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☀️ DiSoCon2 — Differential Solar Controller V2
A fully open-source, ESP32-S3 based differential solar thermal controller with dual-tank switching, PT1000 sensing, TRIAC pump control, and Ethernet/MQTT connectivity.
🌡️ What Is This?
DiSoCon2 is a modular, open-hardware differential solar thermal controller designed to intelligently manage a domestic solar hot water system with multiple storage tanks. It replaces proprietary "dumb" controllers with a fully integrated, home-automation-aware solution built around the ESP32-S3 and ATmega32U4.
The system reads four PT1000 platinum resistance sensors (solar collector, primary tank, secondary tank, return line), computes temperature differentials, and drives pumps and 3-way valves accordingly — all while reporting live data to Home Assistant via MQTT or Ethernet.
V2 integrates the control logic and the analog temperature measurement circuitry onto a single board, eliminating the inter-board wiring of V1 and improving signal integrity.
🎯 Key Features
| Feature | Detail |
|---|---|
| Temperature sensing | 4× PT1000 via precision OPV front-end (MCP6004, ±0.1% resistors) + DS18B20 OneWire bus |
| Pump control | 2× TRIAC outputs (BTA41-600B + MOC3021) for 230V AC pumps, with zero-cross synchronization |
| Valve control | 2× relay outputs (SRD-05VDC) for 3-way motorized valves, optoisolated via PC817C |
| Connectivity | W5500 Ethernet (SPI) + ESP32-S3 onboard WiFi/BLE |
| Field bus | RS-485 half-duplex via MAX13487E (auto-direction), with 120Ω termination and SMBJ6.8A TVS protection |
| User interface | 20×4 I2C LCD + 5-button keypad (UP / DOWN / LEFT / RIGHT / OK) read via ADS1115 |
| Power supply | HLK-PM01 mains-to-5V module + BL1117-33CX LDO for 3.3V logic; USB-C 5V input as alternative |
| Safety | Anti-freeze logic, overheating protection, fuse-protected AC input, snubber networks on all TRIAC outputs |
| Firmware | ESPHome YAML on ESP32-S3; ATmega32U4 Arduino firmware for analog front-end |
🧩 System Architecture
The project is split into two physically separate boards that connect via a 10-pin IDC ribbon:
┌─────────────────────────────────────────────────┐
│ CONTROL BOARD (Full Board V2) │
│ │
│ ┌──────────┐ ┌──────────┐ ┌───────────────┐ │
│ │ ATmega │ │ ESP32-S3 │ │ W5500 │ │
│ │ 32U4 │◄─► (main │ │ Ethernet │ │
│ │ PT1000 │ │ ESPHome)│ │ Controller │ │
│ │ front-end│ └────┬─────┘ └───────────────┘ │
│ └──────────┘ │ │
│ ┌──────────┐ ┌────┴─────┐ ┌───────────────┐ │
│ │ MCP6004 │ │ MAX13487 │ │ ADS1115 │ │
│ │ 4ch OPV │ │ RS-485 │ │ Button ADC │ │
│ └──────────┘ └──────────┘ └───────────────┘ │
└─────────────────────┬───────────────────────────┘
│ 10-pin ribbon (CN3)
┌─────────────────────┴───────────────────────────┐
│ POWER BOARD (Power Board V2) │
│ │
│ ┌──────────┐ ┌──────────────┐ ┌───────────┐ │
│ │ HLK-PM01 │ │ 2× BTA41 │ │ 2× SRD │ │
│ │ AC→5V │ │ TRIAC pumps │ │ relays │ │
│ └──────────┘ │ + MOC3021 │ │ valves │ │
│ │ + snubber │ └───────────┘ │
│ ┌──────────┐ └──────────────┘ │
│ │ ZeroCross│ MB10S bridge + LTV-817C │
│ │ detector │ → ZEROCROSS signal to ESP32 │
│ └──────────┘ │
└─────────────────────────────────────────────────┘
📐 Hardware Details
Control Board — Full Board V2.0 (6-page schematic)
Page 1 · Microcontroller & Power
- ESP32-S3 (U1) — main application processor running ESPHome. Controls all outputs, reads sensors, manages Ethernet and RS-485.
- ATmega32U4 (U31) — dedicated analog co-processor. Drives the PT1000 OPV front-end, sends converted temperature values to the ESP32-S3 via UART (
TEMP-TX/TEMP-RX). Has its own USB-C (USB4) for independent firmware upload and an ICSP header (H9) for UPDI/ISP programming. - 3.3V power: BL1117-33CX LDO (U33) fed from the 5V rail. Protected by a WPM2015-3 P-FET ORing between USB-C 5V (USB5V) and board 5V (+5VP), with a 1N5819 Schottky for reverse-polarity protection.
