Wow! Who knew that adding a typical 5-volt relay module to your Arduino could be so easy yet at the same time harbour some gotchas that will make you wonder what's going on? If you need to drive some external circuitry from your Arduino module then a relay module is a fantastic item to use - but there are some gotchas to be aware of, and having some knowledge in how they work will help you refine your project. I give you the low down on what makes these tick and how to avoid some potential traps that could leave you scratching your head! PLEASE NOTE: In July 2017, an observant Arduinite (#Alexus) has spotted that the circuit diagram for this device is wrong - there should be no direct connection between the collector of Q1 and JD-VCC as that would be a short circuit whenever Q1 was switched on! Luckily the actual device is fine, just the circuit diagram that is wrong. As always, Thanks For Watching. And my channel is here: null How can I remember this? Memory tip: "See" Ralph Bacon, geddit?