# Traffic light controller

1. Nov 22, 2007

Hi,
I have some problem to design a traffic controller.
the traffic lights are placed at an intersection of two streets, one street running north-south (called NS) and another street running east-west (called EW).
there is 5 clock cycles for Red light, 1 clock cycle for Yellow light , and 4 clock cycles for Green light. EW light is Red when NS light is Yellow or Green. Similarly, NS light is Red when EW light is Yellow or Green.
I need to design the controller that is essentially a counter driven with a clock (without any other external input). I will need 10 states to design the circuit. The six outputs are R-EW, Y-EW, G-EW, R-NS, Y-NS, and G-NS

What I did is that I tried to design a modulo 10 counter (see attached file ). I form the truth table of the system.
A B C D | NS | EW |
0 0 0 0 | R | G |
0 0 0 1 | R | G |
0 0 1 0 | R | G |
0 0 1 1 | R | G |
0 1 0 0 | R | Y |
0 1 0 1 | G | R |
0 1 1 0 | G | R |
0 1 1 1 | G | R |
1 0 0 0 | G | R |
1 0 0 1 | Y | R |

Then I tried the output tables, but I does resemble to anything I dont have the relationship of the table.
please can you help me ?
thank you
B.

#### Attached Files:

• ###### Traffic controller.doc
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2. Nov 22, 2007

### mgb_phys

Normal solution would be to use a memory (ROM)
Treat each possible setup as a location in memory, set the value of the memory so that the output lines are the correct state for each switch.
Then clock through the memory in sequence outputing these values.

You can always think of a memory as mapping a particlular set of inputs (the address lines) onto a particular set of outputs (the data lines)

3. Nov 22, 2007

Hi mgb_phys,

we cannot use memory, only any kind of flip-flop and basic gates.
B

4. Nov 23, 2007

### Staff: Mentor

You are designing a state machine. Have you learned how to do that yet? There is a step-by-step process to define the states, figure out how many FFs the state machine will require, and then write the truth/transition table and implement the logic.

http://en.wikipedia.org/wiki/State_machine

.

5. Nov 23, 2007

### mgb_phys

Also worth googling "Karnaugh map"

6. Nov 24, 2007

Thank you berkeman and mgb_phys,

I have looked at what you suggested me and I got some good things to work with!

B.

7. Dec 9, 2007

### kirubagaran.r

8. Dec 10, 2007

### Astronuc

Staff Emeritus
Last edited by a moderator: May 3, 2017