Introduction to Sentinel

Sentinel is a location monitoring node. One Sentinel is installed at one location, for example a data center rack, server room, shelter, or electrical panel, and continuously measures:

• Temperature: up to four DS18B20 sensors on one cable, plus an SHT41 (room temperature and humidity) if installed.
• AC power: voltage, current, power, energy, frequency, and power factor from a PZEM-004T module.
• Digital inputs: four dry contacts for doors, UPS, water leaks, smoke detectors, or other contacts.
• Modbus RTU devices over RS-485 (power meters, temperature and humidity transmitters), up to 40 devices.

Sentinel also has four digital outputs (for a siren, fan, or relay), a 20×4 LCD with four touch buttons, and a buzzer for local alarms. The current model is SNT-G1: ESP32-S3 with W5500 Ethernet, powered by PoE.

Data flow from the location to Celestian.

Three parts of the system

Part	Role
Sentinel	Measures, computes alarms, runs rules, and sends data. Stores no credentials, workspace, or channel.
DC Gateway	One per data center. Acts as the rack LAN router: provides IP (DHCP), time (NTP), a local MQTT broker, and firmware files; holds data on disk when the WAN is down; forwards everything to Celestian.
Celestian	The IoT platform that receives, stores, and displays data, and sends configuration, rules, and firmware to nodes. See the Celestian documentation.

A node installed without a gateway can also log in to Celestian itself (direct mode).

What makes Sentinel different

• Alarms are computed on the node. The buzzer and LCD keep working when the network is down; alarm events and status are sent as soon as it reconnects.
• All settings can be sent from the server, or via the web interface and USB serial. The node reports back what actually applies and what was rejected.
• Safe firmware updates. A new image is only considered valid once Ethernet and MQTT are stable; otherwise the node rolls back to the previous version by itself.
• No secrets on the node in gateway mode. Only the gateway holds the keys to Celestian.

Note

This documentation applies to SNT-G1 firmware series 0.21. Some new features (Modbus sources on G1, telemetry groups) are not yet fully tested on devices; see Feature status before relying on them in production.

Next

• Want to try it right away? Quick start.
• Need to understand the terms slot, metric, and message types? Basic concepts.
• Want to know which cables and modules you need? Hardware.