Funduino/Keyes relay board doesn't switch off relays
what could cause this behaviour?


Welding the relay contacts together due to a high initial current. 

If your lights on the load side are incandescents, they have a low resistance until the filaments heat up. So a relatively high current can pass for several milliseconds.


Image from Lamptech

Note the 500W lamp has a normal operating current of around 2A but during the first five milliseconds the current is greater than 15A. Your relays are rated 10A. 


  What further steps can I take to debug this?


Measure the current flowing in the first few milliseconds.

Hit the recalcitrant relay with a blunt object so that the mechanical shock can break the contacts free?

Take a relay apart and inspect the contacts?



See also


Relay &quot;sticking&quot;