This circuit was actually very enjoyable to reverse engineer, as it had some unusual components with no Google hits. I had to work out what they did by their circuit location, and then verify that with a logic probe (while powering the PCB from a bench supply at just under 5V.)
Once I knew what they did I searched for things like SOT23-5 schmitt trigger and some pin-compatible components then showed up in the results.

Destroying some components on the PCB by connecting what I thought was a safe low load was also an interesting result that was directly related to the unusual way the circuit creates a power source for the LEDs by using the solenoid load as part of the current limiting circuitry.

Aside from my little incident, the most probable cause of the module's failure would have been the degradation of the electrolytic capacitor. I'll guess that the high voltage ripple was probably retriggering the dedicated reset chip to keep the unit turned off. Quite an interesting and most likely intentional fail-safe system.

It's interesting that the module only needs to switch on the water valve solenoid, and that probably then activates the heater control switches via a pressure diaphragm that doubles up as the safety switch if the water pressure drops too low. Nice design.

If you have a shower with one of these modules then the fix is to replace the capacitor with a new one, and if there is room to safely mount it, it can be replaced with a standard through hole capacitor like the one I soldered on.

Because this unit operates at mains voltage in a wet environment, you do anything like this entirely at your own risk.

Although it seems odd to have a piece of electrical equipment in a shower cubicle, these instantaneous showers are extremely common in Europe. They offer the incredible convenience and efficiency of instant hot water on demand. It's extremely rare that there are electrical accidents involving them.


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