7400 Series Guide: 74HC21/74LS21 (Dual 4-input AND gates)

The 74×21 (ex 74HC21) is a cool little chip that has two 4-input AND gates.

In this guide, you will learn everything you need to know about this chip and how you can use 4-input AND gates in your own projects.

What does the 74HC21 / 74LS21 do?

The 74×21 gives you two 4-input AND gates that you can use individually. An AND gate is a logic gate that outputs 1 (HIGH) only if all of its inputs are 1 (HIGH). In the truth table below, you can see what the output will be for any given input:

How To Use This Chip

The 74HC21 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 74HC21 supports 2V to 6V, while the 74LS21 only supports 5V.

The maximum current you can pull out of one output pin of the 74HC21 is 4 mA. For the 74LS21 it’s 0.4 mA when the pin is high (sourcing) or 8 mA when the pin is low (sinking).

Once you’ve connected it to power, you can use any of the two 4-input AND gates inside.

74×21 Pinout

The 74×21 has 14 pins and contains two 4-input AND gates laid out as shown in the pinout diagram below:

Pin #	Type	Description
1	Input	Input to the first AND gate.
2	Input	Input to the first AND gate.
3	NC	Not in use (Not Connected).
4	Input	Input to the first AND gate.
5	Input	Input to the first AND gate.
6	Output	Output from the first AND gate.
7	Power	Connect to ground (GND).
8	Output	Output from the second AND gate.
9	Input	Input to the second AND gate.
10	Input	Input to the second AND gate.
11	NC	Not in use (Not Connected).
12	Input	Input to the second AND gate.
13	Input	Input to the second AND gate.
14	Power	Positive power supply (VCC). Connect to +5V power.

Alternatives and Equivalents for 74HC21 / 74LS21

There are many versions of the 74×21 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:

• 74HC21 (High-speed CMOS)
• 74HCT21 (High-speed CMOS, TTL compatible)
• 74LS21 (High-speed TTL)
• 74LVC21 (Low Voltage TTL)
• 74AC21 (Advanced CMOS)
• 74ALS21 (Advanced Low-Power Schottky TTL)
• 74F21 (Very High Speed)
• 74C21 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC21 and SN74LS21 by Texas Instruments.

Can’t find the 74×21 anywhere? Then try one of the following IC alternatives:

• CD4082 – Dual 4-input AND gates

If you can’t find the 74×21 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 74LS21 and 74HC21 chips

Download the PDF datasheet for your version of the 74×21 here:

• SN74HC21 (Texas Instruments)
• SN74LS21 (Texas Instruments)