I am just starting to get into Raspberry Pi and I saw a post that said that only a few modules are compatible. I would like to know which modules are compatible and how to tell which ones are in the future.
The point your are looking for is the difference between arduino pins (5V) and rapsberry GPIO (3,3V).
With this in mind, we can deduce that every passive components (switchs, potentiometer) will be compatible, and most part of the active modules won't be compatible.
For more details, you need to look at each module specification, and check for the component operating voltage range.
By exemple, the datasheet of this gaz sensor show that the circuit has to be in 5V, so not compatible.
Edit : a convenient way of solving this point is to actually use a arduino, powered and driven from the RPI USB port. This solution may also lower the RPI CPU usage, as some part of the signal processing may be put on the arduino part.