The SN74HC595N shift register is a practical solution for expanding microcontroller outputs without using many GPIO pins. Converting serial data into eight parallel outputs, it helps control LEDs, displays, relays, and other digital devices efficiently. This article explains its features, pinout, operation, applications, and design tips.
SN74HC595N Shift Register Overview
The SN74HC595N is an 8-bit serial-in, parallel-out shift register that converts serial data into eight parallel output signals. It receives data one bit at a time through its serial input, stores it internally, and then sends it to the output pins.
Its integrated storage register allows the outputs to remain stable while new data is being shifted in. The outputs update only when a latch signal is applied, allowing all eight outputs to change simultaneously. This feature helps prevent unwanted output transitions and makes the device useful in LEDs, displays, relays, and other digital control circuits.
SN74HC595N Features, Specifications, and Pinout
SN74HC595N Features
| Feature | Description |
|---|---|
| 8-Bit Output Expansion | Provides eight parallel outputs from one serial data input. |
| Serial-In, Parallel-Out Operation | Receives data one bit at a time and outputs it through eight pins. |
| Integrated Storage Register | Keeps outputs stable while new data is being shifted in. |
| Simultaneous Output Update | Outputs change only when the latch clock is triggered. |
| Output Enable Control | Allows outputs to be enabled or placed in a high-impedance state. |
| Cascading Support | Supports the connection of multiple devices for larger output systems. |
| Breadboard-Friendly Package | Commonly available in a 16-pin DIP package for easy prototyping. |
SN74HC595N Technical Specifications
| Specification | Description |
|---|---|
| Supply Voltage Range | 2 V to 6 V |
| Logic Compatibility | Works with 3.3 V and 5 V microcontrollers |
| Number of Outputs | 8 parallel outputs |
| Serial Input | 1 serial data input (SER) |
| Maximum Clock Frequency | Typically supports clock rates above 20 MHz under suitable conditions |
| Output Enable | Active-low OE pin |
| Shift Register Clear | Active-low SRCLR pin |
| Cascading Support | QH' serial output allows multiple devices to be connected in series |
| Output Current Capability | Intended for logic-level loads |
| Recommended Output Current | About 6–8 mA per output in typical designs |
| Package Type | Commonly available in a 16-pin DIP package |
Note: Although each output can source or sink current, the SN74HC595N is not intended to directly drive high-current devices. Relays, motors, and high-power LEDs typically require external driver circuits.
SN74HC595N Pinout and Pin Functions
| Pin Number | Pin Name | Function |
|---|---|---|
| 1 | Q1 | Parallel Output 1 |
| 2 | Q2 | Parallel Output 2 |
| 3 | Q3 | Parallel Output 3 |
| 4 | Q4 | Parallel Output 4 |
| 5 | Q5 | Parallel Output 5 |
| 6 | Q6 | Parallel Output 6 |
| 7 | Q7 | Parallel Output 7 |
| 8 | GND | Ground (0 V reference) |
| 9 | QH' (Q7S) | Serial Data Output for cascading multiple SN74HC595N devices |
| 10 | SRCLR (MR) | Shift Register Clear (Active Low). Clears the shift register when driven low. |
| 11 | SRCLK (SHCP) | Shift Clock Input. Moves serial data into the shift register on each clock pulse. |
| 12 | RCLK (STCP) | Latch Clock Input. Transfers data from the shift register to the output register. |
| 13 | OE | Output Enable (Active Low). Enables or disables all outputs. |
| 14 | SER (DS) | Serial Data Input. Receives serial data from the microcontroller. |
| 15 | Q0 | Parallel Output 0 |
| 16 | VCC | Positive Supply Voltage (2 V to 6 V) |
How the SN74HC595N Expands Microcontroller Outputs
The SN74HC595N expands output capacity by converting serial data from a microcontroller into eight parallel outputs. It uses two internal registers: a shift register that receives data one bit at a time through the SER input, and a storage register that holds the data driving the output pins.
During operation, the microcontroller sends each bit to SER and pulses SRCLK to shift the data into the register. After all, eight bits are loaded, a pulse on RCLK transfers the data to the storage register. This updates all eight outputs all at once.
Because the storage register controls the outputs, they remain unchanged while new data is being shifted in. This helps prevent flickering and unwanted transitions. By using only three control lines, the SN74HC595N allows microcontrollers to drive more LEDs, displays, relays, and control signals without needing extra GPIO pins.
