Not so fast with level shifters.
I think I found the board you are using. It would appear the opto part of this can be operated by 3.3V but the relay side needs 5V. The small jumper is to operate the opto and relay from the same 5V supply. Remove to operate from separate supplies.
If using an interface transistor connect as described but connect transistor emitter to opto source (3.3V ???) ground. The board ground has to be connected to relay supply ground. If you want complete opto isolation DO NOT connect grounds together. If you think about it you will see that you can have complete isolation, The opto side has a complete circuit from supply back to its own ground and the same for the relay circuit.
The link on Core web cite for this 4 channel unit is headed " * Customer recommended resource #1" and describes operation and options fairly well.
Re 3.3V relay operation.
- While marked as a 5V supply voltage & input signals, we have found this to work fine at 3.3V
If this is what you are referring to it seems to be the same relay I had some words about a couple of days ago. The relay itself is guaranteed to operate at 75% of 5V which is 3.75V. It may or may not work at 3.3V but is not guaranteed. I notice you have noted 0.1V drop across the transistor on your circuit, presumably you measured this. This 0.1V has to be subtracted from the relay available voltage so that would leave only 3.2V available if using a 3.3V supply. A fair bit short of the required 3.75V required for guaranteed operation.
Core support staff have not yet commented on this reply so I don’t know what their thoughts are.
If you want to use 3.3V as a relay supply go for it but don’t be at all surprised if some relays work and some don’t. Doing this will be operating the relay itself outside its design range.
Another consequence not considered yet is the relay speed. If too slow and sluggish and is switching reasonably substantial current some contact arcing and burning could be expected with uncertain results.
The choice is yours.
I have not had issues with this. Simply I have not got any 3.3V devices and I probably would not consider operating with anything but 5V anyway. If the device says 5V that’s it. otherwise it would be quoted as 3.3V - 5.5V or something like that.