Building Basic Prototypes
If you never built any parallel port prototype before, we suggest you to start with the most basic one: a set of eight LEDs, one connected to each data bit pin from the parallel port. With this very basic prototype you will be able to learn a lot how the parallel port works.
When a data pin is set to “0”, you will find 0 V on it. When it is set to “1”, you will find 5 V on it. This is enough to turn on LEDs, but not enough to turn on lights and home appliances; we will explain later how to drive “heavier” devices.
So, all you need to do is to connect each data pin from the parallel port (pins 2 through 9) to a LED (to its anode terminal, a.k.a. “positive terminal”) and get one ground pin (any one from 18 through 25) to connect to the cathode terminal (a.k.a. “negative terminal”) from all LEDs. You can see the schematics in Figure 3.
Since LEDs have polarity, you should pay attention to correctly locate its anode (positive) and cathode (negative) terminals. If you pay close attention, LEDs are not completely rounded: the cathode side is a little bit flat, as you can check in Figure 4.
As for building circuits, we recommend you to use a breadboard unit. Breadboard units allow you to assemble prototypes without needing to solder anything.