Working with 7-segment displays

Seven segment displays are made up of LEDs combined such that they can be used to display numbers and letters. As their name implies, they are made of seven LEDs plus an additional LED for a dot. Seven segment displays can be in form of common anode or common cathode. They can also be in form of single digit or multi digit seven segment displays.

Single digit displays

Common Anode (CA) displays have the anode of all LED pins connected together to 5v, therefore, a LOW voltage applied individually on each pin will light up the LED of that particular pin. Conversely, Common Cathode (CC) displays have the cathode of all LED pins connected together to ground, therefore, a HIGH voltage applied individually on a given pin will turn ON the LED of that particular pin. Fig. 2 shows the internal structure of a 1-digit 7-segment display.

7 segment displays normally have 8 pins which represents the each LED on the display (7 segment plus the dot making 8 LEDs) (Fig. 3a and b). However, there is also a 7 segment display called a Binary Coded Display (BCD) that has only 4 pins (Fig. 3c. In this case, the binary combination as shown in Table 1 will produce numbers 0 - 9 and letters A - F as shown.

For common anode 7-segment displays, Table 2 shows the binary and hexadecimal value that would produce numbers 0 - 9 and alphabets A - F, this can be traced by using Fig. ??. For common cathode 7-segment displays, the binary and hexadecimal values are shown in Table 3 for numbers 0 - 9 and alphabets A - F. Notice that the binary values can also be produced by carrying out a NOT operation on the respective common anode binary value.

Multi-digit displays

Multi-digit displays do not have pins equal to 8 * number of digits, instead, their number of pins equals 8 + number of digits as shown in Figure 4.

7 segment displays can be connected directly to a microcontroller. 74HC573 Latch IC can likewise be used to drive 7 segment displays, the advantage of using a latch is that only 8 digital pins of a microcontroller can be used for multi-digit displays. If a latch is not used, the number of pins that is equal to the number of pins the LED display has will be used by the microcontroller.

4-digit 7-segment displays

The internal structure of a 4-digit 7-segment common cathode display is shown in Fig 5. The cathode of each digit are connected together, a LOW voltage on the cathode will light up the corresponding LED if its anode is HIGH while a HIGH voltage on the cathode will turn off the LEDs.

The internal structure of a 4-digit 7-segment common anode display is shown in Fig 6. The anode of each digit are connected together, a HIGH voltage on the anode will light up the corresponding LED if its cathode is LOW while a LOW voltage on the anode will turn off the LEDs of the corresponding digit.

The pinout for 1-digit and 4-digit 7-segment displays is shown in Fig. 7. For 1-digit displays (Fig. 7a), pin 3 and 8 are the power pins: in common anode, these pins should be connected to Vcc while for common cathode, they should be connected to GND.