1. Briefly describe

LED segment displays (LED modules) are a common digital information display device today, widely used in various digital products, home appliances, industrial digital instruments and meters, and other fields. It forms a display panel with specific functions through the clever layout and combination of multiple LED light-emitting diodes enclosed in fixed plastic components. Some of these display panels can only display decimal numbers from 0 to 9, some can display 26 uppercase and lowercase English letters from A to Z, and some can also display specific patterns (such as Chinese characters or battery symbols, etc.). So, in a narrow sense,LED digital tubeOnly includes seven segment digital tubes, eight segment digital tubes (with an additional decimal point DP or colon symbol on the basis of seven segments), and meter shaped digital tubes; In a broad sense,LED digital tubeIt also includes bar shaped digital tubes and various segmented LED display modules (panels). The following figure (1) shows various common ones on the current marketLED digital tube.

2. Structure and working principle

As shown in Figure (2), this is a schematic diagram of the structure of a common digital tube. From the diagram, it can be seen that LED digital tubes generally consist of four parts: display panel (thin film sheet, usually black or gray white in color; some do not use thin film sheet, but are directly coated on it), LED wick, PCB (for fixed pins and LED wick), and metal pins (patch type, not needed). Among them, multiple LED light-emitting diodes (corresponding to the letters a, b, c, d, e, f, g, h) are cleverly arranged on the PCB to form an 8-shaped number, and each printable pen segment corresponds to an LED light-emitting diode. In this way, by controlling the on or off of each LED through the exposed pins, specific numerical information can be displayed: 0, 1, 2, 3, 4, 5, 6, 7, 8, as well as the decimal point h. Figure (3) shows the corresponding internal hardware wiring situation.

3. Classification

3-1. Classify by display digits

According to how many "8" can be displayed, it can be divided into one digit, two digit, three digit, four digit, or N-digit LED digital tubes. For example, a four digit digital tube represents a display component (screen) that can display four "8" characters. As shown in the following figure:

3-2. Classified by internal connection method

To reduce the number of pins in the interface, LED digital tubes connect multiple pins together internally through a common cathode or common anode method to form a single pin, abbreviated as a common electrode (COM). There are generally two types: co positive type and co negative type, as shown in the following figure:

3-3. Classified by the height of the digital tube

This is named according to the font height of the LED digital tube, for example: 0.36 inches per digit digital tube, 0.36 inches represents the height of the font, and if converted in centimeters, it is 9.1mm height. The common heights of LED digital tubes include 0.28 inches, 0.3 inches, 0.32 inches, 0.36 inches, 0.39 inches, 0.4 inches, 0.5 inches, 0.56 inches, 0.8 inches, 1.0 inches, 1.2 inches, 1.5 inches, 1.8 inches, and so on. As shown in the figure below, there is a 0.4 inch LED digital tube with an 8-shaped font height of 10.16mm, which is 0.4 inches

3-4. Classify by the luminous color of the digital tube

This classification is based on the colors that LED digital tube components can emit light in, generally including fixed monochrome (such as red, white, yellow green, emerald green, orange, etc.), dual color (supporting 2 colors), and full-color (RGB tricolor). By using LED lights of different colors, different emission colors can be achieved. For example, the following two LED digital tubes are monochrome and two-color types.

3-5. Classified by packaging of digital tubes

It can be divided into patch type and plug-in type.

4. Advantages and disadvantages

Compared to display devices such as LCD/TFT LCD and OLED,The main characteristics and advantages of LED digital tubes are：

(1) Active emission type, therefore it has high brightness, wide viewing angle, and can also be clearly distinguished for outdoor applications.

(2) Fast luminous response time!

(3) Capable of driving light emission under low voltage and low current conditions, compatible with CMOS and ITL circuits.

(4) Small size, light weight, and good impact resistance.

(5) Long lifespan, with a service life of over 50000 hours and low cost.

And its disadvantage is：

(1) High thickness, large volume, and heavy.

(2) High power consumption.

(3) The content that can display information is limited.

5. Drive mode

To display the LED digital tube normally, a driving circuit needs to be used to drive the various segment codes of the digital tube, so as to display the numbers we want. Therefore, according to the different driving methods of the LED digital tube, it can be divided into two categories: static and dynamic.

