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Using 2 to 4 Decoder ICs in Digital Logic Circuits

Using 2 to 4 Decoder ICs in Digital Logic Circuits


Decoders are common digital logic components that convert binary inputs into specific outputs. A 2 to 4 decoder IC takes 2 binary inputs and activates 1 of 4 outputs based on the input code. These devices are useful for address decoding and demuxing signals. In this article, we will examine 2 to 4 decoder ICs in detail, including their operation, schematic diagrams, truth tables, and applications.

How 2 to 4 Decoders Work

A 2 to 4 decoder has 2 binary inputs labeled A and B and 4 outputs labeled Y0, Y1, Y2, and Y3. The function of the decoder is to activate one of the 4 outputs based on the 2-bit input code.

The truth table for a 2 to 4 decoder is:


As shown, when A and B are 00, output Y0 is activated. When A and B are 01, Y1 is activated, and so on. Only one output is ever active for a given input code. This allows the 2 binary inputs to uniquely select 1 of 4 possible outputs.

The logic diagram of a 2 to 4 decoder is:

The AND gates generate the proper outputs based on the inputs. For example, Y3 is active only when A AND B are both 1. This implements the truth table logic in circuitry.

Some common 2 to 4 decoder ICs are the 74HC139 and 74HC238. These contain the decoder logic in a single 14-pin or 16-pin chip. Power and ground pins supply the circuit, while input and output pins connect to external components.

4×4 Keypad Scanning with Decoder

A useful application of 2 to 4 decoders is scanning 4×4 keypad matrices. A 4×4 keypad has 4 rows and 4 columns for a total of 16 keys. We can connect 4 row wires and 4 column wires to detect presses.

To scan the keypad, the decoder activates one row at a time. When a row is activated, we check the 4 column inputs to see if a button is pressed. This allows us to detect which of the 16 keys is pressed using only 8 I/O pins.

The following diagram shows how a 74HC139 decoder can scan a 4×4 keypad:

The 2 decoder inputs A and B scan through the 4 rows. The column wires attach to I/O pins on a microcontroller. When a key is pressed, it will short one column wire to the active row. The microcontroller detects the column, and from the decoder row select inputs knows which key is pressed.

This scanning approach reduces the number of I/O pins required from 16 down to only 8. The decoder handles energizing each row in sequence to detect button presses.

7-Segment Display Decoding

Another useful application for 2 to 4 decoders is driving 7-segment LED displays. A 7-segment display has 8 LEDs arranged in a numeric digit pattern. We can light up the segments to display digits 0-9.

To control a single 7-segment display, we need 8 output pins – one for each LED segment. With multiple displays, we would need 8 pins per display.

We can save pins by time multiplexing the display and using a decoder. The decoder activates one display at a time, while the segment pins are shared across all displays.

The following circuit shows a 2 to 4 decoder used to drive 4 seven-segment displays:

The 74HC139 decoder enables one common anode display at a time. The microcontroller drives the shared cathode patterns to illuminate the desired segments. It scans across the displays quickly to give the appearance the digits are on continuously.

This reduces the pins needed from 32 down to only 14 using the decoder. The microcontroller selects the digit and segments to display in a multiplexed scanning fashion.


2 to 4 decoders are versatile digital logic chips useful for address decoding, keypad scanning, and display multiplexing. Some key points:

  • Decoders convert 2 inputs into 1 of 4 active outputs
  • Truth tables and logic diagrams define the input/output relationships
  • Keypad scanning reduces I/O pins by using the decoder to activate rows
  • 7-segment displays can be multiplexed using a 2 to 4 decoder
  • Common ICs include the 74HC139 and 74HC238 decoder chips

Decoders help reduce circuit complexity and I/O pin requirements in many practical applications. Their binary input selection allows clear implementation of address decoding and time multiplexing schemes. Understanding 2 to 4 decoders provides a solid foundation for more complex digital logic designs.

Frequently Asked Questions

What is the difference between a decoder and an encoder?

An encoder converts multiple inputs into a binary coded output, while a decoder converts a binary coded input into multiple outputs. Encoders are used to compress multiple signals into fewer outputs. Decoders expand binary inputs into a wider selection of outputs.

How do I determine the number of inputs and outputs needed for a decoder IC?

The number of decoder inputs and outputs depends on the number of selectable outputs needed. For example, a 2 to 4 decoder has 2 inputs and 4 outputs, allowing 1 of 4 outputs to be selected based on a 2-bit binary code. In general, if you need N selectable outputs, use log2(N) input bits.

Can unused outputs on a decoder be left disconnected?

Yes, any unused decoder outputs can be left disconnected or tied to ground or logic low. As long as the proper enable inputs are provided, the decoder will activate only the specified output for a given input code regardless of unused outputs. Proper operation is determined by the active output settings.

What is an enable input on a decoder IC?

Many decoders have an enable input that must be activated for the device to operate. This allows the decoder outputs to be tri-stated until the enable signal is applied. Enable inputs are useful for overriding the inputs and shutting off all outputs simultaneously.

How do I determine which decoder IC to use for a given application?

Factors to consider are number of channels, logic family (TTL, CMOS, etc), and package type. For simple decoders, common options are the 74HC139 (2 to 4 decoder) or 74HC238 (3 to 8 decoder). Check that the IC meets the voltage, decoding, and interface needs of your application.

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