Question

Design a PLC ladder logic program to control a stop light. The program should have 3 outputs: RED, YELLOW, and GREEN. The ladder logic should illuminate green for 60 seconds, then yellow for 10 seconds, then red for 60 seconds. The system should also have two sensor inputs: THROUGH_TRAFFIC and CROSS_TRAFFIC which detect when there is through traffic and cross traffic present. If through traffic is detected, and no cross traffic is detected, the system should maintain a green light even after the green has remained lit for 60 seconds.

Answer 1

Explanation : After pressing the start button , the process will and all the outputs go low irrespective of any condition when the stop button is pressed.

Rung 1: After pressing start button memory bit m:0/0 will becomes high.

Rung 2 : ON dealy timer will start and Green LED will ON. After 60 seconds , the green LED will off, if and only if, the through traffic and cross trafic condition across timer1.DN bit switch is not satify. If the through traffic is high and cross traffic is low, then after the 60 seconds delay also the green LED will be in its high state.

Rung 3 : when Timer1.DN is high and green LED go low state , the Yellow LED will ON and Timer T2 will start. After 10 seconds the timer2.DN bit will break the path leads to the yellow LED will go off.

Rung 4 : when Timer2.DN is high and Yellow LED go low state , the red LED will ON and Timer T3 will start. After 60 seconds the timer3.DN bit will break the path leads to the red LED will go off as well as it will break the timer1 path leads to timer1.DN becomes low, that leads to timer 2 will get reset and further timer3 also will get reset.

The process repats continously, till the stop button is pressed.

Note : For interlocking purpose to prevent any short of maloperations in the practical point of view, the Output addressed NC swithes uesd in the 2^nd ,3^rd and 4^th rungs.

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