One topic that a lot of people have some trouble with when taking the Extra Class exam is digital logic. I think one of the reasons for this is that while it is electronics, the logic element is different from the other types of technology we deal with in ham radio.
To help students learn the concepts of digital logic, the Department of Informatics at the University of Hamburg has developed the Hamburg Design System, or HADES for short. HADES is an object-oriented, all-Java, Beans- and Web-based, visual design and simulation environment.
With this system, you can play with digital logic without actually building any circuits or connecting up any test equipment. There are canned demonstrations, such as this digital clock, or you can build your own circuits with HADES’ graphical editor, library of component models, and the discrete-event-based simulation engines.