7400 Series Guide: 74HC51/74LS51 (2/3-input AND-NOR gates)

The 74×51 (ex 74LS51) is a chip that comes with a combination of AND-NOR gates. It contains one 2-wide 2-input AND/NOR gate, and one 2-wide 3-input AND/NOR gate. They can be useful for checking several conditions, for example in an access control system.

Learn how it works and how to use it with this beginner-friendly guide and start using 2-wide AND/NOR gates in your projects.

What does the 74HC51 / 74LS51 do?

The 74×51 gives you two AND-NOR gate combinations. One with 2-input AND gates and another with 3-input AND gates.

An AND gate is a logic gate that outputs 1 (HIGH) if all of its inputs are 1 (HIGH). A NOR gate is a logic gate that outputs 1 (HIGH) if all of its inputs are 0 (LOW).

So to get a low output, all of the inputs to one of the AND gates must be high.

These types of gate combinations are called AND-OR-Invert. They are commonly used in digital circuit design such as counters, decoders, state machines, and much more.

How To Use This Chip

The 74HC51 comes in a 14-pin package, and you need to connect it to power before you can use it. Most 7400 ICs support a VCC voltage of 5V. One difference between the HC and LS version of the chip is that the 74HC51 supports 2V to 6V, while the 74LS51 only supports 5V.

The maximum current you can pull out of one output pin is 4 mA for 74HC51. For the 74LS51, you only have 0.4 mA when the pin is high (sourcing), but 16 mA when the pin is low (sinking). This can differ between models, so check the datasheet of your model to verify.

Once you’ve connected it to power, you can use any of the 2-wide basic and 2-wide 3-input AND-NOR gates inside.

Pinout

The 74×51 has 14 pins and contains 2-wide basic and 2-wide 3-input AND-NOR gates laid out as shown in the pinout diagram below:

Pin #	Type	Description
1	Input	Input to the first AND-NOR gate.
2	Input	Input to the second AND-NOR gate.
3	Input	Input to the second AND-NOR gate.
4	Input	Input to the second AND-NOR gate.
5	Input	Input to the second AND-NOR gate.
6	Output	Output from the second AND-NOR gate.
7	Power	Connect to ground (GND).
8	Output	Output from the first AND-NOR gate.
9	Input	Input to the first AND-NOR gate.
10	Input	Input to the first AND-NOR gate.
11	Input	Input to the first AND-NOR gate.
12	Input	Input to the first AND-NOR gate.
13	Input	Input to the first AND-NOR gate.
14	Power	Positive power supply (VCC). Connect to +5V power.

Alternative Pinout for the S version

If you have the 74S51, 74H51, 74S51 or version (not 74LS51 or 74HC51) – then there is a small difference in the pinout. All the AND gates are 2-inputs, and pins 11 and 12 are not in use:

Alternatives and Equivalents for 74HC51 / 74LS51

There are many versions of the 74×51 chip. They all have the same functionality, but with different specifications such as supported voltages and maximum current output.

Here’s a list of a few equivalents of this chip:

• 74HC51 (High-speed CMOS)
• 74HCT51 (High-speed CMOS, TTL compatible)
• 74LS51 (High-speed TTL)
• 74LVC51 (Low Voltage TTL)
• 74AC51 (Advanced CMOS)
• 74ALS51 (Advanced Low-Power Schottky TTL)
• 74F51 (Very High Speed)
• 74C51 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC51 and SN74LS51 by Texas Instruments.

Can’t find the 74×51 anywhere? Then try one of the following IC alternatives:

• 74×50 – Dual 2-input AND-OR-Invert gate
• 74×54 – 4-Wide 2/3-input AND-NOR gate
• 74×55 – 2-wide 4-input AND-NOR gate
• CD4085 – Dual 2-input AND-OR-Invert gate

If you can’t find the 74×51 IC in your local electronics store, don’t worry, you’ll most likely find it in one of the stores listed on this page of online stores where you’ll find components and tools for all your electronics projects.

Datasheet for the 74LS51 and 74HC51 chips

Download the PDF datasheet for your version of the 74×51 here:

• M74HC51 (STMicroelectronics)
• SN74LS51 (Texas Instruments)
• HD74LS51 (Renesas)
• DM74LS51 (onsemi)