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7400 Series Guide: 74HC42/74LS42 (BCD to Decimal Decoder)

The 74×42 (ex 74HC42) is a chip with a BCD to decimal decoder, also called a 1-of-10 decoder that takes a 4-bit input signal and turns 1 out of 10 outputs high.

In this guide, you’ll learn the things you need to know about this chip in order to use this type of decoder in your own projects.

74HC42 / 74LS42 Package And Pinout

What does the 74HC42 / 74LS42 do?

This 74×42 chip takes four Binary-Coded Decimal (BCD) inputs, ranging from S0 to S3. It outputs one of ten mutually exclusive outputs, Y0 to Y9. The design guarantees that if the input represents a decimal number greater than nine, all outputs will be HIGH.

While it’s designed for BCD to decimal (1-in-10) decoding, it can function as a 1-of-8 decoder by using the most significant input bit (S3) as an enable/disable input.

  • When S3 is LOW, the chip acts as a 1-in-8 decoder using inputs S0-S2.
  • When S3 is HIGH, all outputs are disabled, effectively turning it off.

However, a dedicated 1-in-8 decoder like the 74HC138 might be a more efficient choice in terms of pin count and simplicity.

How To Use This Chip

The 74HC42 comes in a 16-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 74HC42 supports 2V to 6V, while the 74LS42 only supports 5V.

The maximum current you can pull out of one output pin is 4 mA when the pin is high (sourcing) or 8/16 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 1-of-10 inverting decoder/demultiplexer inside.

7400 series IC connected to 5V on a breadboard
The 74HC42/74LS42 IC connected to power

74×42 Pinout

The 74×42 has 16 pins and contains BCD to decimal decoder as shown in the pinout diagram below:

74HC42 / 74LS42 Pinout
Pin NamePin #TypeDescription
Y01OutputOutput from the multiplexer (inverted).
Y12OutputOutput from the multiplexer (inverted).
Y23OutputOutput from the multiplexer (inverted).
Y34OutputOutput from the multiplexer (inverted).
Y45OutputOutput from the multiplexer (inverted).
Y56OutputOutput from the multiplexer (inverted).
Y67OutputOutput from the multiplexer (inverted).
GND8PowerConnect to ground (GND).
Y79OutputOutput from the multiplexer (inverted).
Y810OutputOutput from the multiplexer (inverted).
Y911OutputOutput from the multiplexer (inverted).
S312InputControl signal that controls the multiplexer.
S213InputControl signal that controls the multiplexer.
S114InputControl signal that controls the multiplexer.
S015InputControl signal that controls the multiplexer.
VCC16PowerPositive power supply (VCC). Connect to +5V power.
Pin overview for the 74×42 IC

Alternatives and Equivalents for 74HC42 / 74LS42

There are many versions of the 74×42 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:

  • 74HC42 (High-speed CMOS)
  • 74HCT42 (High-speed CMOS, TTL compatible)
  • 74LS42 (High-speed TTL)
  • 74LVC42 (Low Voltage TTL)
  • 74AC42 (Advanced CMOS)
  • 74ALS42 (Advanced Low-Power Schottky TTL)
  • 74F42 (Very High Speed)
  • 74C42 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC42 and SN74LS42 by Texas Instruments.

Can’t find the 74×42 anywhere? Then try one of the following IC alternatives:

  • 74×138 – 3-to-8 decoder/demultiplexer

If you can’t find the 74×42 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 74LS42 and 74HC42 chips

Download the PDF datasheet for your version of the 74×42 here:

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