Microcontroller projects

Solar_master: Monitor your PV panels

* on hold because my display broke and there is a problem with the dual use of pins for SD CARD ans serial modbus *

last updated: 2025-09-27

Intro

After more then 20 years of use some of my solar panels had to be rewired to fit my new battery system. At the same time, they are cleaned and it would be interesting to get the U/I curve of the panels to compare them and detect faulty or weak panels. I wrote this two years ago and developed and used the solar_master, but had no time to document it. This will be done now :) :

solar_master

I needed a load to change the current and measure the voltage. And after a short search I found an electronic 500W load on Aliexpress with a resistance up to 1000 Ω. The load is from the firm SOUSIM (type14 0-100 V 0-20 A) and you will still find it on Aliexpress in 2025 for about 75€. The challenge was to understand the communication with the load, ass all documentation was in Chinese (and my Chinese isn't so good ;)).

For the rest I used mainly an old Inkplate 6 (from 2020). The Inkplate 6 has a recycled 6 inch e-paper display (800x600), with an ESP32 and an SD card slot. The other cool things of the inkplates is that they are 100% open-source for both software and hardware. I think there is no problem to use a newer Inkplates : https://soldered.com/product/inkplate-6-6-e-paper-board/.

Features

The software measures voltage and current for different loads. It then draws the I-V curve on the e-ink display and calculates the maximum power point. It also saves the data and a graph to the SD card.

iv curve iv curve data

BOM

The only information about the load that I could find was from the this EEVblog forum: https://www.eevblog.com/forum/testgear/chinese-electronic-load-500w-module-sousim-500w/. For the load we need a DC power source (12 V, 1 A) and the communication is done via Modbus RTU (Terminal with A+ and B-). This is a differential signal and needs no ground.

Circuit

The circuit is minimal:

neopixel clock circuit

URL: https://github.com/e-radionicacom/Croduino-Board-Definitions-for-Arduino-IDE/raw/master/packageCroduinoBoards_index.json Add the URL in the File → Preferences → Additional Board Manager URLs text box. Install libraries (zip files) with: Sketch → Include Lirary → Add .ZIP Library... https://github.com/SolderedElectronics/Inkplate-Arduino-library https://github.com/4-20ma/ModbusMaster

Load

Modbus Address = 1, 9600bit/s

Holding Register 0x03
0 1 0, 1-255 (0x16)
1 1 0.CV 1.CC 2.CW 3.CR 4.CV+CC 5. 10. 11-15.
2 0 0.OFF 1.ON (100)
3 10000 CV Overvoltage (10000 = 100V)
4 0 CC Current in mA
5 0 CW Watt
6 0 CR Resistance
7 0
8 5 5 = 9600 (RS458) 600,1200,2400,4800,7200,9600,14400,19200,...
9 20630 120 Over Current ? 5500 = 55A
10 1320 120
11 2000 120
12 1565 100

INPUT Register 0x04
0 1 0, 1-255 (0x16)
1 1 0.CV 1.CC 2.CW 3.CR 4.CV+CC 5. 10. 11-154.
2 0 0.OFF 1.ON (100)
3 21 Temperature in °C
4 0 Temperature in °C CPU
5 9368 DVM?
6 4005 Voltage in mV/10
7 0 Current in mA
8 0 Power
9 0 Resistance
10 0
11 0
12 780 Running Timer in Seconds

Software

URL: https://github.com/e-radionicacom/Croduino-Board-Definitions-for-Arduino-IDE/raw/master/packageCroduinoBoards_index.json Add the URL in the File → Preferences → Additional Board Manager URLs text box. Install libraries (zip files) with: Sketch → Include Lirary → Add .ZIP Library...

https://github.com/SolderedElectronics/Inkplate-Arduino-library https://github.com/4-20ma/ModbusMaster

Housing

The housing is drawn in FreeCAD. It is not my best design, but it works :). I used PETG instead of PLA because of the heat.

solar_master housing

The cover of the inkplate was integrated and can be downloaded here: https://github.com/SolderedElectronics/Inkplate-6-hardware.

solar_master housing solar_master housing

FreeCAD files and stl files are on github.

Problems and To do

The project is working as is, and I don't have the time for the moment to refine the project.

Downloads

Interesting links