There are a lot of different products that use heating elements. Unfortunately, many of them are not adjustable. For instance, most soldering irons are either on or off. It would be much more useful if you were able to change the temperature of the iron. This would allow you to use the soldering iron for lower temperature applications.
So I designed a simple control circuit that will let you adjust the output of a heating element.
Here is a video walkthough of the project.