I suspect that this particular board might give problems, if connected directly to the GPIO. Note that this board is "inverted"; an input has to be pulled low to allow current to pass from emitter to base in the (PNP type) transistor, which again allows current to flow from emitter to collector and on to relay
and LED. If Vcc on the relay
board is connected to 5V and the input to a Raspi GPIO pin, even with the pin pulled high (to 3.3V), enough current might flow through base on the transistor and the clamping diode in the Raspi SOC, that the transistor turns on and the relay
operates. If, on the other hand, Vcc is connected to 3.3V, you get rid of this problem, but the 5V relay
may not operate at all at 3.3V, and besides, will probably overload the 3.3V supply on the Raspi.
If you can find (or make) a relay
board with a schematic that is a mirror
image of this one; that is, uses NPN type transistors with emitter tied to ground, and the relay
connected between Vcc and collector of the transistor, then chances are it will work with the Raspi's 3.3V signals, even if it is designed for 5V.
A transistor buffer could of course also solve these problems, but then again you could also use the same transistor buffer to drive the relays directly
[A lightning transistor reference, for those that feel about to drown in acronyms:
There are two basic types of bipolar transistors (which leaves out FET's and a few other types that I won't cover here); NPN and PNP. PNP transistors go into conduction when a negative voltage of about 0.7V (for silicon transistors) is applied to base, relative to emitter. When that happens, current can flow from emitter to collector. NPN transitors are the same, except polarities are exchanged; they go into conduction when base is positive in relation to emitter, and the main current flow is from collector to emitter. The maximum collector current that will flow is a multiple of the base current. This multiple is known as hfe. It varies from one transistor type to an other, but is commonly about 100 to 200 for small signal transistors. Besides this, there is of course a maximum collector current and power that the transistor can handle. In schematics, the emitter and collector connections are the angled lines on one side of the symbol, while the base is the lone connection on the other side. The emitter is marked with an arrow. The direction of the arrow tells us whether it is an NPN or PNP transistor; it points outwards in an NPN symbol, but inwards is a PNP symbol.]
Best regards, Kári.