4 BIT up Counter and BCD Using Discrete Transistor

Introduction

We are familiar with different digital circuits. Few of them are explained in my previous blogs. In digital electronics the counters always wonders me at the first sight. Because it as a machine which counts the numbers. What an amazing machine. So I search for more detail about it. Then I create several circuits based on counters. But all are made from counter ICs. But I am not satisfying in it. So I create my own 4 bit counter using discrete transistors. It is really interesting for me also for you. Hope you enjoy....

What is a binary counter?

From the name we get that it is a counting machine which counts in binary form.  Binary number system contain only two symbol '0' and '1'. All the numbers represented by using this basic symbols. Here we try a 4 bit counter. So it count up to 16 different numbers or symbols.  That is from 0000 to 1111(in binary form). The counters are made by using flip-flops. Commonly the T flip flop are used for counter design. I created a blog based on T flip-flop. For more details, visit it. The given block diagram represent a practical 4 bit up counter. The up/down counting is based on the circuit configuration. It is also given in the given figure. 

Conversion to BCD counter

The BCD counter is a 4 bit up counter which only count up to the decimal number 9. After it, It resets to zero. Then repeats this process. For resting, we need a combinational circuit by using gates. It reset the counter when needed. Its circuit and design given below in the figure.

Working

Consider above block diagram (counter block diagram). Each flip-flop's complementary out put Q' is connected to next flip flop clk input. It is because we need up counter and here I use the positive triggered flip flop.  The output is taken from each flip flop output Q. The first flip flop clk input is connected to the external clk circuit. I use an astable mutivibrator by using transistors. It is given in the following video. The first transistor side is LSB and the last flip flop output is MSB. All the flip flops T input is connected to the Vcc to activate the toggle property.

Assume that all the flip flop first at zero state. So all the complementary output is at high level 1. First we apply a positive clock pulse. It trigger the fist flip flop and its Q changed from 0 to 1. And the Q' is from 1 to 0. So the next flip flop never triggered(remember that flip flop Q is connected to clock). So we get a stable 0001 output. Then apply a second clock to first flip flop. It trigger the first flip flop and it change its output Q from 1 to 0. And the Q from 0 to one. Bingo!! See that it trigger the second flip flop. It output changes from 0 to 1. And the Q from 1 to 0. So it never trigger the remaining flip flops. So w get a stable 0010 as output. This is work in this manner and reach the 1111 and then reset.  If we add an extra BCD  reset circuit .then it count up to 9 and then reset. OK. Hope you understand. If you have any doubts please comment.  See the diagrams given below. It will give more information about it. Observe it.

Circuit Explanation

Consider the above circuit diagram. I mark each part by using different colors. From the circuit it is clear that it contain 4 identical flip-flop. Its pins are connected to corresponding pins as shown in the block diagram above. The reset circuit is given in a yellow color. It is also designed by using diodes and transistors. To work it as a BCD counter switch on the switch. Otherwise work like a 4 BIT up counter. There add some extra capacitor to the reset transistor base. It is provided for reset all the flip-flop when power is ON. Otherwise the counting starts from any number in a random manner. So it act as a power-on reset function. This is the full details about the above circuit. 
For it working. Watch this video.

If you have any doubt give it in the comment section.

For its making procedure visit my instructables page.

https://www.instructables.com/id/BCD-Counter-Using-Discrete-TRANSISTORS

Thanks for reading......

Also I made a "BCD to 7 segment decoder using discrete components" in my new Blog, please visit it. Link given below,

https://0creativeengineering0.blogspot.com/2019/03/bcd-to-7-segment-display-decoder-using.html