There are two important types of diodes to automotive electronics, the Light Emitting Diode (or LED) and the photodiode. The LED light bulb is used as a light source for a wide range of functions. The most obvious is where the LED light bulb is used as a warming or indicator lamp. Less obvious is in the cable-driven speed meters where an LED (transmitter) and a photo diode are teamed up in the back of the speed meter head to produce a signal directly proportional to vehicle speed. In automotive language, when we speak of diodes we normally think of the devices in the alternator that convert AC voltage into DC. We call them a one-way electronic wave. For our purposes, it might be more appropriate to think of these diodes as a light bulb and a light sensitive switch.
The LED lighting we will most often refer to in this book produces infrared (invisible) light when current is flowing through it. Pair this with an infrared photodiode which allows current to flow when this type of light falls on it, and you have a sensor capable of detecting the presence of an opaque object between them.
