7400 Series Guide: 74HC33/74LS33 (NOR gates)

The 74×33 (ex 74HC33) is a chip with four open-collector NOR gates. This type of output makes it simple to hook it up to other chips that use different logic levels. But it also means you can’t use this chip as a direct replacement for a normal NOR gate chip.

But not to worry – in this guide, you will learn all about this chip and how you can use its NOR gates in your own circuit.

What does the 74HC33 / 74LS33 do?

The 74×33 gives you four open-collector NOR gates that can be used individually. A NOR gate is a logic gate that outputs 1 (HIGH) if all of its inputs are 0 (LOW). In the truth table below, you can see what the output will be for any given input:

How To Use This Chip

The 74HC33 comes in a 14-pin package, and you must connect its power before you can use it. Most ICs in the 7400 series support a VCC voltage of 5V. The main difference between the HC and LS version of the chip is that the 74HC33 supports 2V to 6V, while the 74LS33 only supports 5V.

Once it’s hooked up, you can use any of the four NOR gates inside.

This chip uses open-collector outputs which means you can only sink current, not source it (see How To Use Open-Collector Outputs below).

Each output of the 74HC33 can sink about 4 mA when powered with 5V. While the 74LS33 can usually sink about 8 mA of current. But these values can vary between manufacturers of the chip.

How To Use Open-Collector Outputs

The outputs from the 74HC33 are not your normal high/low outputs. Instead, they use something called an open-collector output. These are outputs that are connected via a transistor. And the collector of the transistor is what is available at the pin. For a NOR gate, it will look like this:

So when the output from the NOR gate is ‘1’, it turns on the transistor that is connected to the output. When the output from the NOR gate is ‘0’, it means the transistor will be off.

To turn on an LED when the NOR gate output is ‘1’, you must connect the LED from your positive supply, via a resistor, to the open-collector output:

If you instead want to get your standard high/low output, you can get this by connecting a resistor from the output up to your positive supply. Then your NOR gate output pin becomes a standard high/low output.

The advantage of using open-collector outputs is that you can choose your HIGH voltage level to make it compatible with whatever the voltage is of the next stage. The resistor acts as a pull-up resistor and makes the HIGH level approximately the same as the VCC voltage that you provide.

It’s important to notice that the output will be inverted: A logical HIGH from the NOR gate turns the transistor ON so that the output voltage on the pin becomes low. A logical LOW leaves the transistor OFF so that the output voltage on the pin becomes HIGH.

74×33 Pinout

The 74×33 has 14 pins and contains four open-collector NOR gates laid out as shown in the pinout diagram below. Each gate has the open-collector symbol, to remind you of the type of output:

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

74HC33 Circuit Example

Here’s a simple project you can build with the NOR gates in the 74HC33 IC.

It’s a circuit that shows how to build a 5V to 3.3V level shifter from one of the NOR gates in the 74HC33 IC. This could be used, for example, to connect an Arduino (which has 5V outputs) to an ESP32 microcontroller that only supports 3.3V inputs:

To build this circuit, you’ll need the following parts:

• 1 x 74HC33 IC
• 1 x 10 kΩ resistor (R3)

Alternatives and Equivalents for 74HC33 / 74LS33

There are many versions of the 74×33 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:

• 74HC33 (High-speed CMOS)
• 74HCT33 (High-speed CMOS, TTL compatible)
• 74LS33 (High-speed TTL)
• 74LVC33 (Low Voltage TTL)
• 74AC33 (Advanced CMOS)
• 74ALS33 (Advanced Low-Power Schottky TTL)
• 74F33 (Very High Speed)
• 74C33 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC33 and SN74LS33 by Texas Instruments.

Can’t find the 74×33 anywhere? Then try one of the following IC alternatives:

• 74×02 – Quad 2-input NOR gates
• 74×7002 – Quad 2-input NOR gates (with Schmitt-trigger inputs).
• CD4001 – Quad 2-input NOR gates.

No worries if you can’t seem to locate the 74×33 IC in your nearby electronic store. There’s a good chance that you’ll find it on one of the online stores listed on this page. These stores have got you covered with all the components and tools you’ll need for your electronics projects.

Datasheet for the 74LS33 and 74HC33 chips

Download the PDF datasheet for your version of the 74×33 here:

• 74LS33 (Texas Instruments)