Review on Huayao Channel Module Optocoupler Raspberry by Sarah Greenwood

This is great value for money. I've found that these relays not only require VCC and a common ground from the microcontroller that sends the switching signal to the relay board, but also require a slightly higher switching voltage than specified when all relay gates are active. Search the internet for articles about these relay boards, they are widely available. There are no instructions or information in the packaging, but these can be found on the manufacturer's website.