CASE / 01closed source

EKNOT Access

Production physical access control deployed across 15 residential entrances. Integrates custom ESP32-C6 firmware in C++ with AES-128-CBC challenge-response authentication, optocoupled relay commutation, and an Android administration app in Kotlin and Jetpack Compose.

SYSTEM MAP / 01

Signal path

The working path from input to physical or measurable outcome.

REQUESTResident / service app

CONTROLLERESP32-C6

PHYSICALDoor relay

↳ DEVICE LIFECYCLEOTA / logs / recovery

Release · diagnostics · deferred lossless synchronisation

RoleFirmware & Android engineering

Year2026

Deployment15 access points

SecurityAES-128-CBC challenge-response

ScopeFirmware + Android service app

Challenge & Requirements

Engineering constraints and goals.

The physical access system required sub-second door unlocks over BLE while maintaining AES-128 cryptographic authentication, reliable power-cycling recovery, and lossless offline logging across 15 access controllers.

Implementation

Architecture and engineering approach.

01
Wrote the ESP32-C6 firmware in C++ for authenticated BLE commands, relay control, diagnostics, logs and OTA updates.
02
Built the Android service application in Kotlin and Jetpack Compose for discovery, access operations and device administration.
03
Implemented an AES-128-CBC challenge-response protocol with role-aware commands and backend-assisted key lookup.
04
Prioritised the door-open path while deferring lossless log synchronisation so background work never blocks a person at the entrance.

Hardware Protection & Pinout

Galvanic isolation, flyback suppression & live boot telemetry.

Production access controllers interface with dirty inductive loads and unshielded cable runs up to 50 meters. The ESP32-C6 core is completely isolated from high-voltage flyback EMF, static human discharge, and switch chatter.

Pin / Line	Net Name	Dir	Logic Level	Protection Mechanism	Component	Engineering Rationale
`GPIO18`	`RELAY_ACTUATE`	OUT	3.3V LVTTL -> 12V	Optocoupler Isolation	`Everlight EL817`	Galvanically isolates 3.3V ESP32-C6 GPIO from 12V lock strike surges; 5,000 Vrms isolation rating.
`COIL_CLAMP`	`STRIKE_SNUBBER`	PWR	12V DC Rail	Flyback Snubber Diode	`1N4007 (1kV / 1A)`	Shunts inductive kickback (>70V EMF spike) generated when electromagnetic door strike de-energizes.
`GPIO19`	`EXIT_BTN_TRIG`	IN	3.3V (10k Pullup)	RC Low-Pass Debounce	`10kΩ + 100nF C0G`	Hardware low-pass filter (τ ≈ 1.0 ms) rejects mechanical contact bounce and EMI from long cable runs.
`GPIO20 / 21`	`USB_PROV_DP_DM`	BUS	3.3V Full-Speed USB	TVS ESD Array Clamp	`USBLC6-2SC6 (SOT-23-6)`	Ultra-low capacitance (<3.5pF) TVS diode array clamping ±15kV air / ±8kV contact ESD on provisioning port.
`GPIO22`	`STATUS_BEACON`	OUT	3.3V Active-High	Current Limiting Resistor	`330Ω 1% + Green 0805 LED`	Limits MCU sink/source current to <7.5mA, preventing GPIO driver thermal stress during long activations.
`VIN / GND_ISO`	`PWR_5V_REG`	PWR	12V -> 5V / 3.3V DC	Schottky Reverse Polarity	`SS34 (40V / 3A)`	Blocks accidental reverse polarity insertion from building power supply; star-point digital/actuator GND split.

Galvanic Isolation: Separate ground planes between 3.3V MCU digital logic and 12V strike coil commutators.

ESD Rating: Tested to IEC 61000-4-2 Level 4 (±15kV air, ±8kV contact discharge).

SERIAL LOG // UART0 · 115200 8N1 · ESP32-C6 (RV32IMC @ 160MHz)

ESP-ROM:esp32c6-20220919
Build:Sep 19 2022
rst:0x1 (POWERON),boot:0x2c (SPI_FAST_FLASH_BOOT)
I (28) boot: ESP-IDF v5.2.1-dirty 2nd stage bootloader
I (28) boot: compile time May 14 2026 10:14:22
I (29) boot: chip revision: v0.0
I (33) boot.esp32c6: SPI Speed      : 80MHz
I (38) boot.esp32c6: SPI Mode       : DIO
I (42) boot.esp32c6: SPI Flash Size : 4MB
I (47) boot: Loaded app partition offset 0x00010000
I (72) cpu_start: Pro cpu up. Frequency: 160MHz
I (88) cpu_start: Starting scheduler on CPU0 (RISC-V RV32IMC)
I (95) main_task: Calling app_main()
I (102) eknot_sys: Initializing EKNOT Access firmware v2.1.4
I (110) heap_init: Initializing. RAM available for dynamic allocation: 498 KB (internal SRAM)
I (125) gpio: GPIO[18]| InputEn: 0| OutputEn: 1| OpenDrain: 0| Pullup: 0| Pulldown: 1
I (138) eknot_driver: EL817 optocoupler galvanic stage armed on GPIO18
I (145) eknot_filter: Hardware RC debounce (10kΩ/100nF) validated on GPIO19
I (154) esp_ble: BLE 5.3 GATT Server initialized. Service UUID: 0xFFE0, TX Power: +6dBm
I (168) eknot_auth: AES-128-CBC hardware accelerator linked
I (175) eknot_log: Non-volatile ring buffer storage mounted on flash partition
I (190) eknot_main: Hardware initialization completed in 190ms. Listening for BLE clients...
I (2450) eknot_ble: Incoming BLE connection from 64:A2:F9:31:0B:C4 [RSSI: -52 dBm]
I (2472) eknot_auth: Generated 128-bit cryptographic nonce; awaiting client response...
I (2514) eknot_auth: Mutual challenge-response verified OK (HMAC valid, latency: 18ms)
I (2518) eknot_relay: ACTUATE PULSE -> GPIO18 HIGH (800ms) -- [DOOR STRIKE UNLOCKED]
I (3318) eknot_relay: ACTUATE PULSE -> GPIO18 LOW -- [DOOR STRIKE LOCKED]
I (3325) eknot_log: Event #0412 persisted to flash: UNLOCK_SUCCESS [UID: #78291, status: 0x00]

Field Evidence

Hardware, wiring and operator interfaces.

**ESP32-C6 Access Controller Core**
ESPC6-32 N4 module, 5V optocoupled relay, switching buck regulator and USB-C provisioning interface.

**Multi-Core Signal & Power Cabling**
4-PUC-A low-current signal cable routed to door strike relay commutators and 12V rails.

**Intercom Access Reader Terminal**
Field interface with buzzer (ZUMM), TM touch memory, exit button, status LED and 12V lock actuation lines.

**FE-100 Exit Button Controller**
Commutated line terminals (K1, K2) with 12V LED status illumination.

Outcome & Deployment