Interfacing the SN74HC595N with Arduino and Other Microcontrollers
The SN74HC595N connects to Arduino and other microcontrollers through a simple serial control interface. Instead of using eight GPIO pins to drive eight outputs, the microcontroller only needs three main control lines: data, shift clock, and latch clock. The data line sends each bit to the SER pin, the clock line shifts each bit into the register through SRCLK, and the latch line transfers the completed 8-bit data to the output pins through RCLK.
| Microcontroller Pin | SN74HC595N Pin | Function |
|---|---|---|
| Data Pin | SER | Sends serial data into the shift register |
| Clock Pin | SRCLK | Moves each data bit into the register |
| Latch Pin | RCLK | Updates all output pins at the same time |
| 5 V or 3.3 V | VCC | Supplies power to the IC |
| GND | GND | Common ground connection |
With Arduino, the SN74HC595N is commonly controlled using the built-in shiftOut() function. The Arduino sends one byte of data to the SER pin, pulses SRCLK for each bit, then pulses RCLK so all eight outputs change together. This makes it useful for LED chasers, LED arrays, seven-segment displays, relay control modules, and digital status indicators.
The SN74HC595N can also be used with ESP32, STM32, Raspberry Pi, and many other embedded platforms. In 3.3 V systems such as ESP32 or Raspberry Pi, the IC should be powered from 3.3 V for proper logic-level compatibility. STM32 and other advanced microcontrollers can also control multiple cascaded SN74HC595N devices when more than eight outputs are needed.
There are two common ways to send data to the SN74HC595N: bit-banging and hardware SPI.
| Method | How It Works | Best Use |
|---|---|---|
| Bit-Banging | Software manually controls the data, clock, and latch pins | Simple projects and flexible pin selection |
| Hardware SPI | Uses the microcontroller’s SPI peripheral to send data faster | High-speed updates, displays, and cascaded shift registers |
Bit-banging is easy to understand and works well for basic Arduino projects. Hardware SPI is better when the circuit needs faster output updates, lower CPU load, or multiple shift registers connected in series. In both methods, the latch signal is still used to update the outputs only after the full data byte has been loaded.
Applications of SN74HC595N
LED Expansion
The SN74HC595N can control large groups of LEDs using only a few microcontroller pins. This makes it useful for LED indicators, light patterns, status panels, and simple display effects.
Seven-Segment Displays
The device is often used to drive seven-segment displays because each segment requires a separate control line. With one or more SN74HC595N chips, multiple digits can be controlled while reducing GPIO usage.
LED Matrix Displays
LED matrix displays, scrolling signs, and information panels often use cascaded SN74HC595N devices. Cascading allows many rows or columns to be controlled from the same data, clock, and latch lines.
Relay Control
In automation circuits, the SN74HC595N is commonly paired with transistor drivers, MOSFETs, or relay driver ICs. This allows a microcontroller to control multiple relays safely without overloading its output pins.
Industrial Automation
The SN74HC595N can add extra digital outputs to control indicators, alarms, small control signals, and interface circuits. This helps expand controller capability without redesigning the main hardware.
Robotics
Robotic systems often need many output signals for LEDs, actuators, motor control inputs, sensors, and peripheral devices. The SN74HC595N provides a compact output expansion method, making it useful in small robots and embedded control systems.
Cascading Multiple SN74HC595N Devices for More Outputs
One of the most useful features of the SN74HC595N is its ability to expand the number of outputs beyond 8 channels by connecting multiple devices in series.
The QH' (Q7S) serial output of one SN74HC595N connects to the SER input of the next device. All devices share the same shift clock (SRCLK) and latch clock (RCLK) signals.
As serial data is shifted into the first device, excess bits automatically pass through QH' into the next device. After all bits have been loaded, a single latch pulse updates the outputs of every connected shift register simultaneously.
This approach allows a microcontroller to control dozens of outputs while still using only three control signals:
• SER (Data)
• SRCLK (Shift Clock)
• RCLK (Latch Clock)
Output Expansion Examples
| Number of SN74HC595N Devices | Available Outputs |
|---|---|
| 1 | 8 |
| 2 | 16 |
| 4 | 32 |
| 8 | 64 |
| 16 | 128 |
Cascading is widely used in LED matrices, multi-digit seven-segment displays, industrial control panels, robotics, and automation systems, where a large number of digital outputs are required.
SN74HC595N Performance, Design Tips, and Troubleshooting
The SN74HC595N performs reliably when operated within its specified limits.
Real-World Performance Considerations
• Output Drive Capability: The device is intended for logic-level loads and should not directly drive high-current devices.
• LED Brightness: Large LED arrays often require dedicated driver circuits to achieve higher brightness while staying within output current limits.
• Noise Immunity: A 0.1 µF decoupling capacitor should be placed close to each IC to reduce supply noise and improve stability.
• High-Speed Operation: At higher clock frequencies, PCB layout becomes more critical. Short traces, solid grounding, and proper signal routing improve reliability and reduce timing issues.
