After looking through many instructables it’s clear that many makers love to use the pre-assembled relay modules to switch large loads or mains voltages using their Raspberry Pi or Arduino. These can be bought from places like eBay for as little as 2. There’s many tutorials on how to use them with your microcontroller, and even how to make your own. But none explain in detail how it works, the importance of each component or why a particular value was selected.
In this instructable I wanted to show you how you can easily make your own module and how to calculate the correct component values. A word of warning, working with mains electricity is dangerous and can be deadly. Although it is possible, I do not recommend using a home made module for switching mains voltage.”