I've been playing a bit with it and I'm afraid I'm using it the wrong way, or else it does not work as expected.
I don't want to fry the duino in case the module is defective, so I'm testing it with a 9V battery instead, via a voltage divider that provides Vo = 5V. I'm using just the 4 pins in the right corner (GND, IN1, IN2, Vcc). I've connected Vcc to Vo, GND to ground and IN1 to GND to try to switch the #1 relay on, but it was not enough. The led turns half on, but there's no click and the multimeter measures no connectivity. The Vcc voltage measures 2.6 V when connected to the voltage divider output Vo. If I disconnect the module, then Vo measures 5V.
So I've replaced R1 in my voltage divider with a potentiometer, and slowly increased Vo. I notched it up untill reaching 8.1 Volts disconnected, then I connected the module's Vcc to Vo, and still it was not enough to turn the relay on (Vcc measures 2.8 V). Then I increased Vo a little bit more to around 8.7 V disconnected, connected again the module and that produces a Vcc around 3V. That switches the relay on, a click can be heard and the led in the module turns fully on. When it connects, Vcc suddenly rises from aorund 3V to 7.8V.
So it seems that this module consumes more energy when the relay is off.
Is this normal? Would a duino output pin have enough punch to power this thing on just 5V? Should I use the alternate power source instead? If so, again, does anyone know which voltage and current are required?