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Understanding Decoder 3 to 8 IC

Understanding Decoder 3 to 8 IC

Introduction to Decoder ICs

Decoder ICs (integrated circuits) are logic chips that convert binary information from 3 inputs to up 8 outputs. They are used in a variety of digital logic applications to reduce the number of individual logic gates required. Some key benefits of using decoder ICs include:

  • Compact size – Decoders pack multiple logic gates into a single chip, saving space.
  • Simplicity – Decoders simplify circuit design compared to using discrete logic gates for decoding.
  • Flexibility – There are many decoder options with different number of inputs/outputs.
  • Cost-effective – A decoder IC is cheaper than using multiple individual logic gates.

How 3 to 8 Decoder ICs Work

A 3 to 8 decoder has 3 inputs and 8 outputs. The 3 input wires, labeled A, B, and C, represent the binary input code. These inputs are decoded to activate one of the 8 outputs labeled Y0 to Y7.

The truth table below shows the operation of a 3 to 8 decoder:


Only one output is active (high) at a time. The 3-bit binary input code determines which output is activated.

This decoding allows the 3 input bits to uniquely select one of 8 outputs. The decoder acts like a 1-of-8 demultiplexer.

Logic diagram of a 3 to 8 decoder IC (Image credit: SparkFun)

Applications of 3 to 8 Decoder ICs

Some common applications of 3 to 8 decoder ICs include:

  • Memory addressing – Decoders help decode address lines to select 1 of 8 memory locations.
  • 7-segment displays – Decoders convert BCD digits to drive individual segments of a 7-seg display.
  • Input multiplexing – Decoders like the 74HC138 can multiplex up to 8 inputs into one output.
  • Output expansion – Decoders expand 3 control lines to drive up to 8 external devices.

Decoders provide an efficient way to minimize logic gates for many digital circuit designs.

Recommended 3 to 8 Decoder ICs

Here are some popular 3 to 8 decoder ICs to consider:

  • 74HC138 – Fast CMOS decoder, active low outputs
  • 74LS138 – Low power TTL decoder, active low outputs
  • CD4514 – CMOS decoder, active high outputs
  • MC74HC238 – Faster CMOS decoder, active low
  • MM74HC238 – Smaller footprint, active low

The 74HC138 or 74LS138 are very common, inexpensive decoder ICs for most applications. Pay attention to voltage levels and output polarity when selecting a suitable decoder.


  • Decoder ICs convert 3 binary inputs into one of 8 outputs, minimizing logic gates.
  • They are useful for memory addressing, multiplexing, driving displays, and other applications.
  • Many logic family options exist like the common 74HC138 or 74LS138 CMOS and TTL decoders.

Decoder integrated circuits will continue to play a key role in efficient digital logic circuit design. Their compact size and usefulness ensures decoders remain popular for a wide range of uses.

Frequently Asked Questions

Q: How many inputs and outputs does a 3 to 8 decoder have?

A: A 3 to 8 decoder has 3 binary inputs and 8 outputs. It converts a 3-bit input code into one of 8 output lines being activated.

Q: What are some typical applications of 3 to 8 decoder ICs?

A: Typical applications include memory addressing, driving 7-segment displays, input multiplexing, expanding control lines, and other logic functions needing 1 of 8 selection.

Q: What is the main difference between the 74HC138 and 74LS138?

A: The 74HC138 is a CMOS decoder that uses less power but has higher speed. The 74LS138 is a TTL decoder that works well for many general applications.

Q: Do decoders require any external components?

A: Minimal external components are needed. Usually just power supply decoupling capacitors and pull-up resistors for open collector outputs.

Q: How do I select the proper decoder IC for my application?

A: Consider specifications like speed, power, logic family (TTL, CMOS), output drive capability, and package style. Make sure voltage levels and polarity match your specific circuit design.