How does the seven-segment indicator work?

The LED (or light-emitting diode) is an optical diode that emits light energy in the form of "photons", if it is shifted in the forward direction. In electronics, we call this process electroluminescence. The color of the visible light emitted by the LED lies in the range from blue to red and is determined by the spectral wavelength of the emitted light, which in turn depends on the various impurities that are added to semiconductor materials during their production.

LEDs have many advantages over traditional lamps and lamps, and perhaps the most important of them is their small size, durability, different colors, cheapness 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 case, forming a so-called seven-segment indicator.

The seven segment indicator consists of seven LEDs (hence its name) located in a rectangle, as shown in the figure. Each of the seven LEDs is called a segment, because when the segment is lit, the segment forms part of the digit (decimal or 12- digit number system). Sometimes an 8th additional LED is used within the same package. It serves to display a decimal point (DP), thus allowing to display a 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 contacts are marked with a to g labels representing each individual segment. Other contacts of the LED segments are connected to each other and form a common terminal.

So, a direct bias applied to the corresponding contacts of the LED segments in a certain order will cause some segments to glow, and the remaining ones will remain darkened, which allows to display the desired symbol of the number pattern that 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 generally used to determine the type of 7-segment display. Each LED display has two connecting leads, one of which is called the "anode", and the other, respectively, is called "cathode". Therefore, the LED seven-segment 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 OK design, the cathodes of all 7 segments are directly connected to each other, and in the circuit with the common (OA) anode, the anodes of all 7 segments are connected to each other. Both schemes work as follows.

• Common cathode (OK) - connected cathodes of all LED segments have a logic level of "0" or are connected to a common wire. Individual segments are displayed by feeding a high logic level signal or a logic "1" signal to their anode output via a limiting resistor to create a direct bias of individual LEDs.
• The common anode (OA) - the anodes of all LED segments are combined and have a logic level of "1". The individual segments of the indicator light when each particular cathode is connected to ground, a logic "0" or low potential signal through a corresponding limiting resistor.

In general, seven-segment indicators with a common anode are more popular, since many logic circuits may require more current than the power supply can provide. Also note that the display with a common cathode is not a direct replacement in the circuit for a display with a common anode. And vice versa - this is equivalent to the inclusion of LEDs in the opposite direction, and, consequently, the emission of light will not occur.

Although the 7-segment indicator can be considered as a single display, it still consists of seven separate LEDs within the same package, and as such these LEDs need protection from overcurrent. LEDs emit light only when they are biased in the forward direction, and the amount of light emitted by them is proportional to the direct current. This means only that the light intensity of the LED increases approximately linearly with increasing current. So, to avoid damaging the LED, this forward current must be monitored and limited to a safe value by an external limiting resistor.

Such seven-segment indicators are called static indicators. A significant disadvantage is the large number of conclusions in the package. To eliminate this drawback, schemes for the dynamic management of seven-segment indicators are applied.

Seven segment indicator has gained great popularity among radio amateurs, because it is easy to use and easy to read.