I am testing a module with opto isolation and High/Low level selectable. I found that for one sample I tested. the required input voltage levels are summarized below.

If Low Level Trigger is selected
Vin to turn on relay = 3.2V
Vin to turn off relay = 3.5V

Conclusion:
Arduino
No problem
Rpi
Problem – always on, cannot turn off relay, because Rpi’s High signal is not high enough
Solution – use a logical level converter (BJT NPN, ULN2003/ULN2803 etc) to convert Rpi’s High level signal from 3.0V to above 4V. Get around: set GPIO to input mode to turn off relay.

If High Level Trigger is selected
Vin to turn on relay = 1.8V
Vin to turn off relay = 1.5V

Conclusion:
Arduino
No problem
Rpi
No problem

Schematic
I used a multimeter to check open/shorts to figure out the schematic as below: