Raspberry Pi Zero PLC for home automation

Summary A small PLC-like unit for home automation based on a Raspberry Pi Zero that can be mounted on a DIN rail in a fuse box.

Introduction

• All sensors are wired to maximize stability and avoid the need to replace batteries (important because the cabin is a remote location).
• The Raspberry Pi runs a very minimal software setup, and only does what really needs to run locally and can't handle an internet outage (like the thermostat logic).
• All logs with significant activity are written to RAM, to minimize flash wear.
• The Raspberry Pi connects to my server using MQTT through 4G.
• The server runs an MQTT broker and an additional instance of Home Assistant in a docker container

Hardware specifications

• Raspberry Pi Zero socket
• 2 x 16 A 230 VAC Omron relays that can handle inductive loads with large inrush current
• 4 x 1 A 230 VAC solid state relays with external snubber circuits, primarily intended to control contactors (and other small but inductive loads)
• 4 x inputs with pull-up and filtering
• 2 x optoisolated inputs
• 2 x optoisolated outputs
• 1 x open collector output
• 1-wire bus transceiver (DS2482) to maximize range for a DS18B20 temperature sensor bus
• Expansion port with M-bus slave (for meter reading)

Software

• Sends all sensor values and states over MQTT
• Accepts setpoints for thermostats and output states over MQTT
• Implements thermostats
• Reads 1-wire sensor bus (DS18B20), M-bus (and parse the messages), and inputs (with debouncing)

Source code

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