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    2021-10-21 17:37:22
    Things You Need to Know about Seven-segment Display

    Topics covered in this post:


    Classification of seven-segment display

    Structure of seven-segment display

    Drive methods of seven-segment display


    Interpretation of common problems


    A seven-segment display or 7-seg display is a kind of semiconductor light-emitting device that is cheap and easy to use. By inputting relative current to its different pins to make it light up, it can show that the numbers can display all the parameters that can be represented by numbers, such as time, date, temperature, and so on.

    It is widely used in electrical appliances, especially home appliances, such as display screens, air conditioners, water heaters, refrigerators, and so on. Most water heaters use digital tubes, and other home appliances also use LCD screens and fluorescent screens.

    seven-segment displays.jpg

    seven-segment displays

    Classification of seven-segment display

    According to the connection mode of the light-emitting diode unit, it can be divided into a common anode seven-segment display and a common cathode seven-segment display. 

    common anode and common cathode seven-segment display

    common anode and common cathode seven-segment display

    The common anode seven-segment display refers to the seven-segment display that connects the anodes of all light-emitting diodes to form a common anode (COM). The common anode seven-segment display should be connected to +5V when the common anode is used. The matching field is lit when the cathode of a field of light-emitting diodes is at a low level, and it is not lit when the cathode of a field is at a high level. Common cathode seven-segment display refers to the seven-segment display that connects the cathodes of all light-emitting diodes to form a common cathode (COM). The common cathode seven-segment display should be connected to the ground line GND when the common cathode seven-segment display is used. When the anode of a certain field of light-emitting diodes is high, the corresponding field is lit, and when the anode of a field is low, the corresponding field is not lit.

    Structure of seven-segment display

    LED Segment Displays is an "8"-shaped device encapsulated by multiple light-emitting diodes. The leads have been connected internally, and they only need to draw out their individual strokes and common electrodes. The number of segments commonly used for led segment displays is generally 7 segments, some plus a decimal point, and the other is similar to the 3-digit "+1" type. There are half digits, 1, 2, 3, 4, 5, 6, 8, 10, and so on. The appearance structure of the seven-segment display is shown in the figure below.

     Seven-segment display pin diagram.jpg

    Seven-segment display pin diagram

    The general seven-segment display has seven light-emitting diodes (three horizontal and four vertical) to display decimal numbers from 0 to 9 plus a decimal point. It can also display English letters, including English A to F in hexadecimal (b, d is lowercase, others are uppercase). At present, most of the seven-segment displays will be displayed in italics.

    In addition to the seven-segment display, there are also displays with additional diagonal strokes such as fourteen and sixteen strokes. However, due to the drop in the price of dot matrix displays, these "multiple strokes" have basically been replaced.

    LED Segment Display 10 Pins.jpg

    LED Segment Display 10 Pins

    Understanding these characteristics of LEDs is very important for programming because different types of seven-segment displays have different programming methods in addition to their hardware circuits. Their light-emitting principles are the same, but their power supply polarities are different. The colors are red, green, blue, yellow, etc. Instruments, clocks, stations, home appliances, and other items all employ LED seven-segment displays. When choosing a product, consider the size, color, power consumption, brightness, wavelength, and other factors.

    Drive methods of seven-segment display


    To display the seven-segment display normally, it is necessary to use a drive circuit to drive each segment of the seven-segment display to display the number we want. Therefore, according to the different driving methods of the seven-segment display, it can be divided into static and dynamic types.

    Static display drive

    The static drive is also called DC drive. Static drive means that each segment code of each seven-segment display is driven by an I/O port of a single-chip microcomputer, or driven by decoding using a binary-decimal decoder such as a BCD code. The advantage of static driving is simple programming and high display brightness. The disadvantage is that it occupies a lot of I/O ports. If driving 5 seven-segment display static displays, 5×8=40 I/O ports are needed to drive. You must know that an 89S51 single-chip microcomputer is available. There are only 32 I/O ports. In actual application, a decoder driver must be added for driving, which increases the complexity of the hardware circuit.

    Dynamic display drive

    The 7-seg display dynamic display interface is one of the most widely used display methods in the single-chip microcomputer. The dynamic drive is to connect the terminals of all the 8 display strokes of the seven-segment display "a, b, c, d, e, f, g, dp" together, and add a bit gating control circuit for the common pole COM of each seven-segment display. The bit gating is controlled by each independent I/O line. When the single-chip microcomputer outputs the font code, all the seven-segment displays receive the same font code, but which seven-segment display will display the font depends on the microcontroller's control of the bit gate COM terminal circuit, so we only need to turn on the gate control of the seven-segment display that needs to be displayed, and the bit will display the font. The strobed seven-segment display will not light up. By controlling the COM terminal of each seven-segment display in turn by time-sharing, each seven-segment display can be controlled and displayed in turn, which is a dynamic drive.

    In the alternate display process, the lighting time of each seven-segment display is 1~2ms. Due to the persistence of human vision and the afterglow effect of light-emitting diodes, although the seven-segment displays are not actually lit at the same time, as long as the scanning speed is sufficiently fast, it gives the impression that a set of stable display data will not flicker. The effect of dynamic display is the same as that of static display, which can save a lot of I/O ports and lower power consumption.


    Seven-segment displays are one type of display screen. By inputting relative currents to different pins, it will make it light up, thus showing that the numbers can display all the parameters that can be represented by numbers, such as time, date, temperature, and so on.

    Because of its low price and simple use, it is widely used in electrical appliances, especially home appliances, such as air conditioners, water heaters, refrigerators, and so on. Most water heaters use seven-segment displays, and other home appliances also use LCD screens and fluorescent screens.

    Interpretation of common problems

    Display effect

    Since the light-emitting diode is basically a current-sensitive device, its forward voltage drop has a great dispersion and is also related to temperature. In order to ensure that the seven-segment display has a good brightness uniformity, it needs to have a constant working current. And it cannot be affected by temperature and other factors. In addition, when the temperature changes, the driver chip must be able to automatically adjust the output current to achieve color balance temperature compensation.


    Even a short-term current overload may cause permanent damage to the LED. The use of a constant current drive circuit can prevent large-area damage to the LED caused by current failure.

    In addition, the very large-scale integrated circuits we use also have cascaded delay switching characteristics, which can prevent the damage of the reverse peak voltage to the light-emitting diodes. The VLSI also has a thermal protection function, which can be automatically shut down when the temperature of any one exceeds a certain value, and the fault display can be seen in the control room.

    Brightness consistency

    There are two major factors that affect the consistency: one is the selection of the raw material chips used, and the other is the control method adopted when using the seven-segment display.

    How to divide common cathode and common anode?

    Find common cathode and common anode: First, we find a power supply (3 to 5 volts) and a 1K (a few hundred ohms) resistors, VCC is connected in series with a resistor, and GND is connected to any 2 pins. There are many combinations, but there is always one LED that will emit light. It is enough to find one, then GND does not move, and VCC (string resistor) touches the remaining pins one by one. If there are multiple LEDs (usually 8), Then it is a common cathode.

    On the contrary, use VCC without moving, and GND touches the remaining pins one by one. If there are multiple LEDs (usually 8), then it is a common anode. You can also use a digital multimeter directly. The red test lead is the anode of the power supply and the black test lead is the cathode of the power supply.

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