A light emitting diode (or light emitting diode) is an optical diode that emits light energy in the form of "photons" if it is biased in the forward direction. In electronics, we call this process electroluminescence. The color of the visible light emitted by the LEDs ranges from blue to red and is determined by the spectral wavelength of the emitted light, which, in turn, depends on various impurities that are added to the semiconductor materials during their production.
LEDs have many advantages compared to traditional lamps and luminaires, and perhaps the most important of them is their small size, durability, various colors, low cost and easy accessibility, the ability to easily interact with various other electronic components in digital circuits.
But the main advantage of LEDs is that due to their small size, some of them can be concentrated in one compact housing, forming a so-called seven-segment indicator.
A seven-segment indicator consists of seven LEDs (hence its name), arranged in a rectangle, as shown in the figure. Each of the seven LEDs is called a segment, because when lit, the segment forms part of a digit (decimal or decimal number system). Sometimes the 8th additional LED is used within the same package. It serves to display the decimal point (DP), thus allowing you to display the decimal fraction if two or more 7-segment indicators are connected together to represent numbers greater than ten.
Each of the seven LED segments of the display is connected to the corresponding pad of the contact row located directly on the rectangular plastic housing of the indicator. LED pins are labeled a through g, representing each individual segment. Other contacts of the LED segments are interconnected and form a common output.
So, the direct bias applied to the corresponding contacts of the LED segments in a certain order will make some segments glow, and the rest will remain darkened, which allows you to highlight the desired character of the number pattern, which will be displayed on the display. This allows us to represent each of the ten decimal digits from 0 to 9 on a 7-segment indicator.
The general output is typically used to determine the type of 7-segment display. Each display LED has two connection pins, one of which is called an anode, and the other, respectively, is called a cathode. Therefore, the seven-segment LED indicator can have two types of circuit design - with a common cathode (OK) and with a common anode (OA).
The difference between these two types of displays is that in the design with OK, the cathodes of all 7 segments are directly connected to each other, and in the circuit with a common (OA) anode, the anodes of all 7 segments are interconnected. Both schemes work as follows.
- Common cathode (OK) - interconnected cathodes of all LED segments have a logic level of "0" or connected to a common wire. Individual segments are highlighted by applying to their anode output a signal of a "high" logic level or a logical "1" through a limiting resistor to create a direct bias of individual LEDs.
- Common Anode (OA) - the anodes of all LED segments are combined and have a logical level of "1". Separate segments of the indicator light when each specific cathode is connected to ground, a logic "0" or a low-potential signal through the corresponding limiting resistor.
In general, seven-segment indicators with a common anode are more popular, since many logic circuits may require more current than a power source can supply. Also note that a common cathode display is not a direct replacement in the circuit for a common anode display. And vice versa - this is equivalent to turning on the LEDs in the opposite direction, and, therefore, light emission will not occur.
Although the 7-segment indicator can be considered as a single display, it still consists of seven separate LEDs within a single package, and as such, these LEDs need protection against overcurrent. LEDs emit light only when they are displaced in the forward direction, and the amount of light emitted by them is proportional to the forward current. This means only that the intensity of the LED glow increases approximately linearly with increasing current. So, in order to avoid damage to the LED, this direct current must be monitored and limited to a safe value by an external limiting resistor.
Such seven-segment indicators are called static. Their significant drawback is the large number of conclusions in the package. To eliminate this drawback, dynamic control schemes for seven-segment indicators are used.
The seven-segment indicator has gained great popularity among hams, as it is convenient to use and easy to read.