The 74×138 (ex 74HC138) is a chip that contains a 3-to-8 line decoder/demultiplexer, which is useful for decoding binary coded inputs into a one-out-of-eight output signal.
In this guide, you’ll learn the things you need to know about this chip in order to use decoders/demultiplexers in your own projects.

What does the 74HC138 / 74LS138 do?
The 74×138 is a 3-to-8 line decoder/demultiplexer. It takes three binary inputs and decodes them into one of eight outputs. Each output line corresponds to a unique combination of the input signals.
Here’s how it works:
- It has three inputs (A, B, and C) and eight outputs (Y0 to Y7).
- Depending on the combination of inputs, only one of the eight outputs will be LOW, while the others stay HIGH.
- It also has three enable inputs (G1, G2A, and G2B) that must be set correctly to enable the device. The outputs are only active when G1 is HIGH and both G2A and G2B are LOW.
This chip is useful in applications where you need to select one of several outputs, like in memory addressing or data routing.
How To Use This Chip
The 74HC138 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 74HC138 supports 2V to 6V, while the 74LS138 only supports 5V.
74HC chips can normally supply a maximum of 4 mA from an output pin. If you’re using the 74LS version, the maximum current you can pull out of one output pin is 0.4 mA when the pin is high (sourcing) or 8 mA when the pin is low (sinking).
But these values can differ between models, so check the datasheet of your model to verify.

Build Something Useful This Evening
This gadget lets you use any IR remote-control to control your lamp, garden lights, heater oven, garage door, or anything else.
Once you’ve connected it to power, you can use the Decoder/Demultiplexer inside.

74×138 Pinout
The 74×138 has 16 pins and contains 1-of-8 inverting decoder/demultiplexer laid out as shown in the pinout diagram below:

Pin Name | Pin # | Type | Description |
---|---|---|---|
S0 | 1 | Input | Least significant bit of the select inputs for the decoder/demultiplexer. |
S1 | 2 | Input | Middle bit of the select inputs for the decoder/demultiplexer. |
S2 | 3 | Input | Most significant bit of the select inputs for the decoder/demultiplexer. |
EN3 | 4 | Input | Enable input for the decoder (active low). |
EN2 | 5 | Input | Enable input for the decoder (active low). |
EN1 | 6 | Input | Enable input for the decoder (active high). |
Y7 | 7 | Output | Output (active low) corresponding to select input 111. |
GND | 8 | Power | Connect to ground (GND). |
Y6 | 9 | Output | Output (active low) corresponding to select input 110. |
Y5 | 10 | Output | Output (active low) corresponding to select input 101. |
Y4 | 11 | Output | Output (active low) corresponding to select input 100. |
Y3 | 12 | Output | Output (active low) corresponding to select input 011. |
Y2 | 13 | Output | Output (active low) corresponding to select input 010. |
Y1 | 14 | Output | Output (active low) corresponding to select input 001. |
Y0 | 15 | Output | Output (active low) corresponding to select input 000. |
VCC | 16 | Power | Positive power supply. Connect to +5V power. |
Alternatives and Equivalents for 74HC138 / 74LS138
There are many versions of the 74×138 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:
- 74HC138 (High-speed CMOS)
- 74HCT138 (High-speed CMOS, TTL compatible)
- 74LS138 (High-speed TTL)
- 74LVC138 (Low Voltage TTL)
- 74AC138 (Advanced CMOS)
- 74ALS138 (Advanced Low-Power Schottky TTL)
- 74F138 (Very High Speed)
- 74C138 (CMOS, similar to the 4000-series)
Some manufacturers also add a prefix, such as the SN74HC138 and SN74LS138 by Texas Instruments.
Can’t find the 74×138 anywhere? Then try one of the following IC alternatives:
- 74×137 – 3-to-8 line decoder/demultiplexer.
- 74×139 – Dual 2-to-4 line decoder/demultiplexer.
- 74×154 – 4-to-16 line decoder/demultiplexer.
- CD4028 – 1-of-10 decoder.
- CD4514 – 4-to-16 line decoder/demultiplexer with latches.
- CD4515 – 4-to-16 line decoder/demultiplexer with latches.
If you can’t find the 74×138 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 74LS138 and 74HC138 chips
Download the PDF datasheet for your version of the 74×138 here:
- SN74HC138 (Texas Instruments)
- SN74LS138 (Texas Instruments)
- 74HC138 (Futurlec)
- 74LS138 (Futurlec)
- 74HC138 (Diodes Inc)
- 74HC138 (Nexperia)

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