What is an IC Decoder?
An integrated circuit (IC) decoder is a device that converts binary information from the inputs into a set of outputs. Decoders take in multiple binary inputs and activate one of many output lines. The 2 to 4 decoder is one of the most common types of decoder ICs.
A 2 to 4 decoder has 2 input pins and 4 output pins. The 2 input pins provide 4 possible input combinations (00, 01, 10, 11). Based on the input value, 1 of the 4 outputs is activated. This allows the 2 input pins to uniquely select 1 of the 4 outputs.
Truth Table for 2 to 4 Decoder
| Inputs | Outputs |
|---|---|
| A B | D0 D1 D2 D3 |
| 0 0 | 1 0 0 0 |
| 0 1 | 0 1 0 0 |
| 1 0 | 0 0 1 0 |
| 1 1 | 0 0 0 1 |
Why Use an IC Decoder?
Decoders provide several advantages:
- Save components compared to discrete logic gates
- Faster operation with single IC package
- Reduce complexity by handling logic functions
- Allow microcontrollers to control more outputs
- Available in convenient DIP packages
Common applications include:
- Controlling seven-segment displays
- Selecting 1 of n memory chips or devices
- Multiplexing signals and expanding I/O ports
IC Decoder Packages and Pinouts
Some common 2 to 4 decoder ICs:
- 74HC139 – 16-pin dual 2 to 4 line decoder
- 74HC238 – 3 to 8 line decoder
- CD4514 – CMOS BCD to decimal decoder
The 74HC139 contains two separate 2 to 4 decoders with these pinouts:
The 74HC238 3 to 8 decoder has 8 outputs controlled by 3 select inputs A,B,C.
How to Use a 2 to 4 Decoder
To use a 2 to 4 decoder IC:
- Connect power supply to VCC and ground
- Connect input pins to digital outputs from microcontroller, shift register, etc
- Connect desired output pins to other circuits like LEDs, relays, etc
- Activate decoder by enabling output with output enable pin
- Set input states to activate corresponding output
Minimal connections require connecting power, inputs, 1 output, and output enable. Unused pins can be left disconnected. Consult datasheets for full specifications.
Decoder Circuits
Here is a simple circuit using a 74HC139 to decode 2 binary inputs and enable 1 of 4 LEDs:
The 74HC139 contains two separate 2 to 4 decoder circuits. This uses one decoder with inputs A1 and A2, and activates one of the outputs Y0 to Y3.
The output enable pins OE1 and OE2 are active low, so OE1 is grounded to enable the outputs. When A1 and A2 are 00, output Y0 LED will turn on. Setting A1 and A2 to 11 activates output Y3.
Conclusion
- 2 to 4 decoders convert 2 inputs to 4 outputs using simple logic
- Widely used IC decoder packages provide convenient decoding
- Enable microcontrollers or logic circuits to control more devices
- Available in through-hole DIP and surface mount packages
- Useful for driving seven segment displays, memory addressing, and more
IC decoders provide an easy way to implement binary decoding functions. The simple 2 to 4 decoders serve as a basic building block for more complex logic.
Frequently Asked Questions
What are the advantages of using a decoder IC vs discrete gates?
Decoder ICs simplify circuit design by implementing internal logic functions. This saves components, cost, board space and design time compared to building decoders from multiple logic gates. Decoder ICs also have faster performance than discrete solutions.
How do I select the correct decoder IC for my application?
Consider the required number of inputs and outputs. Many decoders include enable pins for simplified control. Review datasheets for voltage levels, output drive capacity, and other specifications.
Can the unused output pins on a decoder be left disconnected?
Yes, a decoder IC will operate correctly with the unused output pins disconnected or floating. Just connect the active inputs, outputs, and power/ground as needed.
How do I activate multiple outputs from a decoder IC?
Some decoders have separate output enable pins allowing groups of outputs to be enabled together. Otherwise, the outputs of multiple decoders can be connected in parallel to activate multiple outputs as needed.
What logic family should I choose for a decoder IC?
CMOS decoders like the CD4000 series are very common and work well for low power and general purpose applications. HC CMOS provides fast switching along with CMOS’s noise immunity and low power.
