A local educational maintenance monitor for Arduino Nano 33 IoT. A magnetic door contact counts cycles, an LDR reports ambient light, and an RGB LED shows healthy/dark/service states. “Predictive maintenance” here is a transparent rule-based reminder, not machine learning or a diagnosis of wear.
Learn switch debounce, event counting, rollover-safe timing and service thresholds. Reject contact bounce for 50ms; count closed-to-open transitions; warn after 1000 cycles or 30 seconds continuously open; refuse counter reset while open. Serial telemetry works without Wi-Fi or accounts.
The Nano samples the inputs about every 10ms and executes the shared policy. Output uses PWM RGB channels; JSON prints every two seconds. Architecture and editable wiring SVG describe the same circuit. Nano I/O is 3.3V, although USB power is 5V. Its Wi-Fi radio is unused.
| Quantity | Item |
|---|---|
| 1 | Arduino Nano 33 IoT and USB data cable |
| 1 | Magnetic reed contact, closed when magnet/door is closed, plus magnet |
| 1 | LDR photoresistor |
| 1 | 10kΩ divider resistor |
| 1 | Common-cathode four-pin RGB LED |
| 3 | 330Ω LED resistors |
| 1 each | Breadboard and jumper set |
Install Python 3.12 and PlatformIO 6.1.18; use a USB data cable. Identify LED pinout from its datasheet. No external power supply is needed.
| Nano pin | Connection | Electrical role |
|---|---|---|
| 3V3 | LDR first leg | 3.3V rail |
| A0 | LDR second leg and 10kΩ to GND | 10-bit ADC; 0–1023 |
| D2 | Reed first terminal; second to GND | INPUT_PULLUP; LOW closed/HIGH open |
| D5 | 330Ω to red anode | PWM |
| D6 | 330Ω to green anode | PWM |
| D9 | 330Ω to blue anode | PWM |
| GND | LED common cathode, divider and contact | Common return |
The circuit is authoritative. Disconnect USB before wiring. Mount the magnet so the contact closes with the door shut; confirm continuity. Build the LDR divider exactly as above (more light usually raises A0). Give every LED anode a separate resistor; common cathode goes to GND. Inspect for shorts before USB connection. Use a miniature demonstration door, not a security installation.
python -m pip install platformio==6.1.18
pio run -e nano_33_iot
pio run -e nano_33_iot -t upload
pio device monitor -b 115200If upload fails, double-tap the board reset button to enter its bootloader and select the enumerated USB port. Firmware is here.
Defaults in maintenance.h: debounce 50ms, service count 1000, open duration 30000ms, darkness ADC below 150. Adjust only after characterizing the contact and light divider; thresholds are demonstration values. The initial door state is accepted without adding a cycle. Open and close the door, then hold it open for 30 seconds. Green means healthy; blue dark; amber service; red invalid ADC (host-injected values outside 0–1023). A disconnected contact looks open, not a separately diagnosed wiring fault. Send newline-terminated RESET at 115200 after closing the door to clear the cycle reminder. A currently open door refuses reset.
Sample JSONL has one object per line: ms unsigned uptime in milliseconds (wraps), open boolean, cycles saturating unsigned count, light_adc raw ADC, and state healthy/dark/service/invalid. Example:
{"ms":32000,"open":true,"cycles":1,"light_adc":500,"state":"service"}USB RESET responses are plain text and should be filtered separately by JSON consumers. Counts reset on reboot; there is no wall clock or flash persistence. Service takes priority over darkness.
Run results record cloud evidence. CI runs meaningful C++ host tests, Python image transport tests, repository validators, PNG/SVG/link/MIT/credential checks and an actual Nano target build. Hardware test plan is provided. Physical hardware, ADC calibration and door installation have not been tested.
No serial output: select 115200, correct USB port and data cable. Door always open: check contact polarity, magnet alignment and GND; internal pull-up requires no 5V. Dark state in daylight: check divider orientation and tune threshold. Wrong colors: identify the common cathode and anode order. Service persists: close the door then RESET, or reboot; an open-door warning reappears after 30s.
The light sensor does not measure mechanical health. These rules cannot predict failures, protect an entryway or replace maintenance inspections. Long-open can reflect normal use. All state is volatile; short noise pulses under 50ms are intentionally ignored. An ADC rail value is still a valid raw reading and does not prove sensor health. Never feed 5V into this board’s pins. Avoid door pinch points; secure loose wires. This is an indicator only, not a lock, emergency-egress or life-safety system.
Add qualified nonvolatile cycle storage, calibrated lighting, a timestamped service log, and validation against measured wear before making any predictive claims.
Open an issue with observed inputs and expected behavior; include host tests for policy changes and maintain pin-map consistency. Code is under the full MIT License.