🛠️ Hardware Engineering & Schematics

The IntelliKeep system encompasses two custom printed circuit board assemblies: the ultra-compact, coin-cell-powered BLE Asset Tag and the mains-powered ESP32-S3 Base Station Hub. Both were authored in KiCad, fabricated as multi-layer FR4 PCBs, and packaged inside custom 3D-printed enclosures designed in Autodesk Fusion 360.

1. BLE Tag Hardware Architecture

The BLE tag is the most resource-constrained hardware in the IntelliKeep ecosystem. To attach discreetly to equipment and tools, it must maintain a miniature form factor, minimize sleep current into the low nanoamp range, wake in milliseconds to transmit BLE packets, monitor for enclosure tampering without continuous MCU polling, and allow smartphone NFC onboarding.

Figure 1: Populated IntelliKeep prototype tag PCB featuring the ESP32-C3-MINI module, planar NFC coil, boost converter, and optical sensor.

Key Component Selection:

• Microcontroller: Espressif ESP32-C3-MINI-1U-N4. Chosen for its compact footprint, RISC-V 32-bit core, integrated 4MB SPI flash, certified 2.4 GHz RF front end, and versatile deep-sleep RTC wakeup modes.
• RF Interface: U.FL coaxial connector routed directly to an external 2.45 GHz antenna, minimizing parasitic detuning on a dense mixed-signal PCB.
• Coin-Cell Retainer: Linx BAT-HLD-002-SMT surface-mount battery holder accommodating CR2032 or rechargeable LIR2032 coin cells.

Tag Subsystem Circuit Analysis

A. Ultra-Low Iq Boost Regulator (TPS610994)

A standard CR2032 coin cell drops in terminal voltage from 3.0V down to ~2.0V as it depletes, which falls below the stable operating voltage of the ESP32-C3 (3.0V–3.6V). The tag uses the Texas Instruments TPS610994YFFR synchronous boost converter in a miniature DSBGA package, paired with a high-saturation 4.7 µH inductor.

Figure 2: TPS610994 boost converter stage showing input/output decoupling capacitor network.

B. Nanopower Optical Tamper Detection

Continuous firmware sampling of an ambient light sensor would consume tens of microamps, depleting a coin cell in weeks. IntelliKeep moves the tamper decision entirely into ultra-low-power analog hardware using a Vishay VBPW34S PIN photodiode and a Texas Instruments TLV3691IDPFR nanopower comparator.

Figure 3: Tamper detection circuit using VBPW34S photodiode and TLV3691 nanopower comparator.

The comparator draws less than 75 nA of quiescent current. While the tag remains sealed within its dark enclosure, the photodiode stays non-conductive and the comparator output remains low. If an unauthorized person removes the enclosure lid or detaches the tag, incoming photons immediately trigger the comparator output high, firing an asynchronous hardware interrupt to wake the ESP32-C3 and transmit an immediate tamper alarm (Reason Code 1).

C. Switched Battery Measurement Divider

Measuring a battery voltage using a direct resistive voltage divider connected to an ADC pin creates a permanent leakage current path from VCC to GND. For a 2MΩ divider, this would leak 1.5 µA permanently—exceeding the entire deep-sleep budget of the system!

Figure 4: Switched battery divider using BSS84 PMOS and 2N7002 NMOS gate.

IntelliKeep isolates the divider using a high-side BSS84 P-channel MOSFET driven by a low-side 2N7002 N-channel MOSFET. When the MCU is sleeping, the divider is completely disconnected (0 nA leakage). During wake, GPIO assertions briefly switch on the PMOS, sample the ADC, and immediately disconnect the path.

D. 13.56 MHz Planar Spiral NFC Subsystem

Tag enrollment is expedited via an NXP NT3H2111W0FHKH NFC tag IC. Rather than purchasing bulky discrete NFC stickers, a custom 5-turn planar PCB spiral coil with an outer diameter of ~30 mm was routed directly onto the outer copper layers.

Figure 5: NT3H2111 NFC tag IC interface with planar loop tuning capacitors.

Using an RF Vector Network Analyzer (VNA), the coil inductance was measured at ~0.5 µH. When combined with the NT3H input capacitance (50 pF) and PCB trace parasitics, the tank resonance was tuned precisely to 13.56 MHz, yielding reliable smartphone NDEF reads and writes at distances up to 2 cm.

Tag Mechanical Packaging (Autodesk Fusion 360)

The asset tag is enclosed in a miniature cylindrical housing measuring 33 mm in diameter and 11.5 mm thick. It uses a snap-together friction-fit rim requiring no metal screws or fasteners, minimizing manufacturing cost and assembly time.

Figure 6: 3D CAD renders showing the cylindrical snap-fit housing, internal PCB shelf, and dedicated optical aperture for the tamper sensor.

The enclosure incorporates a recessed optical window directly above the VBPW34S photodiode. When mounted against an asset, the surface completely blocks ambient light; prying the tag away allows ambient light to strike the photodiode, triggering the alarm instantaneously.

2. Base Station Hardware Architecture

The base station operates as an always-on mains-powered gateway. It prioritizes continuous radio reception, multi-protocol bridging, and high-performance processing over power reduction:

Figure 7: Populated prototype base station PCB with ESP32-S3 module, buck regulator, and USB-C protection stage.

Key Components:

• Processing Core: Espressif ESP32-S3-WROOM-1U-N16 featuring dual Xtensa 32-bit LX7 cores at 240 MHz, 16MB SPI flash, 512KB SRAM, and Wi-Fi 802.11 b/g/n + Bluetooth 5 (LE) mesh radio.
• Power Input: GCT USB4105 USB Type-C receptacle with ESD clamp diodes, TVS surge protection, and a resettable polyfuse.
• Main Buck Regulator: Diodes Inc AP63203WU synchronous buck converter delivering a steady 3.3V rail at up to 2A with high thermal efficiency.
• Serial Flashing Bridge: WCH CH343P USB-to-UART bridge allowing 1-click firmware upload and real-time serial diagnostics over the native USB-C port.

Base Station Mechanical Enclosure

The base station enclosure follows the same cylindrical aesthetic as the tag, measuring 105 mm in diameter and 85 mm tall:

Figure 8: Base station CAD renders showing PCB standoffs, USB-C rear access port, and antenna retention features.

The enclosure lid incorporates a press-fit retention collar that secures the internal 2.4 GHz antenna vertically in the center of the cylinder, maximizing omnidirectional RF reception across the property without external protruding antennas.

Estimated Bill of Materials (BOM) Costs

Cost modeling was conducted across multiple manufacturing quantity tiers to demonstrate commercial feasibility for property operators and small businesses:

BLE Asset Tag BOM Cost

Base Station Hub BOM Cost