At my parents‘ house in the garden there is a pool with an older controller for filter pump and solar heating. Via underground pipes the water is pumped from the skimmer and floor drain into the house, pumped through the sand filter, heated if necessary (depending on the availability of solar heat) and then pumped back into the pool via a few meters of pipe and the inlet nozzles.
The old controller has a very simple design and includes a mechanical timer, an input for a temperature sensor, an output for the circulating pump of the heat exchanger and of course a (contactor) switched output for the filter pump. Most of the time, however, the pump is switched on manually, which means that a walk into the basement is necessary.
At first I created a draft concept. The new controller should be WLAN capable and be able to map the previous functions.
The concept was then „translated“ into a circuit schematic and a circuit board was designed for it. Actually there are two circuit diagrams and boards, one is the basic module and the soldered-on board is the DCDC voltage converter (6-35V –> 5V)
PCB DIN Module 
PCB DCDC Converter 
Schematic DCDC Converter 
Schematic DIN Module
I ordered the two circuit boards to be manufactured and partially assembled in China. But of course I made a mistake with the first version of the circuit, which I already changed in the schematics above: the two optocoupler isolated inputs were wrongly connected to the wemos board, so the board reset itself continuously (D3/D4 via pulldown to ground).
DCDC PCBs 
DIN Rail Module PCBs 
PCBs soldered together
After assembling the two boards and other components like connectors the wemos board was flashed with a tasmota firmware build. Then the module was packed in a DIN-Rail case and installed in a small distribution box with two auxiliary relays, a contactor and a 24V power supply. On the photo you can see the „fix“ of my error with D3 and D4 with enamelled wire near the two optocouplers.
The display, a button for switching the display on and off and a potentiometer for setting the target temperature were installed in the drilled and cut-out sections of the housing. The display flickers with high frequency, so there is always one line black on the pictures – the human eye does not notice anything. A button for manually switching the filter pump on and off was installed in a cable entry of the distribution box.
The filter pump can now be switched on manually by push button, via the web interface and also schedule-controlled with Tasmota. Depending on the temperature of the water and release from the solar system (excess heat), the circulation pump of the heat exchanger is switched on as well.
