Project Background

Helping infants build a calmer sleep routine depends on small, consistent signals that parents can understand and act on. Our client, Toto Sleep, engaged us to develop a low-power wearable device to support exactly that: tracking temperature-related sleep patterns and giving parents and caregivers simple visual indications that help them better time a child’s sleep routine.

For this type of wearable, the device had to operate reliably close to the child, run with very low power consumption, communicate with a mobile app, and exchange data securely. It also needed a maintainable firmware foundation, so future updates and improvements could be supported after deployment.

The Challenge

One of the main challenges was developing a compact wearable device with limited space for electronics, battery, memory, sensing components, and BLE communication. The hardware and firmware architecture had to support reliable temperature measurement, local data logging, time synchronization with the mobile application, standard BLE services for temperature and battery data, and EEPROM data retrieval through a custom BLE service within a very small device format.

Another challenge was inconsistent BLE behavior across different mobile phones. The device did not work reliably on all phones, which made the issue difficult to reproduce and analyze.

Security and maintainability were also important parts of the system. Communication with the mobile application had to be encrypted and limited to bonded peers, while OTA update support was required to enable firmware improvements, field updates, and long-term maintenance after deployment.

Our Contribution

Our team developed both hardware and firmware for a low-power infant sleep wearable based on the nRF52832 Cortex-M4 platform with an integrated BLE radio.

On the hardware side, we designed the schematic and PCB for a tiny form factor. The electronics included a BLE SoC with a Cortex-M4 core, EEPROM memory for local data storage, a digital temperature sensor for body temperature monitoring, and an LED indicator for simple user feedback.

The firmware enabled the device to combine several key functions within one wearable platform:

• temperature monitoring
• wireless communication
• local data storage
• LED indication control
• OTA update capability

The device measured temperature using the on-board precision temperature sensor and stored measurement data in non-volatile EEPROM memory. This allowed historical temperature data to remain available on the device and later be retrieved by the mobile application.

A proprietary algorithm was integrated to control LED indications based on body temperature trends and support detection of the optimal sleep-time window. This allowed the device to provide simple visual feedback to the user without requiring constant interaction with the mobile application.

On the BLE side, encrypted communication with bonded peers was implemented to support secure data exchange between the wearable and the mobile app.

Standard BLE services were implemented to support core device functions:

• Current Time Service client for updating RTC time whenever a connection to the mobile application is established
• Health Thermometer Service server for providing the latest body temperature to the mobile application
• Battery Service server for providing the latest battery level to the mobile application

In addition to standard BLE services, a custom BLE service was implemented to allow the mobile application to access stored temperature measurements from the EEPROM. This enabled the device to support both real-time data access and later retrieval of logged measurements.

To resolve inconsistent BLE behavior across different mobile phones, we performed extensive testing on more than 10 mobile devices until the issue could be reliably reproduced. Once the problem was reproduced, we compared device logs from successful BLE connection setups with logs from phones where the connection flow did not work as expected. This helped identify the difference in behavior and provided a clear basis for resolving the issue.

We also developed OTA update support, allowing firmware updates to be delivered after deployment. This gave the client a more maintainable product platform, with the ability to improve firmware, resolve issues, and extend functionality without requiring physical access to the device.

Results

The client received a functional firmware foundation for a connected infant sleep tracking wearable, built around secure BLE communication, temperature monitoring, local data logging, LED-based user indications, and OTA update support.

The project delivered:

• a low-power embedded firmware platform for an infant sleep-tracking wearable
• continuous body temperature measurement and logging
• secure BLE communication with encrypted bonded peer connections
• standard BLE services for time synchronization, latest temperature data, and battery level reporting
• a custom BLE service for retrieving stored temperature measurements from device memory
• algorithm-based LED indications for sleep-time detection
• OTA update capability for firmware improvements and long-term product maintenance