for everyone, who is interested in the relay shields sold by aliexpress. This is schematics of the one channel. Normally Vcc and JD-Vcc are shortened by a jumper. but it's obvious, that Vcc should be 3.3v for raspberry PI and most of the microcontrollers supplied by 3.3V. And JD-Vcc is a voltage of the relay operation and probably should be at least 5V for proper function. Be careful. Raspberry PI don't allow 5V on it's GPIO!