5-1. Static display

Static drive, also known as DC drive. Static driving refers to each segment code of each digital tube being driven by an I/O port of a main control chip (such as a microcontroller), or decoded using a BCD code binary decimal decoder for driving. The advantages of static driving are simple programming and high display brightness, but the disadvantage is that it occupies a large number of I/O ports. If driving 8 digital tubes for static display, it requires 8 × 8=64 I/O ports to drive! Ordinary microcontrollers or ARM processors do not have so much IO available, and in practical applications, decoding drivers must be added for driving, which increases the complexity of hardware circuits. The following is a static driving example:

5-2. Dynamic display

The dynamic display interface of LED digital tubes is one of the most widely used display methods in microcontrollers. Dynamic driving connects the same named terminals of all 8 display strokes "a, b, c, d, e, f, g, dp" of digital tubes together. In addition, a bit gate control circuit is added to the common pole COM of each digital tube. The bit gate is controlled by independent I/O lines. When the microcontroller outputs character code, the microcontroller controls the bit gate COM terminal circuit. Therefore, as long as we turn on the gate control of the digital tube that needs to be displayed, the character will be displayed. Digital tubes without gate control will not light up. By alternately controlling the COM terminals of each digital tube in a time-sharing manner, the display of each digital tube can be controlled alternately, which is called dynamic driving. During the rotating display process, the lighting time of each digital tube is 1-2ms. Due to the phenomenon of human visual persistence and the afterglow effect of light-emitting diodes, although the digital tubes are not lit at the same time, as long as the scanning speed is fast enough, it gives the impression of a stable display data without flickering. The dynamic display effect is the same as static display, which can save a lot of I/O ports and lower power consumption. The following figure shows the application of an LED digital tube in a thermometer. This circuit adopts a dynamic driving method. The pins D0~D6 of the microcontroller output the required digital code, while the pins D8~D11 control which position to display which digital code is needed.

6. Selection of LED digital tubes

A、 Font height or size, such as 0.4 inches, 0.56 inches, 0.8 inches, etc

B、 The number of digits on a digital display determines the maximum amount of content that can be displayed on a display component.

C、 The luminous color (wavelength) of the digital tube, such as single red, yellow, blue, green, white, RGB rainbow, etc;

D、 The brightness of the digital tube, including normal brightness or high brightness.

E、 The working voltage and current of the digital tube.

F、 The color of surface silk screen printing (panel) generally includes milky white, black, gray, and so on.

G、 Internal connection method: co yin or co yang.

H、 Working lifespan.

I、 Working temperature and humidity.

J、 Other properties, such as waterproof rating, dustproof, UV resistant, crack resistant, flame resistant, super strong impact aging resistance, etc

7. Testing and usage precautions

7-1. Appearance inspection

The appearance of LED digital tubes requires uniform color, no local discoloration, and no bubbles.

7-2. Electrical performance testing

The detection of digital tubes is the same as detecting LED light-emitting diodes. Generally, a digital multimeter can be used for preliminary testing. Taking the following image as an example, introduce the detection method:

Fix the common probe (black wire) of the multimeter in contact with the common cathode of the LED digital tube, and place the red probe in contact with the electrodes of other probe segments. If certain segment codes can be lit, it indicates that this is a common cathode LED digital tube. Conversely, if the two probes need to be swapped, it is a common anode LED digital tube.

By using this simple method, the corresponding relationships (a~g) between the pins of the digital tube can be detected, And whether there are broken strokes (a certain stroke cannot be displayed), or connected strokes (i.e. short circuits, some strokes are connected together), and the relative comparison of the brightness performance of different strokes can be made.

7-3. Precautions for use

① A typical LED digital tube has a working current I of 5-30mA per drawing. If the current is too high, it will damage the digital tube. Before use, it is necessary to determine the working voltage and current, and add appropriate current limiting resistors!

② LED digital tubes are sensitive to static electricity, especially blue, emerald green, and white LED digital tubes. Anti static measures should be taken during operation and transportation.

③ Try to avoid mechanical vibration and impact as much as possible.

④ The welding temperature should not be too high (around 260 degrees), and the welding time should be completed within 3 seconds.

⑤ The surface of LED digital tubes is sometimes coated with protective film to prevent scratches on the display screen during processing and before use. Therefore, after processing, the protective film should be peeled off to avoid affecting the display effect.