What is a 7-Segment Display?
A 7-segment display is an electronic component made up of 7 individual LEDs arranged in a specific pattern to display digits (0-9) and some letters. These LEDs are named a, b, c, d, e, f, g, and the display typically includes an optional decimal point (dp).
The 7 segments are controlled through two main types of configurations:
- Common Anode: All the anodes are connected to a common pin, and each segment is illuminated when the corresponding pin is set to a low voltage.
- Common Cathode: All the cathodes are connected to a common pin, and each segment is illuminated when the corresponding pin is set to a high voltage.
Each segment is a separate LED that can be independently lit or turned off, allowing it to display numbers and some letters.
7-Segment Display Pinout
As we said above, the pinout of a 7-segment display varies based on its type, which is mainly divided into two categories: common anode and common cathode.
Pinout Configuration
The following is an example pin assignment for a typical 7-segment display (note that actual pinouts may vary depending on the manufacturer and model):
Common Anode Configuration
- g, f, a, b, e, d, c, dp: These are the segment pins, corresponding to the seven segments and the decimal point, respectively.
- COM: The common anode pin, connected to VCC.
Common Cathode Configuration
Similarly, the segment pins (g, f, a, b, e, d, c, dp) correspond to the segments and decimal point, but the common cathode pin (COM) is connected to GND.
Example Connection to Arduino
When interfacing a 7-segment display with an Arduino, the segment pins are typically connected to different digital pins on the Arduino board. For example, in a common anode configuration, the segment pins might be connected as follows:
- a – Arduino Pin 7
- b – Arduino Pin 8
- c – Arduino Pin 4
- d – Arduino Pin 3
- e – Arduino Pin 2
- f – Arduino Pin 6
- g – Arduino Pin 5
- COM – Arduino Pin 9 (connected to VCC)
In this configuration, to illuminate a specific segment, the corresponding Arduino pin is set to LOW, while the common anode pin remains HIGH.
Wiring the 7-Segment Display
To operate a 7-segment display, the individual segments (a, b, c, d, e, f, g, dp) need to be connected to specific output pins on a microcontroller. For example, for a common cathode 7-segment display, the negative end of each segment is connected to a common ground pin, while the positive end of each segment is connected to a different output pin on the microcontroller.
Basic Wiring Setup
- 7-segment display has 8 pins (7 for the segments and 1 for the decimal point).
- Each pin corresponding to a segment (a to g) is connected to the microcontroller.
- For the common cathode configuration, connect the common cathode pin to the ground.
How to Control a 7-Segment Display
The key to controlling a 7-segment display is to provide the correct voltage to the individual segments to form the desired number or symbol.
Displaying Numbers
To display a number, the microcontroller needs to turn on the appropriate segments. For example:
- To display 0, segments a, b, c, d, e, and f should be illuminated, while segment g remains off.
Example Code for Displaying Numbers
int pin_a = 7, pin_b= 6, pin_c = 5, pin_d = 10, pin_e = 11, pin_f = 8, pin_g = 9, pin_p = 4;
// Segment data for digits 0-9
int numTable[10][8] = {
{1, 1, 1, 1, 1, 1, 0, 0}, // 0
{0, 1, 1, 0, 0, 0, 0, 0}, // 1
{1, 1, 0, 1, 1, 0, 1, 0}, // 2
{1, 1, 1, 1, 0, 0, 1, 0}, // 3
{0, 1, 1, 0, 0, 1, 1, 0}, // 4
{1, 0, 1, 1, 0, 1, 1, 0}, // 5
{1, 0, 1, 1, 1, 1, 1, 0}, // 6
{1, 1, 1, 0, 0, 0, 0, 0}, // 7
{1, 1, 1, 1, 1, 1, 1, 0}, // 8
{1, 1, 1, 1, 0, 1, 1, 0}, // 9
};
void setup(){
for (int i = 4; i <= 11; i++){
pinMode(i, OUTPUT);
}
}
void loop(){
for (int i = 0; i < 10; i++){
digitalWrite(pin_a, numTable[i][0]);
digitalWrite(pin_b, numTable[i][1]);
digitalWrite(pin_c, numTable[i][2]);
digitalWrite(pin_d, numTable[i][3]);
digitalWrite(pin_e, numTable[i][4]);
digitalWrite(pin_f, numTable[i][5]);
digitalWrite(pin_g, numTable[i][6]);
digitalWrite(pin_p, numTable[i][7]);
delay(1000);
}
}
In this example, the numTable array holds the data required to turn on the appropriate segments for each digit.
Applications of 7-Segment Displays
7-segment displays are commonly used in digital clocks to show the time. Multiple displays can be used to show hours, minutes, and seconds.
Multi-Digit Displays
For multi-digit displays, the simplest way to control multiple 7-segment displays is through multiplexing.
Multiplexing Technique
With multiplexing, multiple displays share the same set of control pins, and each display is turned on sequentially. This is done so fast that the human eye perceives all the digits as lit at the same time.
Example Code for Multiplexing 7-Segment Displays
const int digitPins[3] = {2, 3, 4}; // Digit selection pins
const int segmentPins[8] = {6, 7, 8, 9, 10, 11, 12, 13}; // Segment pins
int numTable[10][8] = { {0, 0, 0, 0, 0, 0, 1, 1}, {1, 0, 0, 1, 1, 1, 1, 1}, ... };
void setup(){
for (int i = 2; i <= 4; i++) {
pinMode(i, OUTPUT);
digitalWrite(i, LOW);
}
for (int i = 6; i <= 13; i++) {
pinMode(i, OUTPUT);
digitalWrite(i, HIGH);
}
}
void loop(){
for (int i = 0; i < 10; i++){
for (int j = 0; j < 3; j++) {
showdigit(i, j);
}
delay(500);
}
}
void showdigit(int num, int digit){
digitalWrite(digitPins[digit], HIGH);
for (int i = 0; i < 8; i++) {
digitalWrite(segmentPins[i], numTable[num][i]);
}
delay(500);
digitalWrite(digitPins[digit], LOW);
}
Using Libraries to Control 7-Segment Displays
For convenience, libraries like SevSeg are available to simplify the control of 7-segment displays. This library is useful for controlling multi-digit displays, as it abstracts much of the complex wiring and control logic.
Example Code Using the SevSeg Library
#include "SevSeg.h"
SevSeg sevseg;
void setup() {
sevseg.begin(COMMON_CATHODE, 3, digitPins, segmentPins);
}
void loop() {
int readValue = analogRead(A0);
int showValue = map(readValue, 0, 1023, 0, 999);
sevseg.setNumber(showValue, 3);
sevseg.refreshDisplay();
}
Advanced Control with Shift Registers
If you’re dealing with multiple 7-segment displays and are limited by the number of available microcontroller pins, you can use a shift register like the 74HC595 to control multiple displays with fewer pins.
The 74HC595 shift register allows serial data input that is converted to parallel data, thus controlling multiple displays or LEDs with only a few pins.
Conclusion
7-segment displays are versatile components that allow for the simple display of numeric data. Understanding their wiring, control methods, and applications can greatly enhance the functionality of your projects, from simple clocks to complex displays. Whether you’re using multiplexing, libraries, or shift registers, there are many ways to control 7-segment displays effectively, each suited to different project requirements.
