A traffic light control at an intersection uses a binary counter to produce the

Question

A traffic light control at an intersection uses a binary counter to produce the sequence of combinations on lines A, B, C, and D. After 1000, the sequence repeats, beginning again with 0000. Each combination is present 5 seconds before the next one appears. These lines drive combinational logic with outputs to lamps RNS (Red- North/South), YNS (Yellow- North/South), GNS(Green- North/South), REW (Red- East/West), YEW (Yellow- East/West), and GEW(Green- East/West). The lamp controlled by each output is ON for a 1 applied and OFF for a 0 applied. For a given direction, assume that green is ON for 30 seconds, yellow is ON for 5 seconds, and red is ON for 45 seconds. (The red intervals overlap for 5 seconds.)
Divide up the 80 seconds available for the cycle through the 16 combinations into 16 intervals and determine which lamps should be lit in each interval based on expected driver behavior. Assume that, for interval 0000, a change has just occurred and that GNS= 1 and REW= 1, and all other outputs are 0. Design the logic to produce the outputs.

Explanation / Answer

There are many applications for controller circuits. One of them is a highway intersection traffic light. In this experiment you will design and implement a 4-way traffic light controller that uses pressure activated sensors embedded into the roadway to determine the sequence of light changes used to optimize the flow of traffic. System requirements: The system has 4 input sensors, north (N), south (S), east (E) and west (W), and 6 output lights, N/S (red, yellow and green), and E/W (red, yellow and green). Four data switches are used to simulate the sensors embedded on the road, and six LEDs are used to simulate the lights. A counter will be used to determine the amount of time each lamp stays on/off, and a flip-flop is used to determine which direction the traffic is flowing, i.e. N/S red is the complement of E/W red. Details about the design: 1. The traffic light cycle is as follows, assuming traffic is flowing in both directions, i.e. the road sensors detect cars in both directions: • Starting with N/S light being red, the counter should count 12 pulses to change the E/W green light to yellow, then count another 4 pulses before simultaneously turning the E/W light red and the N/S light green. The cycle repeats but this time the N/S light will be green during 12 pulses, then yellow for four pulses, and finally it will turn red simultaneously with the E/W light turning green, which brings the cycle back to the beginning. 2. Even though the information provided below may seem obvious, it is always better to have all facts stated explicitly: • At any given instance both directions may not be red, i.e., assume one direction is red and the other green or yellow. • When cars are detected flowing in only one direction, that direction should remain green. • When cars are not detected in either direction, the lights should not change. • If a car stops at a red light and makes a right turn, then the count should start and as soon as the car leaves it should stop. • If any yellow light is ON, the count should continue even if there are no cars waiting. 15 • When cars are flowing in both directions the lights should toggle every 16 counts as described above. • Since the 74LS93 doesn’t have a stop count feature, a gate should be used to turn the clock going to the counter on or off as needed.

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