Electronics Projects: How to Use a Diode to Block Reverse Polarity
Although diodes look a little like resistors, they behave very differently in electronic circuits. Diodes have one ability that sets them apart: They let current flow freely in one direction, but block current if it tries to flow in the other direction.
In other words, a diode is like a turnstile gate that you can walk through in one direction but not the other. This characteristic turns out to be incredibly useful in electronic circuits.
This project demonstrates a diode's ability to conduct current in only one direction. For this project, you wire up a rectifier diode in series with a 3 V flashlight lamp and a pair of AA batteries.
You use a DPDT knife switch between the battery and the diode/lamp circuit so that when the switch is changed from one position to the other, the polarity of the voltage across the diode and lamp is reversed. Thus, the lamp lights in only one of the two switch positions.