The 74LS series of integrated circuits (ICs) was one of the most popular logic families of transistor-transistor logic (TTL) logic chips. 74LS series is a bipolar, low-power Schottky IC. The 74LS138, decode one of eight lines dependent on the conditions at the three binary select inputs and the three enable inputs. Two active-low and one active-high enable inputs to reduce the need for external gates or inverters when expanding

74LS138 Features
- Decodes One-of-Eight Lines Based Upon the Select Inputs
- Inputs Clamped with Schottky Diodes
- Designed for Memory Decoders and Data Transmission Systems
- Operating Temperature up to 70°C
- Standard TTL Switching Voltages
74LS138 Specifications
Supply Voltage | 4.75 – 5.25Vdc |
Maximum Clock Frequency | 40Mhz |
Power Dissipation | 2mW/gate @100kHz |
Minimum Output Current | 8mA |
Propagation Delay | 10nS |
Fan Out (TTL Loads) | 20 |
74LS138 Pinout Diagram

74LS138 Pin Description
Pin No | Pin Name | Description |
1 | A0 | Data Input Select Pin |
2 | A1 | Data Input Select Pin |
3 | A2 | Data Input Select Pin |
4 | E1 | Enable Pin |
5 | E2 | Enable Pin |
6 | E3 | Enable Pin |
7 | O7 | Output 7 |
8 | GND | Ground |
9 | O6 | Output 6 |
10 | O5 | Output 5 |
11 | O4 | Output 4 |
12 | O3 | Output 3 |
13 | O2 | Output 2 |
14 | O1 | Output 1 |
15 | O0 | Output 0 |
16 | VCC | Positive Supply |
74LS138 Circuit
Applications
- Decoders are used to input data to a specified output line as is done in addressing core memory where input data is to be stored in a specified memory location.
- It is used in code conversions.
- In high-performance memory systems, this decoder can be used to minimize the effects of system decoding
74LS138 Alternative Equivalent
74LS137
Download 74LS138 3 to 8 Decoder/Demultiplexer Datasheet from the link given below.