74HC595 Shift Register IC – Detailed Guide with Circuit Example

Introduction

The 74HC595 is one of the most popular serial-in, parallel-out (SIPO) shift register ICs used in electronics projects. It is widely used with microcontrollers like Arduino, ESP32, PIC, AVR (ATtiny) to expand output pins using only 3 control wires.

If you are working on projects such as:

• LED displays (7-segment, LED matrix)
• Relay control boards
• Digital output expansion
• LCD / indicator driving

…the 74HC595 is an excellent and cost-effective solution.

What is 74HC595?

The 74HC595 is an 8-bit shift register combined with an 8-bit storage (latch) register. Data is shifted serially into the IC and then latched to output pins simultaneously.

Key Features

• Converts serial data to parallel output
• Only 3 MCU pins required
• Output latch for stable output
• Cascadable (multiple ICs can be chained)
• Operates at 2V – 6V (HC version)
• High-speed CMOS logic

Pin Configuration of 74HC595

Pin No	Pin Name	Description
1–7	QA–QG	Parallel data outputs
8	GND	Ground
9	QH'	Serial data out (for cascading)
10	MR	Master Reset (Active LOW)
11	SH_CP	Shift Clock
12	ST_CP	Storage (Latch) Clock
13	OE	Output Enable (Active LOW)
14	DS	Serial Data Input
15	QH	Parallel output
16	VCC	+5V Supply

Internal Block Diagram (Conceptual)

The IC consists of:

1. Shift Register – receives serial data bit by bit
2. Storage Register (Latch) – holds data before output
3. Tri-state Output Buffers – drive external loads

This architecture ensures glitch-free output updates.

How 74HC595 Works (Step-by-Step)

1. Data Input (DS) receives 1-bit data
2. Shift Clock (SH_CP) shifts data into the shift register
3. After 8 clock pulses, data fills the shift register
4. Latch Clock (ST_CP) transfers data to output register
5. Outputs (QA–QH) update simultaneously

Truth Table (Simplified)

SH_CP	ST_CP	Action
↑	0	Shift data
0	↑	Latch data

Basic Circuit Example – Driving 8 LEDs Using 74HC595

Components Required

• 1 × 74HC595 IC
• 8 × LEDs
• 8 × 220Ω resistors
• Arduino / ATtiny / ESP32
• Breadboard & jumper wires

Circuit Connections

74HC595 Pin	Connected To
DS (14)	MCU Data Pin
SH_CP (11)	MCU Clock Pin
ST_CP (12)	MCU Latch Pin
OE (13)	GND
MR (10)	VCC
QA–QH	LEDs via resistors
VCC (16)	+5V
GND (8)	Ground

Example Arduino Code

int dataPin  = 8;   // DS
int latchPin = 9;   // ST_CP
int clockPin = 10;  // SH_CP

void setup() {
  pinMode(dataPin, OUTPUT);
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
}

void loop() {
  digitalWrite(latchPin, LOW);
  shiftOut(dataPin, clockPin, MSBFIRST, 0b10101010);
  digitalWrite(latchPin, HIGH);
  delay(1000);
}

Cascading Multiple 74HC595 ICs

One major advantage of the 74HC595 is expandability.

How Cascading Works

• Connect QH' (Pin 9) of IC1 to DS (Pin 14) of IC2
• Share clock and latch pins
• Each IC adds 8 more outputs
• Example
• 2 ICs → 16 outputs
• 3 ICs → 24 outputs

Driving High-Power Loads

The 74HC595 cannot drive relays or motors directly.

Safe Solutions

• Use ULN2803 transistor array
• Use NPN transistors / MOSFETs
• Add flyback diodes for inductive loads

Common Mistakes

• Forgetting pull-up on MR pin
• Leaving OE floating
• Driving high-current loads directly
• Not using latch pin correctly

Applications of 74HC595

• 7-segment display driver
• LED matrix control
• Relay expansion board
• Digital output port expansion
• SCADA / industrial indicators
• Arduino & ESP32 projects

74HC595 vs 74HCT595

Feature	HC	HCT
Logic Level	CMOS	TTL compatible
Speed	High	Moderate
MCU Friendly	Yes	Yes

Conclusion

The 74HC595 shift register IC is a powerful, flexible, and beginner-friendly component that solves one of the biggest limitations of microcontrollers: limited I/O pins. With proper understanding and circuit design, it can be used in everything from simple LED projects to complex industrial control systems.

If you are planning projects like temperature controllers, display systems, or IoT output expansion, mastering the 74HC595 is highly recommended.