As tested in the last log, the ESP32 needs a lot of power. With not too big solar cells, it might only update the display once every two hours. This is the reason I tested another wireless transceiver module, the nRF24L01+.

I got some modules from eBay for or EUR 1.45 each and tried to test it with the example program with two Arduino nanos, as explained in his tutorial.

But it didn't work. After what felt like reading all of the 122,000 Google search results for "nrf24L01+ not working", soldering extra SMD capacitors to the power supply pins, using an external power supply etc., it turns out the GettingStarted script doesn't work. But it works fine with the pingair_ack example. Power consumption was about 15 mA, with some short spikes when sending, much less than what the ESP32 needs. Was no problem to send to the next room, 5 m distance through one wall, with highest transfer power setting and 1 Mpbs. With 10 m and 2 walls sometimes there were (recognized) transfer errors, but it still worked.

Unlike the ESP32, there is no programmable microcontroller on these modules. I plan to use a STM32L433CCT6 microcontroller for it. The 64 kB SRAM is sufficient to buffer a whole image for the 4.2" black/white/red display and it has very advanced power saving modes. While testing the LTC3106 power supply IC, it didn't work when starting from 0 V. With this new microcontroller it should work: The power good pin of the power supply IC can hold reset down until the voltage has reached a good level, and when the microcontroller starts, I can further measure the voltage of the backup capacitor and the solar cell, while running in low-power mode, to determine if there is enough power to do the wireless transfer. I already soldered it to a breakout board:

This would allow a use-case that @Acube asked for: a slim and compact wireless ePaper display with a 2" display, which can be updated from a RF24L01+ module that is connected to a RaspberryPi (or to an ESP32). The RPi can be powered somewhere central in the house with a wall wart and it could update multiple displays as well, or read many sensors over the low-power wireless connection.

I also got some cheaper and smaller displays from Crystalfontz to play with:

This allows to build very compact wireless display modules.

So the plan for the next steps is to modularize the design: The display modules will have a common interface to use whatever display is best for a given project. Then either an ESP32 can be used to control it, or an nRF24L01+ with the STM32L433 microcontroller and a sender module somewhere. The power supply can be a solar cell with all required voltage regulator chips, which I will build as a separate reusable module, or simply a battery or a wall wart, depending on your project.