SunFounder Relay Module for Arduino and Raspberry Pi 5V DC Trigger by HIGHLO (HIGH Trigger) Relays are suitable for driving high power electronic devices such as lights, electric fan and air condition. This relay module can be used to control high voltages with a low voltage by connecting to an MCU Controls DC or AC signals  220V AC load Working voltage: 5v; PCB size: 2.0 x 4.3 cm With power indicator and signal output indicator Working principle: when a high level is given to SIG, the PNP transistor is energized, coil of the relay electrified, normally open contact closed, and the indicator LED lights up; when a low level is given, PNP will be de-energized, the original state is restored  the normally closed contact closed, and the indicator LED goes out then. For more info, please refer to http://wiki.sunfounder.cc/index.php?title=Relay(LOW)_for_Arduino