Common SN74HC595N Design Mistakes
| Design Mistake | Potential Problem | Recommended Solution |
|---|---|---|
| Exceeding output current limits | Overheating or device damage | Stay within datasheet ratings |
| Missing decoupling capacitor | Unstable operation and noise issues | Place a 0.1 µF capacitor near VCC |
| Incorrect OE wiring | Outputs remain disabled | Verify OE logic level |
| Floating SRCLR pin | Unexpected resets | Connect SRCLR to VCC if unused |
| Long clock traces | Data corruption and timing errors | Keep clock traces short and clean |
SN74HC595N Troubleshooting Guide
| Symptoms | Causes | Fixes |
|---|---|---|
| Outputs not changing | Missing clock pulses, missing latch signal, incorrect wiring, or power supply problems | Check the SRCLK and RCLK signals, verify all pin connections, and confirm that VCC and GND are properly connected |
| Random output states | Floating inputs, power-supply noise, or missing decoupling capacitors | Tie unused inputs to a defined logic level, add a decoupling capacitor near the IC, and check power stability |
| Cascaded devices are not working | Incorrect QH' connection, clock timing issues, wrong data sequence, or poor power integrity | Verify the QH' connection between devices, check clock and latch timing, confirm the bit order, and test one stage at a time |
SN74HC595N vs Other Shift Registers
| Device | Type | Inputs | Outputs | Interface | Best For | When to Choose |
|---|---|---|---|---|---|---|
| SN74HC595N | Serial-In, Parallel-Out Shift Register | No | 8 | Serial (Data, Clock, Latch) | Low-cost output expansion | When only additional outputs are needed |
| 74HC164 | Serial-In, Parallel-Out Shift Register | No | 8 | Serial (Data, Clock) | Simple output expansion | When a separate latch is not required |
| SN74HC165 | Parallel-In, Serial-Out Shift Register | 8 | No | Serial | Input expansion | When multiple switches or sensors must be read |
| TPIC6B595 | Power Shift Register | No | 8 | Serial | High-current outputs | For relays, solenoids, lamps, and other higher-current loads |
When Not to Use the SN74HC595N
High-Current Loads
The SN74HC595N provides logic-level outputs and cannot directly drive relays, motors, solenoids, or high-power LEDs. These loads require external transistors, MOSFETs, or a power shift register such as the TPIC6B595.
Systems Requiring Inputs and Outputs
The SN74HC595N expands outputs only. For configurable input and output pins, an I/O expander such as the MCP23017 is a better choice.
I²C-Based Designs
The SN74HC595N uses separate data, clock, and latch signals. In systems that already use an I²C bus, devices such as the MCP23017 or PCF8574 can reduce wiring by using only two communication lines.
Very Large Expansion Systems
Although multiple SN74HC595N devices can be cascaded, large chains increase shift time, wiring complexity, and signal-integrity concerns. Dedicated I/O expanders or distributed control methods may be easier to manage.
Frequently Asked Questions [FAQ]
Why does the SN74HC595N use both a shift register and a storage register instead of updating outputs directly?
The separate storage register allows new data to be loaded into the shift register without affecting the current output states. The outputs change only when the latch clock (RCLK) is triggered, ensuring that all outputs update simultaneously. This prevents flickering, glitches, and unwanted transitions in LEDs, displays, and control circuits.
How many outputs can be controlled by cascading multiple SN74HC595N devices?
The SN74HC595N can be daisy-chained through its QH' serial output, allowing virtually unlimited output expansion. Each additional IC adds eight more outputs while using the same data, clock, and latch lines. For example, two devices provide 16 outputs, four devices provide 32 outputs, and eight devices provide 64 outputs.
Can the SN74HC595N directly drive relays, motors, or high-power LEDs?
No. The SN74HC595N is designed primarily for logic-level loads and has limited output current capability. While it can drive standard indicator LEDs with proper current-limiting resistors, high-current devices such as relays, motors, and high-power LEDs typically require external transistors, MOSFETs, or dedicated driver circuits.
When should an I/O expander be chosen instead of the SN74HC595N?
An I/O expander such as the MCP23017 or PCF8574 is often a better choice when both inputs and outputs are needed or when minimizing microcontroller pin usage is important. The SN74HC595N excels at simple, low-cost output expansion, while I/O expanders offer configurable pins and communication through interfaces such as I²C.
What are the most common causes of unreliable SN74HC595N operation in real-world projects?
Most issues result from wiring mistakes, missing decoupling capacitors, floating control pins, excessive output current, or poor clock signal integrity. Reliable operation can usually be achieved by placing a 0.1 µF decoupling capacitor near the IC, keeping clock traces short, properly connecting OE and SRCLR, and ensuring the device operates within its specified electrical limits.
