OverWatch is a smart safety bracelet for inmates and detainees that includes an STM32L4 microcontroller, an LIS2DS12 accelerometer, and an ST25DV NFC tag. It monitors the user’s heart rate and other vital signs, such as oxygen levels, to alert personnel in an emergency. It protects people in detention by ensuring they get immediate medical attention. It also benefits the staff who has access to data remotely rather than having to constantly be physically close to the person under their supervision. 4Sight Labs, the company behind OverWatch, contacted C3 Medical Device Consulting, which helped define requirements and design the product. Let’s see why C3, a member of the ST Partner Program, chose our devices for such an important task.
How to tailor a wearable to a specific use case
OverWatch: New tradeoffs
The wearable from C3 is interesting because it has a different set of requirements from traditional smartwatches or smartbands. For instance, while a typical smartwatch must be waterproof and dustproof, OverWatch must be extremely rugged. It must still work despite users banging it against a wall or after a violent altercation with the personnel. Hence, C3 had to work on a design that was small enough to live in a very large and thick enclosure. Making the PCB bigger would require even bigger housing, making the whole thing unwieldy. Additionally, the system must be low-power. Authorities can’t ask inmates and detainees to charge their OverWatch by their bedside table every night.
STM32L4 and LIS2DS12 for 2 weeks between charging, one minute between readings
It’s for these reasons that C3 settled on the STM32L4 and the LIS2DS12. The microcontroller is famous for its ultra-low power modes ranging from 340 nA to only 8 nA, depending on the amount of memory powered and whether the real-time clock is active. Additionally, it has a wake-up time of only 5 µs. C3 explained that their teams found this highly appealing. Indeed, it means the MCU can quickly wake up, get data from the sensors, and go back to sleep, thus offering significant power savings. The OverWatch systems can last two weeks between charging, greatly facilitating the work of the staff responsible for putting the wearable on a person and then taking it off.
The choice of MEMS accelerometer was also challenging. Indeed, the measurements must be accurate enough to be relevant even if the application is not meant to provide a diagnosis. To save battery, the MCU only wakes up every minute to read sensor data, but the sensor takes data more often than that. There must thus be a buffer that fills up between readings. C3 chose the LIS2DS12 because of its 256-level FIFO, its up to ±16g full scale, and high shock survivability. The sensor can help determine if a person is sleeping, fighting, or in an abnormal situation, and the MCU can provide a lot more data than the hourly physical rounds that are common in these situations.
How to account for the primary and secondary users
ST25DV: Provisioning wearables instantly
Overwatch is also an interesting challenge because it highlights the importance of intelligent device management. A facility will have many systems to manage at once. To put things in perspective, C3 aims to manufacture about 2,000 units per month soon. Supervising personnel must, therefore, be able to quickly provision hundreds of devices and use them at a moment’s notice. That’s why C3 added the ST25DV. The NFC tag can talk to a smartphone or tablet and immediately provision a system. It also enables the use of an NFC antenna on a flexible PCB, which increases the overall ruggedness of the product.
STM32 Ecosystem: Developing fluidly
Looking back, C3 shared how everything came together more fluidly thanks to the STM32 ecosystem. It’s easy to look at each component individually. And that’s often how engineers work. Unfortunately, too many overlook the ability to have a cohesive ecosystem. For C3, it meant easily getting development boards and documentation and talking with our experts when they had questions about our devices. Especially when the use case is so sensitive, being able to reuse a lot of our source code and having this level of support from ST meant that the teams at C3 could focus on the features that would change the lives of detainees, inmates, and the staff that supervises them.
