A programmable logic controller (PLC) is an instrument used for many types of machine control. Instead of having to design complex circuitry, a PLC can be configured with software to do the job more efficiently. PLCS.net is a site with loads of useful tutorials and instructions that make the programming required to use these gadgets much easier. Many of the sections have animations that further illustrate the concepts. In addition to the online tutorials, there is also a very active forum where beginners can ask questions or experienced users can answer them.