors we analyzed, there’s a pairing button (2) on the top of the device.
• The back of the device has a removable cover (3) with four strips of 3M removable adhesive attached (4).

The shape of the sensor and position of the adhesive strips allows the user to either mount the device flat on a wall, or in a corner. Neat!

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Battery cover removed

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• Removing the rear cover (1), we can see pass-through features for the adhesive strips, so the user can remove the adhesive if they need to move the sensor later.
• Four countersunk holes (2) are also located above the strips for fastening this cover to a wall using drywall screws. (Fun fact: It’s generally recommended to avoid countersunk screws on plastic as they’re more likely to crack!)
• Similar to the other sensors, we can spy some regulatory information embossed on the rear housing (3). This is a great technique to save costs on labeling (no need to buy labels and pay for someone to apply them) and reduce the impact on the product design. (I’ll be stealing this for sure!) But, you do need to determine where you’re selling the product ahead of time.
• There’s a switch hidden back here (4) that allows the user to change between low and high sensitivity. This likely adjusts the threshold at which the PIR sensor reports back to the hub, saving battery and preventing false alarms.
• Lastly, we can spy a CR123 battery (5), a powerful 3V battery commonly used for camera flashes. It seems like this device draws more power than the other sensors, or they wanted it to last longer. </aside>

Opening it up

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Opening up the Motion Sensor was tricky!

• I was able to pry the two housings apart a bit by hand and spotted a good entry point on the bottom housing (1).
• Shoving a flathead screwdriver inside allowed me enough leverage to pry the housing open, but it seemed to permanently break a few of the internal snap hook features (2) and cut my finger. (I’m fine now, thanks for asking!)
• Two snap hooks in particular caught my eye. They were integrated into the curvature of the battery holder (3).
• On the front housing, two very beefy features (4) were added to engage with snaps formed against the rear housing wall.
• There are a handful of circular “bosses” (5) peppering both housings. These shapes fit inside each other and help to locate the housings during assembly, but I haven’t seen something like this before.

I wonder if the design initially had screws holding the housings together, which were later eliminated by closing up the holes on the rear housing.

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Inside the Motion Sensor

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• Inside we’re greeted by a sensor PCBA (1) held in place by a single thread-forming screw, a button PCBA (2), and a front housing subassembly (3). This is the only screw in the entire device, and could probably have been replaced by a few snap hooks and locating ribs. Still, bravo for reducing fastener count!
• Here’s where we get to the first thing that bothers me on this device. The team designing the sensor opted to solder wires from the battery terminals to the main board (4) instead of using a bent wire or extending the PCBA in this area and soldering the terminals directly to the board.
• Wires need to be cut to size, stripped, tinned, and soldered to the board and the terminal, adding cost and potential failure points along the way. If wires have to be used in a design, I always recommend using connectors such as the JST-PH series (my favorite) (5). These connectors can be soldered to the board with the rest of the electronics components, and the battery terminals can be soldered up during a subassembly process. Installation is easier too, as you simply plug in the cable versus having to through hole solder manually. </aside>

Cool feature: Fresnel lens

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• The front housing sub assembly has a milky white translucent PIR lens (1) snapped in.
• This plastic part has a tiny Fresnel lens molded into the inside face (2). These lenses work to focus the infrared rays from items being detected to the PIR sensor and shapes the detection area of the sensor.

*This article from MuRata explains the use of lenses in depth and is a great read.*

• Notice how the Fresnel lens pattern stops a little below halfway through the plastic lens? This is intentional, and allows for a “dead zone” on the sensor, limiting the area detected by the sensor to anything roughly 45 degrees below horizontal (3).

Pro Tip: SimpliSafe recommends placing this sensor 6 feet off the ground to detect the largest amount of motion. If you have pets, they recommend flipping the sensor upside down and mounting it 4.5 feet off the ground to ignore anything crawling around on the ground.

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