PressLog Edge Node

PressLog — Industrial IoT OEE Downtime Logger

A manufacturing-ready PCB and firmware architecture for capturing machine downtime reasons on a factory floor.

Origin: The original design brief (see Original Case Study Brief.pdf) was given as a second-round technical case study during an internship interview process. I completed the full PCB design and firmware architecture independently and am sharing it here as my own project.

Status: PCB design and firmware architecture complete. DRC/ERC clean, Gerbers and BOM generated for fabrication. Not yet physically built/soldered — this repository documents the design and engineering rationale, not a deployed product.

1. The Problem

OEE (Overall Equipment Effectiveness) is only as good as its data. But capturing downtime reasons at the machine edge is a human-interface problem as much as a software one. The factory floor kills conventional UX.

2. Design Philosophy

• Physical First — Six large 30mm illuminated tactile buttons, one per reason code. No menus, no navigation, no scrolling. One press = one logged event, confirmed by an LED flash, an 85dB buzzer, and vibration.
• Offline First — Every press is timestamped via a DS3231 RTC and written to 8MB LittleFS flash before any network attempt. When Wi-Fi returns, the device silently replays the queue.
• OEE Ready — Each event carries device_id, machine_id, reason_code, operator_uid (NFC badge tap), UTC timestamp, and a sequence number. Sequence gaps are auto-detected downstream so any connectivity hole is visible to plant management.

3. Hardware — Custom 2-Layer PCB (KiCad)

Designed specifically for high-noise industrial environments.

Physical Specifications

4. Firmware Architecture

Offline-first data flow:

Button Press → RTC Timestamp → Flash Queue (always written first) → Wi-Fi Check → Cloud Sync (batch HTTPS POST)

Defense-in-depth: the hardware RC filter kills high-frequency spikes; a 50ms software debounce is the secondary safety net.

// Button ISR with secondary 50ms software debounce
void IRAM_ATTR isr_btn_0() {
  uint32_t now = millis();
  if (!pressed[0] && (now - last[0]) > 50) {
    pressed[0] = true;
    last[0] = now;
  }
}

// Queue write — always before network attempt
queueWrite(seq, rtc.now().unixtime(), buttonIdx); // flash first
confirmFeedback(buttonIdx);                       // LED + buzzer + vibration
if (wifiOnline) syncQueue();                       // attempt immediate sync

Event payload schema:

{
  "device_id": "DL-CNC07-A3",
  "seq": 1042,
  "ts": "2026-04-08T14:22:08Z",
  "ts_source": "rtc",
  "reason_code": "MATERIAL_JAM",
  "reason_label": "Material Jam",
  "operator_uid": "a3f9d82c",
  "machine_id": "CNC-07"
}

5. Repository Structure

├── Hardware_Source/          # KiCad project — schematics + PCB layout
│   ├── ESP32_Core.kicad_sch
│   ├── HMI_IO.kicad_sch
│   ├── Power_Delivery.kicad_sch
│   ├── RTC_SubSystem.kicad_sch
│   └── PressLog_Edge_Node.kicad_pcb / .kicad_pro
├── Manufacturing/            # Fabrication-ready outputs
│   ├── PressLog_Gerbers/
│   └── PressLog_BOM.csv
├── Documentation/            # Renders, design rule reports, app mockups
│   ├── PressLog_Edge_Node.pdf
│   ├── DRC.rpt / ERC.rpt    # Design Rule Check / Electrical Rule Check — board is verified manufacturable
│   ├── PressLog_Front.png / _Back.png / _Side.png
│   └── App Interface Designs.pdf
└── Original Case Study Brief.pdf   # Full original case study writeup

6. What's Verified vs. Designed-Only

• ✅ Schematic capture across 4 subsystems (ESP32 core, HMI/IO, power delivery, RTC)
• ✅ PCB layout, routed and DRC/ERC clean
• ✅ Gerbers + BOM generated — ready to send to a fab house
• ⬜ Physical assembly / soldering — not yet built
• ⬜ Firmware — architecture and key patterns designed; not yet flashed to hardware

This was a design and engineering reasoning exercise, not a built product. The PCB design itself is real, complete, and manufacturable.

Tools used: KiCad, ESP32-S3, LittleFS, DS3231 RTC, PN532 NFC