This is the best part of studying engineering, we improvise. Have you experienced at one time where you want to make your own decoder? Since the available ‘decoders’ were already fixed on what they do? If yes, don’t worry I’m here to help you design a decoder suitable for what you need.

Decoder, it is a device which decodes/outputs binary signal depending on the binary input signal which enters it, and the output of our decoder is a 7-segment display. Let’s say for example, I have an input of 1001 (‘1’ means input is high or on and ‘0’ means low or off) and I want the 7-segment to display a character of letter ‘H’ or letter ‘J’ so it’s up to us what output we want. So let’s start.

       First plan: know the maximum outputs you will have. In binary, maximum output can be determined by this simple formula:

                         Maximum output = 2^n ; where n is equal to your input

So, with this if we have a 4-input binary, we could make a 16 different output combinations in our decoder. If 2-input we could have 4 and so on. In this tutorial I’ll just use a 2-input binary to our decoder.

       Second plan: what character you want to display in our 7-segment display. As what the displays name is, it has a 7-segment input where we will put the output of our decoder namely, a b c d e f g. This letters is arrange from top to the center, in clockwise rotation. The 7-segment display looks like this:

Since we could only have 4 maximum output to display, I will just display my name, P – A – U – L.

       Third plan: make a table. Looking at the pin configuration of the 7-segment display, at input = 0 0, I want to display a letter P, so I want the letters a b e f g to be set on high, and the rest is low. Meaning I want the supply voltage to turn on these letters (a b e f g) and the rest is off in order to produce a letter ‘P’ character. Do this for the next characters. 

Input		a        b        c        d        e        f        g	Desired output  (Character)
A	B
0 0		1        1        0        0        1        1        1	P
0 1		1        1        1        0        1        1        1	A
1 0		0        1        1        1        1        1        0	U
1 1		0        0        0        1        1        1        0	L

       Fourth plan: make a k-map. Ok, ok what is a k-map? K-map is a short term for Karnaugh Map. It is a method use to make this binary into a mathematical equation for our decoder and turn them into circuit, by using logic gates. There are lots of program about k-mapping you can download it and have an instant equation. But nevertheless, I’ll teach you the basic mapping.

First we want to get the first equation of our letter ‘a’ part of the 7-segment display. Which means we will k-map the binary numbers a= 1 1 0 0 at inputs 00 to 11.