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How to crack 74hc decoder

How to crack 74hc decoder

Introduction to 74hc decoder

A 74hc decoder is a digital logic chip that takes a binary input and converts it to a decimal output. Decoders are used in a wide range of electronics applications to convert between binary codes and control signals. The 74hc series of CMOS logic chips are a popular family of decoders, notable for their high speed and low power consumption.

In this article, we’ll walk through the basics of how 74hc decoders work, how to wire them up in circuits, and how to “crack” or decode their outputs.

How 74hc Decoders Work

The basic function of any decoder is to take an N-bit binary number and convert it to 1-out-of-2^N unique output lines. For example, a 2-to-4 line decoder like the 74hc139 takes 2 input bits and activates 1 of 4 outputs.

Decoder logic symbol

The 74hc139 has two select input pins (A and B) and four output pins (Y0 through Y3). The select input encodes a 2-bit binary number, while each output corresponds to a unique decimal number between 0 and 3.

ABActive Output

So for example, when A=1 and B=0, output Y2 will be activated while the other outputs remain off. This converts the binary input “10” into the decimal output “2”.

Wiring and Using 74hc Decoders

To use a 74hc decoder chip like the 139 in a circuit, you’ll need to provide power, connect the select inputs, and wire up the desired output pins. Here are the basic steps:

  1. Connect pin 16 (VCC) to 5V and pin 8 (GND) to ground.
  2. Connect the binary selector inputs A and B to switches, logic outputs, etc.
  3. Connect the desired output pins (Y0-Y3) to LEDs, transistors, or other devices you want to activate.
  4. Enable the chip by connecting pin 1 (E1) to logic high.

Now when you toggle the A and B inputs, you can decode them and activate the corresponding output! You can also daisy chain decoders to handle larger numbers of outputs.

Cracking the 74hc Decoder

To “crack” a 74hc decoder, we just need to work backwards from the outputs to determine the select input states. Here are some tips:

  • Monitor the chip’s outputs – for example with LEDs. Note which output is active.
  • Check the decoder truth table to see which input (A/B) corresponds to the active output.
  • Use a logic probe or multimeter to measure the levels on the selector inputs and determine the binary input number.
  • For larger decoders, continue decoding each set of selector inputs stage-by-stage.

With some basic tools and logic analysis, you can crack the input-output relationships of 74hc decoders.


  • 74hc decoders convert binary inputs to unique 1-out-of-N outputs.
  • To use them, provide power, connect inputs, and wire outputs to desired devices.
  • You can decode the inputs by working backwards from the active output pin(s).

Decoders are extremely useful for binary-to-decimal conversion and activating multiple circuits. Understanding how to utilize and crack 74hc decoder ICs is key knowledge for working with digital logic systems.

Frequently Asked Questions

What tools do I need to crack a 74hc decoder?

You’ll need basic electronics equipment like a multimeter, logic probe, oscilloscope, or LEDs to monitor the decoder’s outputs. You’ll also need a basic understanding of digital logic and Boolean algebra to analyze the input-output relationships.

How do I tell which output is active?

Attach LEDs to the output pins – the one lit up is the active output. You can also monitor them with a logic probe or oscilloscope.

What if multiple outputs are active?

That indicates the decoder chip is being used incorrectly. There should only be one active output pin at a time. If you see multiple outputs, check your wiring and logic levels.

Can I use a 74hc decoder for analog signals?

No, 74hc decoders are designed for digital logic signals only. For analog decoders, look at chips like the CD4051.

Are all 74hc decoder pins compatible with 5V CMOS logic?

Most are, but check your decoder’s datasheet to be sure. The power and ground pins should always be 5V tolerant. Some inputs and outputs may require level shifting.