This project is a modular differential solar thermal controller designed for systems with multiple thermal storage tanks. It supports:
- A primary pump for the solar panels
- A 3-way valve to switch between two storage tanks
- A secondary pump for an additional tank, in my case the one heated by a normal gas boiler
- A final 3-way valve to manage recirculation between solar tank and normal hot water
The controller is designed to operate autonomously based on temperature differentials and includes logic protection features like anti-freeze and overheat modes.
I have a solar panel system that generates hot water at home in the warm months. The current controller is a classic type without any type of integration with home automation control systems. The system works great but it was time for me to make it smart...
I decided to take inspiration from the manual of my controller, from a triac control of 230V motors, and from a PT1000 probe reader, like the one I currently have installed on the panels and cannot remove or replace. For this reason this project was born and now, in the final stages of testing, I would like to share it for inspiration or comments.
V2: This new release integrate into one board the controller and the temperature measurement board. the functionality is the same, the code a little bit different.
- Based on a microcontroller (ESP32-S3)
- Controls triacs for pumps and relays valves (check your voltage and characteristic based on your system, this one is designed for a normal 230V power supply)
- Accepts analog/digital inputs (buttons, override switches, etc.)
- Manages logic for differential temperature thresholds, hysteresis, timing, and safety
- Interfaces with up to 4 PT1000 temperature sensors and DS18B20 probes for others temperature. The PT1000 are designed for the solar panels, tank and normal hot water
- Provides real-time thermal data for control logic
- Possibility to connect also with RS-485 interface
- Drives relays and triacs for high-current devices
- Features optoisolated inputs and snubber circuits
All hardware schematics are provided in PDF format:
This board is the core controller of a modular solar thermal system. It processes temperature readings, applies control logic, and operates pumps and 3-way valves for dynamic thermal management across multiple storage tanks.
The main board is designed to:
- Read temperatures from solar collectors, buffer tanks, and return lines
- Convert sensor values to analog signals via operational amplifiers
- Execute differential logic (ΔT) to optimize energy transfer
- Drive up to 3 pumps and 2 3-way valves
- Provide optional control over final delivery circuits
- Connect to network systems via Ethernet (W5500) or WiFi
- Display system status on an I2C LCD
- ESP32-S3 module for logic processing
- USB Type-C for programming and debugging
- I2C ADC (ADS1115) for buttons inputs
- RS485 transceiver (MAX13487) for remote sensor integration
- W5500 Ethernet controller with integrated magnetics
- USB-C for firmware upload
- 20x4 I2C LCD with backlight control
- 5-button interface (UP, DOWN, LEFT, RIGHT, OK)
This board provides the power interface for a differential solar thermal controller. It manages relays, TRIACs, and optoisolators to drive AC pumps, 3-way valves, and other inductive loads safely and efficiently.
The power board is designed to:
- Switch high-current AC loads (pumps) via TRIACs
- Drive DC relays for directional valve control
- Provide galvanic isolation via optocouplers
- Detect zero-crossing for synchronized TRIAC firing
- Convert AC to DC power with onboard switching regulator for all boards
- HLK-PM01 module for converting AC mains to 5VDC
- EMI filter (inductor + varistor) for surge suppression
- Fuse protection on AC input
- 2 x Relay outputs for 3-way valve actuation (VALVOLA1 / VALVOLA2)
- 2 x TRIAC circuits (with MOC3021 + BTA41) for motor/pump control
- Optical isolation on both relays and TRIAC gates
- LED indicators for relay status
- Optocoupler + RC filtering network to detect AC phase crossing
- Signal
ZEROCROSSsent to the logic board for phase-synced control
- Modular input/output management
- Support for dual-tank switching and secondary heating loops
- Temperature-based control logic with configurable thresholds
- Integration-ready with MQTT, Openhab, and standalone LCD display
- Configurable via analog buttons or MQTT
- Anti-freeze and overheating protections included
- Control board: V2
- Power board: V2
- Date: December 2025
This project is provided for educational purposes only.
The author(s) of this repository do not take any responsibility for:
- Incorrect use of the design or schematics
- Damage to equipment, persons, or property
- Installation in unsafe or non-compliant environments
Any implementation of this project in real-world systems should be done under the responsibility of a qualified professional, and in compliance with local regulations and safety standards.
By using this material, you acknowledge that it is offered as-is, without warranty of any kind, and you agree to use it at your own risk.