- RESET and BOOT tactile buttons for ESP32-S3; RESET button also present for ATmega32U4.
- Power LED (XL-2012SURC red) on 3.3V via 1kΩ.
Page 2 · Ethernet
- W5500 (U12) — hardware TCP/IP offload chip connected to ESP32-S3 via SPI1 (
SPI1_MOSI/MISO/SCK/CS). Reset line (RST_NET) and interrupt line (INT_NET) connected to dedicated ESP32-S3 GPIOs. - Magnetics: HR911105A integrated RJ45 with built-in transformer (1CT:1CT) and LEDs — LINK (yellow) and ACT (green).
- 25MHz crystal (X1) for W5500 clock. Pi-filter (HCB2012KF-121T50 ferrite beads L1/L2) on the 3.3VNET rail for noise isolation.
- Proper termination: 49.9Ω series resistors on TX pairs; 82Ω series on RX pairs; 6.8nF and 10nF AC coupling capacitors; 22nF center-tap capacitor.
- W5500 PMODE pins floating (auto-negotiate 10/100 with auto-MDIX).
Page 3 · I/O, PCF8574 & LCD
- 5-button keypad: UP, DOWN, LEFT, RIGHT, OK (KH-6X6X8H-TJ tactile switches) wired to an ADS1115 (U3) via a resistive voltage divider (1kΩ + 1MΩ pull-down). The ADS1115 reads button combinations on AIN0; ALERT/RDY can generate interrupts.
- PCF8574M (U9) — 8-bit I2C I/O expander that buffers output signals (MOTORE1-3, VALVOLA1-2, ZEROCROSS) to the Power Board via the SN74HCT14 (U10) Schmitt-trigger hex inverter. Note: this path is not suitable for PWM signals (as marked on the schematic).
- 20×4 I2C LCD (LCD1) connected via a standard PCF8574-based I2C backpack module.
- Two 5-pin I2C expansion headers (H1, H2) provide 3.3V / 5V, SDA, SCL for external I2C peripherals.
- 10-pin IDC connector (CN3) carries all inter-board signals: MOTORE1, MOTORE2, ZEROCROSS, VALVOLA1, VALVOLA2, 3.3V, 5V, +5VP (relay coil), GND.
Page 4 · RS-485
- MAX13487EESA+T (U5) — half-duplex RS-485 transceiver with integrated auto-direction control (no DE/RE GPIO needed). VCC at 5V, with a
SN74LVC1G125(U4) level shifter on the RO output to convert from 5V to 3.3V logic for ESP32-S3 RX. - 120Ω termination resistor (R21) across A/B lines.
- 4.7kΩ fail-safe bias resistors (R20, R22) on A and B.
- SMBJ6.8A TVS diodes (U13, U14, U15) on A line, B line, and GND for surge/ESD protection on the RS-485 bus.
- Self-resetting polyfuses (F1, F2: nSMD005) on A and B lines.
- Screw terminal connector (CN10: WJ2EDGRC-5.08-3P) for field wiring.
Page 5 · ATmega32U4
- ATmega32U4-MU (U31) in QFN-44 package. Clocked at 16MHz (crystal X2 with 22pF load caps U29/U30).
- USB (D+/D-) protected by a DALC208SC6 (D8) ESD array with 22Ω series resistors (R97, R98).
- ICSP/SPI header H9 exposes MOSI, MISO, CLK, RESET for external programmer access.
- Communicates with ESP32-S3 via UART:
PD2(RXD1)→TEMP-RXon ESP32;PD3(TXD1)→TEMP-TX. - ADC pins PF0–PF7 connected to the OPV amplifier outputs (TEMP_OUT1–4) and enable signals (TEMP_ENB1–4) controlled by PC7/PC6/PB6/PB5.
OPV_ENBline (PC7 / PE2) allows the ATmega to power-gate the entire OPV section.- OneWire DS18B20 bus on a separate 3-pin screw terminal (CN5) with 4.7kΩ pull-up (R66).
Page 6 · PT1000 Amplifier
- MCP6004T-I/SL (U32) — quad rail-to-rail op-amp, all four channels used, one per PT1000 sensor.
- Each channel implements an inverting amplifier with gain set by a ±0.1% tolerance 9.1kΩ feedback resistor (R90/R85/R74/R81) and 1kΩ input resistor (R91/R86/R75/R80). This gives a gain of –9.1.
- 1MΩ resistors (R87/R88/R71/R72) provide a DC bias path to the non-inverting input (tied to GND reference).
- Each output is low-pass filtered: 1kΩ (R89/R76/R78/R73) + 1µF (C46/C48/C47/C45) forming a ~160Hz corner frequency to reject switching noise.
