Buck regulator near top left, 3.3V PSU under it, then the Atmega328P microcontroller, and the ULN2003A relay driver chip. The ethernet board connects via SPI, and enables an embedded webserver for status and configuration. The battery-backed Real-Time Clock is on I2C off to the right. Below/between them is the white DHT22 Temperature/Humidity sensor, which uses a proprietary 1-Wire 40-bit protocol. There are five yellow coloured relays, two of which are dedicated for the furnace and fan, the other three are for future expansion. The 3.3V serial port and ethernet share a single run of Cat5e via the special 4w/4w splitter block. The 3.3V LCD display connects via a single wire (tx) serial interface, and the four coloured push-button switches all share a single GPIO using a resistor ladder.

Dagnabbit! Forgot to wire in a reset switch. Which means I'll probably need one.