74HC138 3-to-8 Decoder

Name: 74HC138 3-to-8 Decoder
Brand: Texas Instruments
Availability: InStock

SN74HC138

Texas Instrumentsic logic

The 74HC138 is a high-speed CMOS 3-to-8 line decoder/demultiplexer with active-low outputs. Three select inputs and three enable inputs choose one of eight outputs, easing memory decoding and cascading.

Manufacturer datasheet

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In stock

Specifications

Logic Function: 3-to-8 line decoder/demultiplexer
Operating Voltage: 2 to 6 V
Operating Temperature Range: -40 to 85 C
Supply Current Max: 80 uA
Output Drive Current: 4 mA
Input Current Max: 1 uA
Propagation Delay Typical: 15 ns
Input Capacitance Max: 10 pF
Output Current Continuous: 25 mA
Package Types: SOIC-16, SSOP-16, PDIP-16, SO-16, TSSOP-16, CDIP-16, CFP-16, LCCC-20

Pinout

Pin	Name	Functions	Notes
1	A	GPIO	Select input A (LSB)
2	B	GPIO	Select input B
3	C	GPIO	Select input C (MSB)
4	G2A	GPIO	Active-low enable A
5	G2B	GPIO	Active-low enable B
6	G1	GPIO	Active-high enable
7	Y7	GPIO	Output 7 (MSB)
8	GND	GND	Ground
9	Y6	GPIO	Output 6
10	Y5	GPIO	Output 5
11	Y4	GPIO	Output 4
12	Y3	GPIO	Output 3
13	Y2	GPIO	Output 2
14	Y1	GPIO	Output 1
15	Y0	GPIO	Output 0 (LSB)
16	VCC	POWER	Supply voltage

Interactive pinout

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SN74HC138

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Starter code

Community starter examples — minimal, unofficial, and provided to get you wiring fast. Verify against the manufacturer datasheet before relying on them.

Arduino (C++)

arduino

// 74HC138 3-to-8 line decoder — community starter example
// A/B/C select which ONE of Y0..Y7 goes LOW (active-low outputs).
#if defined(ESP32)
  #define A_PIN 25
  #define B_PIN 26
  #define C_PIN 27
#else
  #define A_PIN 2
  #define B_PIN 3
  #define C_PIN 4
#endif

void selectOutput(uint8_t n) {       // 0..7 -> Yn goes LOW
  digitalWrite(A_PIN, n & 1);
  digitalWrite(B_PIN, (n >> 1) & 1);
  digitalWrite(C_PIN, (n >> 2) & 1);
}

void setup() {
  pinMode(A_PIN, OUTPUT); pinMode(B_PIN, OUTPUT); pinMode(C_PIN, OUTPUT);
}

void loop() {
  for (uint8_t n = 0; n < 8; n++) { selectOutput(n); delay(200); }
}

MicroPython

python

# 74HC138 3-to-8 decoder (ESP32) — community starter example
from machine import Pin
import time

a = Pin(25, Pin.OUT); b = Pin(26, Pin.OUT); c = Pin(27, Pin.OUT)

def select(n):                       # Yn goes LOW
    a.value(n & 1); b.value((n >> 1) & 1); c.value((n >> 2) & 1)

while True:
    for n in range(8):
        select(n); time.sleep_ms(200)

Wiring

Pin	Arduino Uno R3	ESP32 DevKitC (WROOM-32)	Notes
VCC	+5V	3V3	2-6 V
GND	GND	GND
A	D2	GPIO25	Select bit 0
B	D3	GPIO26	Select bit 1
C	D4	GPIO27	Select bit 2
G1	+5V	3V3	Enable: tie HIGH
G2A/G2B	GND	GND	Enable: tie LOW
Y0..Y7	-	-	Selected output goes LOW (active-low)

Typical uses

Selecting 1 of 8 lines (chip-select, mux address)
Driving common-anode displays / banks