LED digital tube pin identification and multimeter detection method

1. Introduction of LED digital tube

LED digital tubes, also known as semiconductor digital tubes, are one of the most commonly used digital display devices that arrange and package several light-emitting diodes in a certain pattern. There are many types of LED digital tubes, and here we only introduce the identification and use of the most commonly used small "8" shaped LED digital tubes to beginners.

At present, the commonly used small LED digital tubes are mostly "8" shaped digital tubes, which are composed of 8 light-emitting diodes, of which 7 light-emitting diodes (a~g) are used as 7-segment strokes to form the "8" structure (so it is also called 7-segment LED digital tube), and the remaining 1 light-emitting diode (h or dp) forms the decimal point, as shown in Figure 1 (a). Each light-emitting diode is connected according to the method of common cathode or common anode, that is, the negative pole (cathode) or positive pole (anode) of all light-emitting diodes are connected together as a common pin; and the positive pole or negative pole corresponding to each light-emitting diode is used as an independent pin (called "pen segment electrode"), and its pin name corresponds to the light-emitting diode in Figure 1 (a), namely, pin a, b, c, d, e, f, g and pin h (decimal point), as shown in Figure 1 (b). If the LEDs on certain pen segments are lit as specified, the 10 numbers "0-9" and the 6 letters "A-F" as shown in Figure 1 (c) can be displayed, and a decimal point can also be displayed. It can be used to display binary, decimal and hexadecimal numbers and is widely used.

Figure a

Figure b

Figure c

2. Appearance and type

The packaging form of commonly used small LED digital tubes almost all adopts a dual in-line structure, and 1 to more "8"-shaped characters are packaged together as needed to form digital tubes with different display digits. If divided according to the number of displayed digits (that is, the number of all digital characters), there are 1-digit, 2-digit, 3-digit, 4-digit, 5-digit, 6-digit... digital tubes, as shown in Figure d. If divided according to the different internal light-emitting diode connection methods, there are two types of common cathode digital tubes and common anode digital tubes; divided according to different character colors, there are red, green, yellow, orange, blue, white and other digital tubes; divided according to different display brightness, there are ordinary brightness digital tubes and high brightness digital tubes; according to different display fonts, they can be divided into digital tubes and symbol tubes.

Figure d

3. Model and pin identification

Since the model naming of LED digital tubes is not unified among manufacturers, each manufacturer does its own thing and there is no rule to follow. If you want to know the structural characteristics and related parameters of a certain model product, you can generally only check the manufacturer's manual or related parameter manual. For LED digital tubes whose models are unclear, you can only find out the internal circuit structure and related parameters through measurements such as multimeters. Table 1 lists the main parameters of some domestic BS××× series LED digital tubes for reference.

The pin arrangement rules of small LED digital tubes are shown in Figure 4. That is, facing the display surface of the product, with the pin surface facing the magazine, starting from the upper left corner (left and right double-arranged pins) or the lower left corner (upper and lower double-arranged pins), count in a counterclockwise direction (i.e., the arrow in the figure), 1, 2, 3, 4 pins in turn... If you turn it over and look at it from the back (such as the soldering surface of the printed circuit board), that is, the pin surface is facing you and the display surface is facing the magazine, you should count in a clockwise direction. It can be seen that this is consistent with ordinary integrated circuits.

The pin arrangement of commonly used LED digital tubes is double-row 10-pin, 12-pin, 14-pin, 16-pin, 18-pin, etc. Table 2 shows the pin arrangement diagram and internal circuit diagram of commonly used LED digital tubes compiled by the author, hoping to provide readers with simple and quick help for application. There is generally such a rule when identifying the pin arrangement: for a single digital tube, the most common pins are arranged in double rows of up and down, usually its 3rd pin and 8th pin are connected, which are common pins; if the pins are arranged in double rows of left and right, then its 1st pin and 6th pin are connected, which are common pins. But there are exceptions, and specific models must be treated specifically. In addition, the "decimal point" of most LED digital tubes is connected to the common pin internally, but the "decimal point" pin of some products is independently led out. For digital tubes with 2 digits or more, the 8 data lines a to h of each "8"-shaped character are generally connected together, and the common pin of each character is led out separately (called "dynamic digital tube"), which not only reduces the number of pins, but also provides convenience for use. For example, a 4-digit dynamic digital tube has 4 common terminals, plus pins a to h, for a total of 12 pins. If a "static digital tube" with independent "8" characters is made, the number of pins can reach 40.

