BLE IoT Wearable Child Safety Device Development Using nRF52840 & MAUI

Entity: Child Safety Technology Startup

Product: Wearable BLE device designed to monitor and alert in real time on the whereabouts of children

Components: nRF52840 System on Chip (SoC), rechargeable battery, BLE connectivity to the child's phone, mobile app

Deliverables: Full hardware (PCB design, building the prototype, and writing all of the firmware) and software (MAUI BLE app) as well as investor readiness package.

Our client requested a wearable prototype to present to their investor base that is a fully functioning product and not a development platform populated with wires, etc.

The project consisted of the creation of a custom PCB incorporating the nRF52840 SoC, battery management system, haptic feedback, and an emergency button all embedded within a compact, child-safe wearable enclosure.

In addition, the companion MAUI mobile app communicates with the prototype via BLE to provide real-time positioning data as well as to send alert notifications.

The final prototype model is completely functional and recharges via USB-C and will operate for 48 hours on a single charge. The investment group received an attractive production ready concept rather than an experiment from the laboratory.

Phase 1: Prototype & Design of Printed Circuit Board

Design of schematic and selection of components.

The nRF52840 was selected because of its following characteristics:

• Integrated Bluetooth Low Energy (Bluetooth 5.0) Radio - Low Power Consumption
• Has ARM Cortex-M4 32-bit Processor, Providing Enough Processing Power for Real Time Processing
• Has Built in NIC for Simple Pairing Using NFC Technology
• Broad Supply Voltage Levels (1.7V to 5.5V) - Battery Operated

Supporting components added to schematic:

Component	Part Number	Purpose
Battery charger	MCP73831	LiPo charging with thermal regulation
Boost converter	TPS61099	Regulates 3.7V battery to 3.3V for nRF52840
Emergency button	Tactile switch (6x6mm)	User trigger for SOS alerts
Haptic motor	Linear resonant actuator	Silent vibration alerts
LED indicator	WS2812 (mini)	Status (power, pairing, alert)
Antenna	Chip antenna (Johanson 2450AT18)	BLE range up to 30m
USB-C connector	Type-C receptacle	Charging + firmware updates

Layout constraints: Wearable form factor — maximum board size 35mm x 35mm. All components placed on a single side (reduces manufacturing cost). 4-layer board (signal, ground, power, signal) for noise isolation, especially critical for BLE antenna performance.

PCB Fabrication & Assembly

Process:

• Schematic capture and PCB layout in KiCad (open source, client adoptable)
• Design rule check (DRC) – clearance, trace width, via sizes
• Gerber files generated and sent to JLCPCB for fabrication (5-piece prototype run)
• Stencil ordered for solder paste application
• Components sourced from Mouser and DigiKey (all in stock)
• Assembly performed using reflow oven (hot air for small batches)

Essential Design Considerations

• The antenna must be kept clear of copper and components under the chip antenna to ensure proper BLE range.
• The battery connector is a common and dependable JST-PH 2 pin connector.
• Test points will be created using exposed pads on the PCB for debugging (SWD, UART, power rails).
• ESD protection is included using TVS diodes on USB-C and button inputs.

Prototypes have been assembled and lived to see the first tests where we had three boards made and assembled, one of which had a solder bridge on the charger IC and was corrected during rework; thus both boards worked correctly confirming their proper connections (continuity) and powering on as expected. At 10 m we achieved -85 dBm of BLE Sensitivity, which conforms to the manufacturing criteria of -85 dBm (at 10 m).

Core Firmware Architecture

We built the firmware using Zephyr RTOS (nRF Connect SDK) – chosen over bare-metal for faster development and built-in BLE stack.

Firmware modules:

Module	Function	Memory usage
BLE stack	Advertising, connection, GATT services	48KB
Battery monitoring	ADC reads, percentage calculation, low-voltage cutoff	4KB
Button handler	Debounced interrupt, long-press detection (3 seconds for SOS)	2KB
Haptic control	PWM-driven vibration patterns	3KB
LED driver	WS2812 one-wire protocol	6KB
Power management	Sleep modes, wake on BLE or button	8KB

BLE Services:

• Battery Service (UUID: 0x180F) provides battery level information to the app.
• Device Information Service (UUID: 0x180A) provides device serial number and firmware version information to the app.
• Alert Service (Custom UUID) receives SOS triggers and notifies the app when the button is pressed.
• Location Service (Custom) receives approximate location from the GPS on the phone when the child is wearing the device and the parent uses the app to push the approximate location.

Power Optimization:

• nRF52840 enters System OFF mode after being disconnected for longer than 5 minutes.
• The nRF52840 wakes from System OFF Mode via BLE advertising every 2 seconds and maintains an advertising window of 0.5ms.
• The average current consumption will be 12µA when Idle, 6mA when Connected, and 18mA when providing Haptic feedback.
• 200mAh LiPo battery will provide approximately 48 hours of use depending on usage patterns.

Testing:

Firmware was flashed to the device using SWD via a J-Link EDU Mini. BLE advertising packets, connection parameters, and service discovery were verified using a BLE packet sniffer (Nordic Dongle).

