74HC595 Shift Register: Pins, Limits and Part Selection
The 74HC595 is an 8-bit serial-in, parallel-out shift register with a separate output storage register. It lets a controller update several logic outputs using a small number of signal wires. It is not automatically a high-current LED or relay driver. Check the part family, package, control pins and load before adding it to a project.
Understand the two-register arrangement
Serial data is shifted into an internal register by the shift clock. A separate storage-clock edge transfers that data to the output register, allowing the visible outputs to change together after the byte is loaded. Keeping those clocks distinct avoids showing each intermediate shifting state at the outputs. An output-enable input can place the outputs in a high-impedance state.
Identify the package that fits the build
A DIP package can straddle a breadboard’s center gap; an SOIC part needs a compatible PCB or adapter. Read the complete part marking and the manufacturer package drawing. The pin table below describes the common 16-pin package, not every possible package in the broader family. Do not assume a similar-looking IC is a 595.
Know the essential control pins
| Signal | Common 16-pin number | Role |
|---|---|---|
| VCC / GND | 16 / 8 | Logic supply and return |
| SER | 14 | Serial data input |
| SRCLK | 11 | Shift-register clock |
| RCLK | 12 | Storage-register or latch clock |
| OE | 13 | Active-low output enable |
| SRCLR | 10 | Active-low shift-register clear |
| QH′ | 9 | Serial cascade output |
| QA through QH | 15, 1–7 | Eight parallel outputs |
Keep unused control inputs at valid defined levels. In a simple demonstration, OE can be held low to enable outputs and SRCLR high to avoid clearing the shift register. Check the datasheet before assuming that clearing the shift register immediately sets every visible output; the storage register has its own clock.
Match input thresholds to the supply
The HC family uses CMOS input thresholds that depend on supply voltage. A 3.3 V controller output is not automatically guaranteed to meet the high-input requirement of a 74HC595 powered at 5 V. Use a supported common-voltage arrangement or a suitable interface solution. HCT and other logic families have different characteristics, so confirm the exact device rather than treating suffixes as cosmetic.
Budget output current and total package current
A shift register output can only drive a limited load while meeting its electrical specifications. Do not use an absolute-maximum current as a normal operating target. Give every LED its own resistor, keep currents conservative and check the total package budget. For relay coils, motors, long LED strips or higher-current displays, use appropriate driver stages rather than connecting the load directly.
Plan a predictable startup state
Do not assume the output register powers up with all bits cleared. Systems that must remain inactive during boot should use a deliberate hardware enable strategy and suitable defaults. A simple LED experiment can tolerate a brief undefined state, but a motor-enable or other consequential output cannot be designed on that assumption. Keep output enable controlled until the intended data is loaded.Newest in the store
Include decoupling and a first-test plan
Place a local 100 nF bypass capacitor near the supply pins and use short clock and latch connections for an initial breadboard test. Start with a small resistor-limited LED load, verify bit order, then add outputs gradually. If more than one device is cascaded, check serial order and clock timing. Save the actual schematic and part suffix with the project so a later replacement is not assumed to be identical.
Epro parts and availability
These are relevant Epro listings, not independent authenticity or performance certifications. Prices below were checked on 19 October 2026; they are a dated snapshot rather than a historical price implied by this article’s publication date. Confirm the exact variant, included accessories, stock and current checkout price before ordering.
| Listing | Checked price | Stock at check |
|---|---|---|
| Shift Register 8 Bit Dip SN74HC595 IC in Pakistan | PKR 68 | In stock |
| Smd 74hc595 74hc595d Sop16 Patch 8 Bit Serial Shift Register | PKR 40 | In stock |
Frequently asked questions
Can 74HC595 drive eight relay coils directly?
No. Use appropriate output drivers and inductive-load protection, with a suitable power supply.
Is QH′ the same as output QH?
No. QH′ is the serial cascading output, while QH is one of the latched parallel outputs.
Related Epro guides
Technical references and verification
The guidance above is based on the manufacturer documentation linked here and the listed product information. Example procedures and sketches are provided for a controlled low-voltage bench setup; no independent hands-on test of your particular board or adapter is claimed. Check the markings and documentation for your actual revision.