Great value

This is a fantastic product at a great price. Just one thing! Do NOT set the jumper to GND-VCC! This will cause a short! This is not a design error! You can use the board non-isolated the way it comes. This jumper is used to separate the opto-coupler circuit from the relay side. To fully isolate the inputs from the relays: 1) Pull the jumper and use secondary 5 volt power to the relay using GND and VCC next to JD-VCC. Do NOT use the VCC and GND pins next to the IN1 and IN2 input pins! 2) The JD-Vcc is the common for all inputs. The input side consists of the indicator LED and opto-coupler-LED and a 100 ohm resistor in series. 3) Apply Arduino 5 volt to JD-VCC and IN1 or IN2 to the Arduino outputs (LOW) and the relay will be triggered. Hope this helps! The board is missing one star for the mounting hole issue! Thanks to Eric for the schematic! You can find a schematic on amazon.