In this blog post, we’ll explore the structure and operating principles of the 7-segment display—the most basic device that converts digital signals into numbers humans can understand—as well as the 7-segment driver that controls it.
Everyone has probably heard at least once that digital signals consist solely of 0s and 1s. However, this concept may not resonate much with the general public. This is because people primarily interact with machines that have optimized user interfaces. A user interface refers to a device or program that facilitates the exchange of information between a user and a machine. For example, thanks to a smartphone’s user interface, people can instantly see on the screen—without much effort—how much battery life is left or what time it is. Sometimes, it even alerts users to incoming calls via vibrations or ringtones. Devices that help machines convey information to users through senses such as sight or hearing are called display devices. One of the most basic types of display devices is the “7-segment display.” While the name itself may sound unfamiliar at first, once you understand what it is, you’ll realize it’s actually a very familiar device. Of all the components I encountered while studying my major, this one felt the most familiar to me. If you look at the 7-segment displays used in digital clocks, you’ll see that a single 7-segment display shows one digit. By combining multiple displays, you can represent numbers with multiple digits, and digital clocks display numbers in this very way. Here, a “segment” refers to each of the bars that make up a digit. Since this device uses seven bars to represent numbers, it is called a 7-segment display. You can see this device not only in watches but also in movie theaters, at banks when you take a number and wait your turn, when riding an elevator, and when tapping your transit card on a bus.
The principle behind the 7-segment display, which is used in so many places, is simple. 7-segment displays are classified into common-cathode and common-anode types based on their operating principles. Since the basic principles of the two types are not significantly different, this explanation will focus on the common-cathode type. If you examine the circuit diagram, you can see the symbol representing a light-emitting diode (LED). An LED is a type of diode that must be connected with its anode and cathode distinguished. In the circuit diagram, you can see that the cathodes of the LEDs corresponding to each segment are connected together. Since the cathodes of multiple LEDs are connected in common, this configuration is called the common-cathode configuration. Conversely, the common-anode configuration is one in which the anodes of multiple LEDs are connected together.
A light-emitting diode emits light when a voltage is applied such that the potential at the anode is higher than that at the cathode, causing current to flow. Looking at the circuit diagram again, you can see that the terminals labeled with letters are connected to the anodes of the light-emitting diodes corresponding to each segment. Therefore, to illuminate a specific segment, a high logic level must be applied to the terminal connected to that segment. Using this principle, you can display any desired digit on a 7-segment display. For example, to display the number 1, apply a high logic level to terminals b and c, and a low logic level to the remaining terminals. Here, a high logic level is represented as 1, and a low logic level as 0. If you apply a 1 to pins a, b, and c, and a 0 to the remaining pins, what number will appear? The answer is “7.” Using this device, you can display numbers from 0 to 9. The common anode configuration operates in the opposite manner. It generally works by applying a 0 to turn on a segment and a 1 to turn it off.
However, in actual circuits, the specific driving conditions may vary depending on the logic type and electrical characteristics of the driver used.
So far, we’ve examined the operating principles of 7-segment displays. However, it’s difficult to immediately display the desired value on a 7-segment display using only the knowledge covered above. This is because a 7-segment display is not a device that displays the desired number immediately upon input. To display a specific number, electrical signals must be applied to the appropriate pins. For example, to display the number “0,” rather than directly inputting the value “0,” you must apply a “1” to all pins except the g pin. A 7-segment driver is a device that facilitates the use of 7-segment displays. A “driver” refers to a device that controls or regulates another device. A 7-segment driver receives the BCD value of the number to be displayed, converts it into the necessary signals for the 7-segment display, and transmits them, thereby ensuring that the entered number is displayed. In practice, integrated circuits such as BCD-7-segment latches, decoders, and drivers perform this role. A BCD value is a representation of each decimal digit as four binary digits, which differs slightly from the standard binary representation. For example, the number 64 is represented as 1000000₂ in standard binary. However, in BCD, each digit is converted into four binary digits. The number 6 is 0110₂, and 4 is 0100₂. When concatenated, this becomes 01100100.
Now, let’s take a closer look at the role of a 7-segment driver. Once again, I will focus on the common-cathode configuration. A 7-segment driver typically consists of four input pins and seven output pins. There are four input terminals—A, B, C, and D—which correspond to each bit of the BCD value represented by a four-digit binary number. D corresponds to the 2³ position, C to the 2² position, B to the 2¹ position, and A to the 2⁰ position. The seven outputs from the 7-segment driver are connected to the corresponding input terminals of the 7-segment display. A truth table is a table that shows how the output value changes based on the combination of input values. Through this, we can understand what operations the 7-segment driver performs. For example, consider a situation where 1, 0, 0, and 0 are applied to the D, C, B, and A terminals, respectively. If you locate the row in the truth table where the input is “1000” and examine the outputs on the right, you will see that all outputs are 1. When connected to a 7-segment display, this will display the number “8.” Since 8 is represented in binary as 1000₂, this matches the entered BCD value. In this way, the 7-segment driver converts the input BCD value into signals that turn each segment on and off, displaying the number in a format that is recognizable to humans.