- Per-channel enable switches (
TEMP_ENB1–4) built from 4.7kΩ / 4.7kΩ resistor pairs allow the ATmega to multiplex or disable channels individually. - DALC208SC6 (D6) ESD protection array on all four sensor input lines (IN+1 through IN+4) before they reach the OPV inputs.
- OPV supply (VDD/VSS) bypassed with 100nF (C51/C52), power-gated via
OPV_ENB. - PT1000 connectors: CN9 / CN8 / CN6 / CN7 (WJ2EDGRC-5.08-3P screw terminals, one per sensor channel).
Power Board — Power Board V2.0 (2-page schematic)
Page 1 · Power Supply, Connector & Relays
- AC mains input enters through a 5×20mm fuse (F2/F3) and a varistor (R4: RM10D561KD24E100) + EMI filter choke (L1: UU9.8Y-10mH) before reaching the HLK-PM01 (U8) isolated AC-DC module, which outputs 5VDC. A 100nF capacitor (C1) on the AC side suppresses differential-mode noise.
- An additional slow-blow fuse (F4: 5G.300021) provides secondary protection.
- The 5V rail powers relay coils, optocouplers, and feeds through the 10-pin IDC (CN3) to the control board.
- POWER LED indicator on the Power Board side.
- AC line (L, N) also distributed to motor output screw terminals (M1, M2) and to TRIAC load outputs.
- DBT50G-9.5-2P connectors (U2, U3, U6) for L, N, and motor output wiring.
- Four PCB screw posts (SCREW1–4) for chassis/DIN rail mounting.
Relay section (Valve 1 & Valve 2):
- Two identical circuits, each using:
- BC547 NPN transistor (Q1, Q3) to drive the relay coil from a 3.3V logic signal via 1kΩ base resistor.
- SRD-05VDC-SL-C 5V SPDT relay (RLY1, RLY2) switched by the transistor.
- 1N4007 flyback diode (D2, D3) across the relay coil.
- PC817C optocoupler (U11, U14) provides galvanic isolation between logic-side (
VALVOLA1/2signals, 3.3V-fed via 1kΩ) and the relay driver side (5VUSB referenced). Output resistor 1kΩ. - RELE' LED indicators (green XL-2012SURC via 220Ω) for visual relay status.
- 3-pin screw terminal outputs (CN1, CN2) for valve wiring.
Page 2 · TRIAC Firing & Zero-Cross Detection
TRIAC circuits (Pump 1 & Pump 2) — identical topology:
- MOC3021M (U10, U13) — non-zero-crossing TRIAC optocoupler (zero-cross sync is handled in software using the ZEROCROSS signal). LED drive current set by 150Ω / 100Ω resistors (R24/R25) on the logic side.
- Gate current limiting resistor 360Ω (R9, R15) + snubber network: 47nF (C2/C4) + 100Ω (R11/R17) in series across the TRIAC, plus 100nF (C3/C5) across the main terminals.
- Gate-side 470Ω resistor (R10, R16) to limit peak gate current into the TRIAC.
- BTA41-600BRG (Q2, Q4) — 40A/600V TO-220 TRIAC. Rated far above typical domestic pump currents for long-term reliability and inrush tolerance.
- XSD1226-307 (U12, U15) — TRIAC thermal fuse (307°C) in series with the gate resistor for thermal runaway protection.
- Output screw terminals for motors M1 and M2 (Line + Neutral).
Zero-cross detection:
- MB10S bridge rectifier (D1) connected directly across Line (LP) and Neutral (NP) through four 47kΩ resistors (R1/R2/R5/R6) as a current limiter (total ~188kΩ into a 230V mains, ≈1.2mA peak).
- LTV-817-C (U4) optocoupler with 10kΩ pull-up (R3) on the output side (3.3V). Generates the
ZEROCROSSsignal — a pulse train at twice the mains frequency (100Hz for 50Hz mains) that is routed to the ESP32-S3 for phase-synchronised TRIAC firing.
🔌 Inter-Board Connector Pinout (CN3 — DC3-2.54-10PAS)
| Pin | Signal | Direction | Description |
|---|---|---|---|
| 1 | MOTORE2 | PB → CB | Pump 2 control signal |
| 2 | MOTORE1 | PB → CB | Pump 1 control signal |
| 3 | ZEROCROSS | PB → CB | Zero-cross detection pulse |
| 4 | 5V | PB → CB | 5V power from HLK-PM01 |
| 5 | 3V3 | CB → PB | 3.3V logic reference |
| 6 | 5VUSB | CB → PB | USB 5V (relay coil supply) |
| 7 | — | — | (reserved/GND) |
| 8 | VALVOLA1 | CB → PB | Valve 1 control |
| 9 | VALVOLA2 | CB → PB | Valve 2 control |
| 10 | GND | — | Common ground |
CB = Control Board, PB = Power Board
🌡️ PT1000 Sensing: Recommended Wiring
Each PT1000 connects to a 3-pin screw terminal on the Control Board (CN6–CN9). The sensor is wired in a 2-wire configuration to the OPV input. For best accuracy, use matched cable runs and consider the per-channel gain/offset calibration available in the ATmega firmware.
