Description of 4 Channel Relay Module: 4 Channel Relay Module Relay Board Arduino Relay Module allows a wide range of micro-controllers such as Arduino, AVR, PIC, ARM with digital outputs to control larger loads and devices like AC or DC Motors, electromagnets, solenoids, and incandescent light bulbs. 4–channel relay interface board, and each channel need a 15-20mA driver current. We can use it to control various appliance SPECIFICATIONS of 4 Channel Relay Module: 5V 4-Channel Relay interface board and each one needs 15-20mA Driver Current Equipped Equipped with high-current relay, AC250V 10A ; DC30V 10A Standard interface that we can control directly by microcontroller (Arduino, 8051, AVR, PIC, DSP, ARM, ARM, MSP430, TTL logic active low) Opto-isolated inputs Indication LED’s for Relay output status Principle From the picture below, you can see that when the signal port is at low level, the signal light will light up and the optocoupler 817c (it transforms electrical signals by light and can isolate input and output electrical signals) will conduct, and then the transistor will conduct, the relay coil will be electrified, and the normally open contact of the relay will be closed. When the signal port is at high level, the normally closed contact of the relay will be closed. So you can connect and disconnect the load by controlling the level of the control signal port.