An LED (or Light Emitting Diode) is an optical diode that emits light energy in the form of "photons" when it is forward biased. In electronics, we call this process electroluminescence. The color of visible light emitted by LEDs ranges 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 the semiconductor materials during their manufacturing process.
LEDs have many advantages over traditional lamps and fixtures, and perhaps the most important of them is their small size, durability, various colors, low cost and easy availability, the ability to easily interface with various other electronic components in digital diagrams.
But the main advantage of LEDs is that, due to their small size, some of them can be concentrated in one compact package, forming the so-called seven-segment indicator.
The 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 hexadecimal number system). Sometimes an 8th additional LED is used within one package. It serves to display a decimal point (DP), thus allowing a decimal to be displayed if two or more 7-segment displays 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 case of the indicator. The LED pins are marked with labels a through g representing each individual segment. The other pins of the LED segments are interconnected and form a common terminal.
So, a forward bias applied to the corresponding pins of the LED segments in a certain order will cause some segments to light up and the rest to remain dimmed, thus highlighting the desired number pattern character to be shown on the display. This allows us to represent each of the ten decimal digits from 0 to 9 on a 7-segment display.
Common output is generally used to determine the type of 7-segment display. Each display LED has two connecting leads, one of which is called the "anode" and the other, respectively, is called the "cathode". Therefore, a seven-segment LED indicator can have two types of circuit design - with a common cathode(OK) and 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, while in the common anode (OA) design, the anodes of all 7 segments are connected to each other. Both schemes work as follows.
- Common cathode (OK) - interconnected cathodes of all LED segments have a logical "0" level or are connected to a common wire. Individual segments are illuminated by driving their anode output to a logic "high" or logic "1" through a limiting resistor to forward bias the individual LEDs.
- Common anode (OA) - the anodes of all LED segments are combined and have a logic level of "1". The individual segments of the indicator glow when each specific cathode is connected to ground, logic "0" or a low-potential signal through the appropriate limiting resistor.
In general, common anode seven-segment displays are more popular, as many logic circuits can draw more current than the power supply can supply. Also note that the common cathode display is not a direct replacement in the circuit for the common anode display. And vice versa - this is equivalent to turning on the LEDs in the opposite direction, and therefore no light emission will occur.
Although the 7-segment indicator can be considered as a single display, it is stillconsists of seven individual LEDs within one package, and as such these LEDs need to be protected from overcurrent. LEDs only emit light when they are forward biased, and the amount of light they emit is proportional to the forward current. This only means that the intensity of the LED increases approximately linearly with increasing current. So, to avoid damaging the LED, this forward current must be controlled and limited to a safe value by an external limiting resistor.
Such seven-segment indicators are called static. Their significant disadvantage is the large number of outputs in the package. To eliminate this shortcoming, schemes for dynamic control of seven-segment indicators are used.
The seven-segment indicator has gained great popularity among radio amateurs because it is easy to use and easy to read.