Pin arrangement and internal circuit diagram of commonly used LED digital tubes

1) CPS05011AR (1-bit common cathode/red 0.5 inch), SM420501K (red 0.5 inch), SM620501 (blue 0.5 inch), SM820501 (green 0.5 inch)

2) SM420361 (1-bit common cathode/red 0.36 inches), SM440391 (red 0.39 inches)

3) SM420322 (1-bit common cathode/red 0.32 inches), SM220322 (green 0.32 inches)

4) SM410561K (1-bit common anode/red 0.56 inches), SM610501 (blue 0.5 inches), SM810501 (green 0.5 inches)

4. Detection Methods

A quality-assured LED digital tube should have fine workmanship, uniform luminous color, no local discoloration, no light leakage, etc. For digital tubes whose performance, product model and pin arrangement are unclear, the following simple method can be used for testing.

1) Dry cell battery test method.

As shown in Figure 5 (a), connect two ordinary 1.5V dry batteries in series (3V), and connect a 100Ω, 1/8W current limiting resistor in series to prevent overcurrent from burning the LED digital tube under test. Connect the negative lead of the 3V dry battery (both leads can be connected to a small alligator clip) to the common cathode of the digital tube under test, and move the positive lead to contact each segment electrode (a~h pins) in turn. When the positive lead contacts a certain segment electrode, the corresponding segment will light up and display. This method can quickly detect whether the digital tube has broken pens (a certain segment cannot be displayed) or continuous pens (some segments are connected together), and can relatively compare whether the light intensity of different segments is consistent. If testing a common anode digital tube, just swap the positive and negative leads of the battery, and the method is the same as above.

If all the pen segment electrodes (pins a to h) of the digital tube under test in Figure 5 (a) are short-circuited and then connected to the test dry battery, the digital tube under test can be made to emit light in all pen segments. For a digital tube with guaranteed quality, its luminous color should be uniform, without any missing pen segments or partial color change.

If you are not sure about the structure type (common anode or common cathode) and pin arrangement of the digital tube being tested, you can start from the first pin on the left side of the digital tube being tested, and test each pin one by one in a counterclockwise direction, so that each segment can be illuminated, and the pin arrangement and internal wiring of the digital tube can be measured. Note that when testing, as long as a certain segment is illuminated, it means that one of the two pins being tested is a common pin. Assuming that one pin is a common pin and does not move, change the other test pin. If the other segment is illuminated, it means that the assumption is correct. In this way, according to the polarity of the power supply connected to the common pin, it can be determined whether the digital tube being tested is a common anode or a common cathode. Obviously, if the common pin is connected to the positive pole of the battery, the digital tube being tested is a common anode; if the common pin is connected to the negative pole of the battery, the digital tube being tested should be a common cathode. Next, test the remaining pins to quickly determine the corresponding segments.

2) Multimeter detection method.

Here, we take the MF50 pointer multimeter as an example to explain the specific detection method: First, as shown in Figure 5 (b), set the pointer multimeter to the "R×10k" resistance block. Since the forward conduction voltage of the light-emitting diode inside the LED digital tube is generally ≥1.8V, the resistance range of the multimeter should be set to the "R×10k" block with an internal battery voltage of 15V (or 9V), and should not be set to the "R×100" or "R×1k" block with an internal battery voltage of 1.5V, otherwise the forward and reverse resistance of the light-emitting diode cannot be measured normally. Then, perform the test. When measuring the common cathode digital tube shown in Figure 5 (b), the red test lead of the multimeter (note: the red test lead is connected to the negative electrode of the battery in the meter, and the black test lead is connected to the positive electrode of the battery in the meter) should be connected to the "-" common terminal of the digital tube, and the black test lead should be connected to each segment electrode (a~h pins); for the common anode digital tube, the black test lead should be connected to the "+" common terminal of the digital tube, and the red test lead should be connected to the a~h pins respectively. Under normal circumstances, the pointer of the multimeter should deflect (generally the reading is within 100kΩ), indicating that the LED of the corresponding segment is turned on and the corresponding segment will emit light. If the pointer of the multimeter does not deflect when a certain pin is measured, and the corresponding segment does not emit light, it means that the LED of the measured segment has been damaged by an open circuit. Like the dry cell detection method, the multimeter detection method can also be used to quickly detect digital tubes whose structure type and pin sequence are unclear.