This is the 555 timer This is the 74163 binary counter This is the 74138 decoder

Question

This is the 555 timer

This is the 74163 binary counter

This is the 74138 decoder

Design and build a traffic light circuit that makes use of the micro-computational capability of the 555/74163/74138 three-chip controller circuit.

Traffic lights continuously cycle the illumination of three different colored lights: green, yellow, and red. Use green, RGB, and red LEDs to create the green, yellow, and red output. Use the RGB LED to display yellow light (roughly) by turning on both the red and green emitters. The output of your traffic signal must show green for 4 seconds, followed by yellow for 1 second, followed by red for 3 seconds, and then repeat in this order.

Design three combinatorial circuits, one to compute the signal for each traffic light color. The controller circuit provides eight outputs from the demultiplexer and three outputs from the counter. Any of these outputs may be helpful in developing the signals for the traffic light. You may also use NAND and NOR gates to implement your computational logic, as you see fit.

Create a truth table for each color of the traffic light. Use outputs from either the counter or decoder as inputs for the truth tables. Think about the operation of a traffic light and think about which of the counter or decoder outputs are similar in timing and logic level to the signal you need for a given traffic light. Using an existing signal that is similar to the LED signal needed for a given light means a simpler computation to perform to create the actual LED signal and, thus, a simpler combinatorial circuit to build.

For your traffic light design, write the Boolean expression for each traffic light color, and simplify to 2-input NAND and/or NOR gates.

Draw the schematic diagram of your traffic light circuits, following the expression you just derived

Explanation / Answer

Digital clocks have been built by countless electronics hobbyists over the world. So why have I chosen to implement that? Well usually clock circuits available on the internet (all circuits I have seen) use the 7490 counter (I have used 7493 but I will show why), microprocessors or Arduino boards. But not all of us have the means to buy microprocessors or Arduino boards (as far as I am concerned they are expensive). I wanted to try a different circuit for the same clock and I also chose it because it requires a lot of counters, and counters are based on sequential logic. When I say digital clock, you should expect something like the one in the picture!

It's my stand that just looking at the circuit diagram and replicating it on a bread-board is not what electronics is about. Almost all digital circuits from traffic lights etc. to even computers are all based on sequential logic (its importance). Therefore, I have included the theory of flip-flops and sequential logic design in hope that it would help the reader to design circuits of their own.

I have done my best to explain sequential logic design between Step 3 to 8. However, if all you want is the clock, then please skip everything from step 3 to step 8. But if you go through those steps you will understand how to work with flip-flops etc. and though it takes a lot of time I can assure that you will be left with a wealth of knowledge.

Since this is a circuit 'of my own', I know that I have to show a novelty factor. Usual clocks based on decade counters have a hour counter from 0 -23. I have only used IC's but still got a 12 hour clock, which I have not seen elsewhere. I have also added a small alarm module. The alarm is again achieved using IC's not by programming boards (which quite frankly is comparatively easy). It is not much but I did whatever extra nicks I could do. The main emphasis however, is learning sequential logic and developing a breadboard based clock using that knowledge.

For your kind attention: I am assuming that you know basic high school level digital logic - the fundamental logic gates and binary numbers.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.