What is a payload decoder?
A bit of background, LoRaWAN is an open protocol that defines communication between low powered sensor devices and the Internet. It is great but it does not define data payload formats. It’s much like HTTP, it defines a mechanism for communication but not what that communication is.
The current state of play is that the various sensor manufacturers devise their own payload formats for data sent from or to their sensors. There are some efforts to define standards (such as Cayenne LPP) but none are widespread. The difficulty is that there are so many different types of sensors that designing a protocol to fit inside a few Bytes and yet cover all eventualities isn’t simple.
So at the moment when you add a sensor to a LoRaWAN network, you need to translate the payload into something useful.
I will keep adding more sensors to it as time goes on.