The client requested that the application be developed with a single codebase for both iOS and Android. The developer chose MAUI because of the following capabilities:

• Ability to share C# business logic (BLE, data model, network)
• Platform-specific support for BLE, using dependency injection
• Rapid iteration without maintaining two separate applications

The application consists of the following layers

App Architecture

Layer 1: BLE Abstraction Layer

• Wraps native BLE APIs provided by each platform (CoreBluetooth for iOS, BluetoothLeScanner for Android)
• Manages device discovery and connecting to a device; device service discovery and devices characteristics (read/write/notify); and automatic reconnection up to 30 seconds

Layer 2: Data Models

• ChildDevice - contains device name, battery level, signal strength and last known location
• Alert - contains SOS event, including time and GPS coordinates
• LocationReport - contains latitude, longitude, accuracy and time

Layer 3: ViewModels (MVVM)

• DashboardViewModel – connection status, battery indicator, last location
• MapViewModel – displays child's position on embedded map control
• SettingsViewModel – device pairing, alert tones, notification preferences

Layer 4: Views (XAML)

• Main dashboard (clean, investor-friendly design)
• Map view with real-time position updates
• Pairing wizard (guided BLE bonding)
• Alert history list

BLE Integration Details

Discovery: App scans for devices advertising with custom manufacturer ID (assigned to client). Filters out non-relevant BLE devices.

Pairing & Bonding: First connection requires user confirmation (to prevent unauthorized tracking). Bonding stores keys for automatic reconnection.

Live updates:

Here is how the app receives updated information (notifications):

• The app subscribes to notifications on the Alert Characteristic.
• When the child presses the SOS button, the device sends a notification to the app, which then generates a local push notification (as well as an optional SMS) to emergency contacts (if configured).
• Battery status is updated every 30 seconds by the app reading the device's battery status using read requests.

When location information will be provided:

• The device does not use GPS to save battery and space on the device.
• The app requests location access on the parent's phone.
• The app will read the phone's GPS and send that information to the device's Location Characteristic.
• When any person in the family has a phone connected with the device, they can read the Location Characteristics.

Test Results:

The app was tested via the iPhone12 (using iOS 16) and Google Pixel6 (using Android 13). The range of the BLE connection was confirmed to be good for the hardware at approx 30m in straight line sight. The vibration and LED were confirmed to work with app-based (device) functionality.

Mechanical Enclosure Design

The PCB alone is not investor-ready. We designed a two-part silicone + polycarbonate enclosure:

Part	Material	Function
Inner chassis	Clear polycarbonate (3D printed)	Holds PCB, battery, USB-C port alignment
Outer skin	Medical-grade silicone (overmolded)	Soft, child-safe, waterproof (IP65), available in colors
Clip attachment	Stainless steel spring clip	Attaches to belt, backpack, or wristband

Assembly design:

• USB-C connection covered in silicone flap (self-sealing)
• An emergency switch is slightly exposed through silicone (tactile feel)
• LED visible through translucent silicone window
• No exposed screws or sharp edges

Prototype fabrication:

3D printed polycarbonate (SLA finish) silicone skin made using simple mold (two-part silicone poured, hand finished) production version will use injection molded; prototype costs $380 each (investor demo acceptable).

Investor Deliverables Package

Item	Format	Purpose
Working prototype (2 units)	Assembled wearable + charged battery	Live demonstration
MAUI app installed on demo phones (iOS + Android)	.IPA and .APK files	Cross-platform proof
Technical specification sheet	PDF (4 pages)	Components, battery life, BLE range, certifications pathway
PCB design files	KiCad (source) + Gerbers (fabrication)	Future production
Firmware source + build instructions	GitHub private repo	Developer handoff
App source + documentation	GitHub private repo	Maintenance and updates
Investor presentation deck (hardware section)	PowerPoint (15 slides)	Client's use for fundraising
Demonstration video	3-minute MP4	Remote investor sharing

Test	Method	Result
Battery life	Constant BLE connection, 50 daily button presses	46 hours (within 48h spec)
BLE range	Open field, line of sight	32 meters (exceeds 30m spec)
Water resistance	Submerged 0.5m for 5 minutes	No ingress (IP65 confirmed)
Button durability	10,000 presses	No mechanical failure
Charging time	USB-C, 5V/500mA	2.5 hours (0% to 100%)
App memory usage	iOS and Android profiles	48MB average (acceptable)

Failure caught:

Early firmware was experiencing BLE disconnections every 4 minutes. The reason was the short connection interval (7.5ms). Connection intervals were changed from 7.5ms to 30ms and the connection update procedure was also enabled.

The client received two complete (as in physically completed) wearable prototypes and a dual-platform MAUI BLE app to demonstrate to potential investors.

Within 2 weeks of the delivery, the client received a $250,000 angel investment round based largely on the "the functioning hardware and cross-platform app". The client is now preparing for a pilot production run of 500 units.

Requirement	Our Solution	Investor Readiness
Small BLE wearable	Custom 35x35mm 4-layer PCB	Demonstrated
nRF52840 integration	Full schematic + Zephyr firmware	Source delivered
Battery + charging	MCP73831 + LiPo + USB-C	48-hour runtime
Companion app	MAUI (iOS + Android)	Installed on demo phones
Reliable BLE	Chip antenna + connection management	32m range
Investor presentation	Working units + spec sheet + video	Client funded

All files, source code, and documentation handed over. Client now owns full intellectual property and manufacturing pathway.