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Introduction
Last fall I came across an interesting article in Hackaday, where Arne van Itersen made an interesting solar power display with an analog gauge and an e-paper screen.
It immediately grabbed my attention because it solves the biggest problem with e-paper displays—they need a few seconds to refresh, making them unsuitable for displaying continuously changing data like solar power. A simple analog needle takes care of that problem but is limited to displaying one thing only. But if you put the needle in front of an e-paper screen, you get continuous power information from the needle. You can then use the e-paper display for displaying things that only change every few minutes, like storage battery level, daily production, and system operation mode.
But using an analog dial from a multimeter seemed a bit fiddly to me, and I thought about how to improve on that. Using a small servo instead would be better, because I could use needles of any size. Controlling the servos is quite simple too, I needed the ESP32 to control the e-paper display anyway. It took a weekend to put together a simple prototype for a one gauge display, which I used for my DIY EV-charger.
The thing worked like a charm and got me thinking about how to improve it. I was thinking about updating my solar system display for a while now, and this seemed like a good solution.
Thinking About The Design
One needle wasn’t going to cut it this time—I wanted one for solar power, one for power coming from the grid, and one for the power going into the house. After thinking about it for a while, I decided to put the grid and house power servos on the left and right side of the display. The solar servo would be on the bottom. This provides some spatial logic too—power flows from the grid (left) or solar (bottom) to the house (right). I guess putting the solar need on top could also work, but that didn’t look right to me for some reason.
I wanted to put the display outside, where it would be exposed to the elements, so it needed a weatherproof housing. So I had to think a bit on how to make a housing that would be both weatherproof and easy to assemble. It’s easy to build a two part shell, but how to design it in a way you can assemble and then disassemble it easily while keeping it watertight? I decided to approach it differently. I designed a mounting plate for the servos and the e-paper display first and gave it a flange on the sides and on the bottom. Then I designed a one-piece housing with a window and a simple top cover. This housing had simple guides to hold the flanges of the mounting plate. The window can be closed using some acrylic glued with silicone to make it weatherproof. The top cover is designed to have rainproof downslopes and only needs a drop of superglue on each side to keep it in place. This makes the assembly and testing really simple, you can do everything with the mounting plate alone.
The Software
I’m used to Arduino, so this is what I used. E-paper in Arduino is IMO best used with the GxEPD2 library, which includes the Adafruit GFX functions, the servos are controlled using the Servo library, data exchange is done using Adafruit MQTT, and network connectivity uses WiFi.
The program is subscribed to several MQTT topics, containing the values for grid power, house power, solar power, solar production, battery level, and system mode. Each of these topics is linked to a callback function that triggers on topic updates. The callback functions for grid, house and solar power all move the respective servo needles. The callbacks for battery level and solar production only update the relevant global variables. Screen refresh is only directly triggered by a system mode topic change or every 10 minutes, but this updates the display with the latest data stored in the globals.
Startup makes a full sweep of all the needles, serving as a kind of servo calibration check, then starts the main loop. The main loop is really short and only keeps the MQTT connection alive and resets the whole thing if Wi-Fi drops.
Getting the Data To the Topics
The one remaining job on the software side was to get the data from my solar system to the MQTT topics. The solar system is connected to my Home Assistant installation, so it was really easy to set up some automations that publish the relevant data to MQTT topics when the relevant sensor states change. I used state changes for triggers and the MQTT Publish service to send the data to the MQTT topic. I already had a MQTT broker set up for my other projects, so I just used that one.
Build
Once I was satisfied with the results, it was plain sailing:
- 3D-print the enclosure using ABS
- Cut the acrylic to size and glue it to the enclosure using some silicone glue
- Drill a hole for the power cable on the appropriate spot, then use some mounting glue to fix the enclosure to the wall
- Connect the power and insert the mounting plate in the housing.
You can find all the relevant code and files on Github
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