Smart Litter Box

An ESP32-C6 based smart litter box monitor that tracks cat visits, measures waste weight, and monitors air quality. Integrates with Home Assistant via MQTT autodiscovery. The project goal was to use cheap electronics to modify an off the shelf litterbox into a smart litterbox.

Disclaimer: This project implemented with support of CoPilot. Architecture is mine, but some of the methods are fully implemented by CoPilot.

Hardware

Component	Model	Role
Microcontroller	ESP32-C6	Main MCU
Gas sensor	MICS-5524	Air quality / odor detection (A1)
PIR motion sensor	HW740	Presence detection (GPIO 2)
Load cell amplifier	HX711	Weight measurement (DOUT=20, SCK=19)
Load cells	4x 50 kg	Sensing elements for weight

In addition to this, I designed two PCBs for the project. One that is mounted to the ceiling of the litterbox. It houses the ESP32, the MICS-5524 and the PIR sensor. The second board is mounted to the back of the litterbox and houses the HX711 as well as a micro-usb port for powering. A cable goes into the litterbox to connect to the first PCB.

Features

• Cat visit detection — PIR motion sensor triggers weight analysis
• Cat weight estimation — calculates cat weight as the difference between peak stable weight and post-visit weight
• Air quality monitoring — low-pass filtered MICS-5524 readings with a configurable baseline delta
• Cleaning mode — detected automatically when weight drops sharply below a threshold
• Home Assistant integration — full MQTT autodiscovery with 20+ entities
• Runtime configuration — all key parameters adjustable via MQTT at runtime
• OTA updates — firmware updates over WiFi via ArduinoOTA

Configuration

Fill in your credentials in config.h:

#define WIFI_SSID ""
#define WIFI_PASSWORD ""

#define MQTT_BROKER ""
#define MQTT_PORT 1883
#define MQTT_USER ""        // Leave empty if no authentication
#define MQTT_PASSWORD ""    // Leave empty if no authentication
#define MQTT_CLIENT_ID ""

#define OTA_HOSTNAME ""
#define OTA_PASSWORD ""

Adjust the timezone if needed in smart-litterbox.ino:

const long gmt_offset_sec = 3600;       // UTC+1
const int daylight_offset_sec = 3600;   // DST offset

Calibrate the load cell by setting the calibration factor:

const float HX711_CALIBRATION = 27.96;

MQTT Topics

All topics are prefixed with esp/smart-litterbox/.

Published by device

Topic	Payload
weight	{"weight": x.xx} — live weight (g) during a visit
cat_analysis	{"cat_weight": x, "waste_weight": x, "highest_stable_weight": x}
presence	{"detected": true/false}
airquality/mics5524	{"raw": x, "baseline": x, "delta": x}
last_use	ISO 8601 timestamp of last cat visit
last_cleaned	ISO 8601 timestamp of last cleaning
cleaning_mode/state	ON / OFF

Commands (subscribed)

Topic	Payload	Effect
calibrate	CALIBRATE	Re-baseline air quality sensor
tare	TARE	Tare the scale
filter_alpha	0.01–1.0	Set low-pass filter alpha
analyzer/stability_threshold	5–100 (g)	Set weight stability threshold
analyzer/stability_samples	3–20	Set stability sample window
analyzer/sampling_rate	1–10 (Hz)	Set weight sampling rate
analyzer/min_weight_threshold	500–5000 (g)	Set minimum cat weight
motion_timeout	10–300 (s)	Set motion clear timeout
cleaning_threshold	-5000–0 (g)	Set cleaning trigger threshold

Dependencies

• PubSubClient — MQTT client
• HX711 — load cell library
• ArduinoOTA (included with ESP32 Arduino core)

Hardware installation

The internal PCB is mounted in a small casing and attached to the ceiling of the litterbox. It houses the ESP32-C6, the MICS-5524 gas sensor, and the PIR motion sensor.

From it, a cable with communications and power goes to the external PCB on the back of the litterbox. The external PCB houses the HX711 load cell amplifier and a micro-USB port for power.

The four load cells are mounted under the base of the litterbox in a cross pattern, one at each corner.