Light-emitting Diodes

A light-emitting diode (LED) is a specially manufactured diode that is designed in such a manner to glow, or emit light from it, when it is forward-biased. When it is reverse-biased, it will act just like any common diode; it will neither emit light nor it will allow considerable amount of current flow. LEDs can be designed to emit any color of the visible light spectrum we want, including “white” light. Red is found to be the most common color. For certain physical reasons, semiconductor material is especially efficient and sensitive to near-visible light in the infrared region. Consequently, many photoelectric eyes used for presence detection and industrial control functions operate in the infrared region.

LEDs are widely used commonly as indicator devices. The brightness, or intensity, of an LED is relative to the forward current flow through it. Most LEDs are low-voltage, low-current devices, but thr recent developments in optoelectronics have directed to a family of high-intensity LEDs that reach the light intensity levels of incandescent bulbs.

Most commonly available LEDs ranges from 5- to 50-milliamp and these usually drop about 1.4 to 2 volts when these are in the forward-biased mode. In most uses,

series resistor are used  to limit the maximum current flow to meet LEDs requirement.

LEDs are available in wide range of case styles, many shapes, and different colors to describe in detail within this context, but each type will use some physical method to point out the cathode lead. You will come accross many of the indication methods through your  experience, but when you fall in doubt, simply use your DVM in the “diode test” mode to check lead identification. (Most DVMs, when in diode test mode, will make an LED to glow very faintly when checked in forward-biased orientation.)

A widespread alpha-numeric type of LED indicator device is the seven-segment display. (The term alphanumeric means to display devices that are capable of displaying some, or all, of the characters of the alphabet as well numeric characters.) Seven-segment LED displays are in fact seven individual elongated LEDs arranged in a “block 8” pattern. Seven-segment LEDs will have a common connection point to all seven diodes. This common connection might connect all of the cathodes together, as a result creating a common-cathode display, or all of the anodes together that make a common-anode display. When to use a common-cathode, or a common-anode display, depends  on the circuit configuration and the polarity of voltages used. In addition to this, there are many types of “decoder” integrated circuits (these integrated circuits are designed in order to convert logic signals into seven-segment outputs), that will identify the case in which you make use of either common-cathode or common-anode displays.

A seven-segment LED has 8 connection pins to the case. In these pins one pin is the common connection point to all of the cathodes or anodes. The rest of seven pins connect with each individual diode with-in the package. Therefore, by connecting the common pin to the appropriate polarity, and forward-biasing we can achieve various combinations of the LEDs with the rest of seven pins, any seven-segment alphanumeric character can be displayed.