1-to-4 Demultiplexer

Build a 1-to-4 demultiplexer from gates—route one input to any of four outputs using two selector switches!

Overview

The 1-to-4 DEMUX extends the 1-to-2 version with two selector lines that address four possible outputs. The selector lines form a 2-bit address (00, 01, 10, 11) that determines which output receives the data input. When the input is 0, all outputs are 0. When the input is 1, exactly one of the four outputs goes HIGH based on the selector. This is the complement of the 4-to-1 MUX and is fundamental to address decoding in memory systems.

Components Needed

  • 2x SN74LS08 IC (Quad AND)
  • 1x SN74LS04 IC (Hex Inverter)
  • 3x Switch
  • 4x LED
  • severalx Wires

Instructions

  1. Gather Your Components

    Collect two SN74LS08 (AND) ICs, one SN74LS04 (NOT) IC, three switches (one data input, two selectors), four LEDs, wires, breadboard, and 5V power supply.

  2. Identify the SN74LS08 AND Gate IC

    The SN74LS08 contains four 2-input AND gates. Two of these ICs give you eight AND gates—enough for the four 3-input AND functions needed (by cascading two 2-input gates).

  3. Identify the SN74LS04 Inverter IC

    The 74LS04 provides the NOT gates to create S̄0 and S̄1 from the selector switches.

  4. Wire the 1-to-4 DEMUX

    Place all three ICs on the breadboard and connect power. Create S̄0 and S̄1 using NOT gates. For each output, AND the data input with the appropriate selector combination: Y0 gets D·S̄1·S̄0, Y1 gets D·S̄1·S0, Y2 gets D·S1·S̄0, Y3 gets D·S1·S0. Connect each output to an LED.

  5. Test: Select 00, Input 0 → Output 0000

    Selector: 00, data input: 0. No data to route—all four LEDs are OFF. Output: 0000.

  6. Test: Select 01, Input 0 → Output 0000

    Selector: 01, data input: 0. Still no data—all outputs OFF. Output: 0000.

  7. Test: Select 10, Input 0 → Output 0000

    Selector: 10, data input: 0. Output: 0000.

  8. Test: Select 11, Input 0 → Output 0000

    Selector: 11, data input: 0. Output: 0000. All four selector states confirmed with input 0.

  9. Test: Select 00, Input 1 → Output 0001

    Now set input to 1! Selector: 00 routes data to Y0. Only the rightmost LED lights. Output: 0001.

  10. Test: Select 01, Input 1 → Output 0010

    Selector: 01 routes data to Y1. The second LED lights. Output: 0010.

  11. Test: Select 10, Input 1 → Output 0100

    Selector: 10 routes data to Y2. The third LED lights. Output: 0100.

  12. Test: Select 11, Input 1 → Output 1000

    Selector: 11 routes data to Y3. The leftmost LED lights. Output: 1000. All four outputs verified—the data appears at exactly one output based on the selector!

Challenges

  • Extend to a 1-to-8 DEMUX by adding a third selector line
  • Build a complete MUX-DEMUX communication link
  • Use the DEMUX as a binary decoder (tie input permanently HIGH)
  • Compare your gate-based design to a dedicated 74HC139 decoder IC