| Connector | Sensor Location | Signal |
|---|---|---|
| CN9 | Solar collector (panel) | IN+1 → TEMP_OUT1 |
| CN8 | Primary storage tank | IN+2 → TEMP_OUT2 |
| CN6 | Return / secondary circuit | IN+3 → TEMP_OUT3 |
| CN7 | Secondary tank / boiler outlet | IN+4 → TEMP_OUT4 |
⚙️ Control Logic Overview
The ESP32-S3 implements the following logic layers in ESPHome:
- Temperature acquisition — ATmega32U4 reads 4× PT1000 channels via the OPV front-end, applies per-channel IIR filtering and gain/offset calibration, and streams the values over UART to the ESP32-S3.
- Differential control (ΔT) — Compares solar collector temperature against tank temperatures. Activates
MOTORE1(solar pump) when ΔT exceeds a configurable threshold; deactivates with hysteresis. - Dual-tank switching — Activates
VALVOLA1/VALVOLA2to direct hot fluid to the tank with the lowest temperature, maximising energy storage. - Secondary pump —
MOTORE2manages heat exchange with the gas boiler loop based on secondary tank differentials. - Protections:
- Anti-freeze: Forces pump on when collector temperature drops below a configurable minimum.
- Overheating: Limits pump runtime or diverts flow when tank temperature exceeds maximum.
- Sensor fault: Detects open-circuit (NaN, T < –10°C) or short-circuit (T > 200°C) PT1000 conditions.
- TRIAC zero-cross firing — Pump activation is synchronised to the mains zero-crossing via the
ZEROCROSSinterrupt for reduced EMI and inrush.
📁 Repository Structure
DiSoCon/
├── 3D Model/ # 3D renders and model files
├── Hardware/ # Schematics, PCB Gerbers, EasyEDA sources
│ ├── SCH_Full_Board_V2.0.pdf # Control board schematic (6 pages)
│ └── SCH_Power_Board_V2.0.pdf # Power board schematic (2 pages)
├── Images/ # Board renders, photos, diagrams
├── Software/ # Firmware source code
│ ├── ESPHome YAML # ESPHome configuration for ESP32-S3
│ └── ATmega32U4/ # Arduino sketch for PT1000 co-processor
├── LICENSE
└── README.md
🚀 Getting Started
1. Hardware
- Order PCBs from Gerber files (JLCPCB)
- Assemble the Power Board first; verify 5V output before connecting the Control Board
- Flash the ATmega32U4 via ICSP header H9 (Arduino Nano as ISP or dedicated programmer)
- Flash ESPHome to the ESP32-S3 via USB-C (USB3 connector)
2. Firmware
# Clone the repo
git clone https://github.com/carletz/DiSoCon
# Flash ESPHome (first time via USB)
esphome run Software/main.yaml
# Subsequent updates via OTA (Ethernet or WiFi)
3. Home Assistant Integration
Add the device to ESPHome dashboard. All sensors, pumps, and valve states are automatically exposed as Home Assistant entities. MQTT topics follow ESPHome conventions.
⚠️ Safety & Disclaimer
This project involves mains voltage (230V AC). Working with mains voltage is potentially lethal.
This project is provided for educational and inspiration purposes only. The author(s) accept no responsibility for damage to equipment, persons, or property arising from the use of this design.
Any real-world installation must be performed by a qualified electrician in compliance with local electrical codes and safety regulations (e.g. IEC 60364 in Europe).
By using this material you acknowledge it is offered as-is, without warranty of any kind.
📅 Version History
| Version | Date | Notes |
|---|---|---|
| V2.0 | March 2026 | Integrated control + analog measurement on single board; ATmega32U4 co-processor; EasyEDA redesign |
| V1.0 | April 2025 | Separate controller and temperature measurement boards |
👤 Author
carletz — @carletz on GitHub
📜 License
This work — hardware schematics, PCB designs, firmware, and documentation — is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License (CC BY-NC-SA 4.0).
You are free to:
- Share — copy and redistribute the material in any medium or format
- Adapt — remix, transform, and build upon the material
Under the following terms:
- Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made.
- NonCommercial — You may not use the material for commercial purposes.
- ShareAlike — If you remix, transform, or build upon the material, you must distribute your contributions under the same license.
See LICENSE for the full license text.