7433 2-Input Positive NOR Buffers with Open collector Outputs

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 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:

Specifications

7433 Datasheet