7-Segment display with 74HC595 shift register | Arduino Projects

Adding a serial-in parallel-out shift register such as the popular one 74HC595 to a 7-segment display will reduce number of pins required to drive it (the display).
Basically the 7-segment display requires 9 pins: 8 segment pins (A, B, C, D, E, F, G and DP) + common pin. By connecting all the segment pins to a shift register, the required number of pins becomes just 3: clock pin and data pin (for the shift register) + common pin.
So for a 4-digit 7-segment display we need just 6 pins: clock, data and 4 common pins (each digit has its individual common pin).

This topic shows how to build a simple digital counter using Arduino, common anode 7-segment display with 4 digits and 74HC595 shift register.

To see how to interface Arduino with 7-segment display (without shift register) visit the following post:
Interfacing Arduino with 7-segment display | 4-Digit counter example

Arduino UNO with 7-segment display and 74HC595 shift register

Hardware Required:

  • Arduino UNO board
  • 4-Digit common anode 7-segment display
  • 74HC595 shift register   —->   datasheet
  • 4 x PNP transistor (2SA1015, 2S9015, 2N3906 …)
  • 8 x 100 ohm resistor
  • 4 x 4.7k ohm resistor
  • Push button
  • Breadboard
  • Jumper wires

Arduino 7-Segment display with 74HC595 shift register circuit:
The image below shows our example circuit schematic diagram.

Arduino 7 segment display 74HC595 shift register

As shown in the circuit diagram above, all segment pins are connected to the 74HC595 output pins, each one through 100 ohm resistor, where:
Segment A … G are connected to 74HC595 pin Q7 … Q1 respectively and segment DP is connected to pin Q0.
The data pin of the 74HC595 shift register is named DS (#14) and it is connected to Arduino pin 6.
ST_CP (or RCLK) and SH_CP (or SRCLK) are connected together which then connected to Arduino pin 7. This is the clock pin.

Since the display has 4 digits there’re 4 common pins: 1 (most left), 2, 3 and 4. Each common pin is connected to collector terminal of one transistor. Emitter terminals of the 4 transistors are connected to +5V which comes from the Arduino board. Base terminals of the four transistors are connected to Arduino through 4.7k resistors.

The 4 transistors are of the same type (PNP).

The push button which is connected to Arduino analog pin 0 (A0) is used to increment the displayed number.

7-Segment display with 74HC595 shift register code:
The Arduino code below doesn’t use any library for the 7-segment display.

The button connection is defined in the code as:

Shift register clock pin and data pin are defined as:

The display needs to be refreshed periodically, for that I used Timer1 module interrupt with the following configuration:

With Timer1 prescaler = 1, we’ve an interrupt every 4096 microseconds. That means every digit is displayed for 4096 us. { 4096 us = 65536 / (16 * prescaler) }
Note that Timer1 module is 16-bit timer and Arduino clock frequency is 16MHz.

Functions used in the code:
ISR(TIMER1_OVF_vect) : is Timer1 interrupt function, when the microcontroller interrupted by Timer1 it will directly execute this ‘function’.

void disp(byte number, bool dec_point = 0) : this function is for printing data on the 7-segment display, it prints the variable number which may vary between 0 and 9. The variable dec_point decides whether the DP will be printed or not, the default value is 0 (don’t print), if dec_point = 1 the DP segment will be ON.

void disp_off() : this function turns off the whole display.

I used Arduino shiftOut function (built-in) to send data serially to the 74HC595 shift register.

Full Arduino code:

The result of this example should be the same as the one shown in the following video (4-digit counter without shift register):

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