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How to crack ic 74ls139

How to crack ic 74ls139

The 74LS139 is a common IC chip used in electronics and digital logic circuits. This guide will show you how to “crack” or bypass the chip’s internal logic to override or control its operation.

Understanding the 74LS139 Chip

The 74LS139 is a 16-pin dual 2-to-4 line decoder/demultiplexer IC. It takes 2 binary inputs and activates one of the 4 outputs based on the input code.

Some key features:

  • Compatible with TTL and CMOS logic families
  • Active low outputs (high impedance when not selected)
  • Output can sink up to 20mA
  • Propagation delay <13ns

74LS139 Pinout

Here is the pinout diagram for reference:

1G2AInput A2
2G2BInput A1
3G1AInput A0
4G1BInput B2
5C0Output 0
6C1Output 1
7C2Output 2
8C3Output 3
10Y3Output 3
11Y2Output 2
12Y1Output 1
13Y0Output 0
14A = BEqual Inputs (Low)
15CD+Disable (High)
16VCCSupply Voltage

Cracking the 74LS139 Chip

There are a few different methods to override or control the operation of the 74LS139 chip:

Method 1: Ground/VCC Override

You can force an output pin high or low by connecting it directly to ground or VCC. This will override the internal logic.

For example, to force output Y1 high, connect pin 12 to VCC. Or to force Y3 low, connect pin 10 to ground.

Method 2: Disable Pin Override

The CD+ “disable” input (pin 15) forces all outputs high when pulled high. This ignores the select inputs.

Connect pin 15 to VCC using a switch or jumper wire. Now all outputs are high regardless of the inputs.

Method 3: Equal Inputs Override

The A=B input (pin 14) activates ALL outputs when pulled low. This also overrides the internal logic.

Connect pin 14 to ground to enable all outputs simultaneously.

Example Circuits

Here are some example circuits using these override methods:

Forcing Single Output High

Enabling All Outputs

Toggle Disable Input


  • Testing/prototyping circuits using the 74LS139
  • Overriding system logic by hacking decoder chips
  • Building custom logic controllers
  • Learning how digital logic ICs can be modified

These principles can also be applied to related decoder and demultiplexer chips like the 74LS138.

Frequently Asked Questions

Is this illegal?

Modifying or “cracking” chips may violate their terms of use. Please check your local laws before attempting these methods. This article is for educational purposes only.

Can I damage the chip doing this?

Yes, directly connecting pins to VCC or ground may damage the IC if current limits are exceeded. Use care when wiring override circuits.

Will this work on all 74LS139 chips?

These methods should work on most 74LS139 ICs. However, some manufacturers may use different designs. Always check the datasheet for your specific chip.

What tools do I need?

Basic electronics equipment – like a breadboard, jumper wires, power supply, etc. An oscilloscope or logic analyzer can also be helpful for testing.

Are there other methods?

Yes, you can also use methods like grounding the enable pins, or modifying the select inputs. The circuits above are just some basic examples.

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