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I connected a SHT30 shield to a Wemos D1 mini (one on top of the other) to measure temperatures.
Wemos D1 mini (2.3.0): 
SHT30 shield: 
It works great but when I added a relay on the top of the stack it doesn't work anymore.
Relay shield: 
Maybe there are a conflict with their pins. So these are their schematics:
This is the solution that I made but I don't know why this works instead of inserting the relay shield on top of the SHT30 shield:
Yes, there is a pin conflict. The relay uses D1, however D1 is (as you can see from looking at the schematics) part of the I2C connection for the SHT30.
You will have to find some way of modifying the relay shield to use a pin that's not part of I2C.
