If you are an engineering student or an automation enthusiast looking to learn PLC ladder logic programming, this guide will walk you through real-world examples. We’ll go step by step, starting with simple control mechanisms and then moving toward tank filling automation using sensors. By the end, you’ll understand how ladder diagrams are applied in industrial automation systems.
What is PLC Ladder Logic?
Ladder logic is one of the most widely used programming languages for Programmable Logic Controllers (PLCs). It uses graphical symbols resembling electrical relay logic diagrams, making it easy to understand and implement for automation control tasks.
Ladder logic typically controls:
- Motors
- Pumps
- Valves
- Lights & Indicators
- Sensors
Problem 1 - Control of an Output Device Using 3 Inputs
Develop ladder logic for controlling an output device through 3 input devices.
Input/output devices list - Problem 1
Inputs: outputs:
0000, 0001, 0002 –switches 0500-fan/light
Ladder Logic - Problem 1
Timing Diagram - Problem 1
Problem 2 - Automatic Tank Filling System with Sensors
Develop a ladder logic program to control the lubricating oil being dispensed from a tank. This is possible by using two sensors one is near the bottom and another one is near the top. The motor has to pump the lubricating oil to fill the tank until the high level sensor turns on and the motor has to turn off when the level falls below the low level sensor. Then the motor should turn on and starts filling the tank.
Input/output devices list - Problem 2
Inputs: outputs:
0000-top sensor 0500-motor
0001-bottom sensor 0501-tank
Ladder Diagram - Problem 2
Timing Diagram - Problem 2
Problem 3 - Boxes on a conveyor belt
Develop ladder logic for a continuous filing operation. The process requires that boxes moving on a conveyor be automatically positioned and filled. This process follows the following sequence-
- Start and stop the conveyor when the start and stop buttons are momentarily pressed.
- Energize the run status light when the process is operating and energize the standby status light when the process is stopped.
- Stop the conveyor when the right edge of box is first sensed by the photosensor.
- With the box in position and the conveyor stopped, open the solenoid valve and allow the box to fit. Filling should stop when the level sensor goes true.
- Energize the full light when the box is full. The full light should remain energized until the box is moved.
Input/output devices list - Problem 3
Inputs: outputs:
0000-start 0500-run
0001-stop 0501-standby
0002-photoswitch 0502-full
0003-level switch 0503-motor
0504-solenoid
Ladder Diagram - Problem 3
Timing Diagram - Problem 3
Problem 4 - LED
Develop ladder logic to blink a LED light with a delay of 5 sec
Input/output devices list - Problem 4
Inputs: outputs:
0000-switch T020, T021-timers
0500-LED
Ladder Diagram - Problem 4
Timing Diagram - Problem 4
Problem 5 - Multi LED
Develop ladder logic to alternate turn ON and turn OFF of a red and green LED’s.
Input/output devices list - Problem 5
Inputs: outputs:
0000-switch T020, T021-timers
0501-Red LED
0502-Green LED
Ladder Diagram - Problem 5
Timing Diagram - Problem 5
Problem 6 - Conveyor System
Develop a ladder logic program for the following process.
A conveyor has to run by switching ON or OFF a motor. The parts on the conveyor were detected by an optical detector. When the optical sensor goes ON, the conveyor has to stop after 1.5 sec. After a delay of 2 sec, the conveyor will start again. Use START & STOP buttons- a light should be ON when the system is active.
Input/output devices list - Problem 6
Inputs: outputs:
0000-sensor T020, T021-timers
0001-Go 0500-conveyor
0002-Stop
Ladder Diagram - Problem 6
Timing Diagram
Problem 7 - Motor System
Develop ladder logic to switch OFF a motor after 30 sec of pressing the stop switch
Ladder Diagram
Timing Diagram
Problem 8 - Solenoid
Develop a ladder logic program for the following sequence of statements-
- Normally open start and normally closed stop push buttons are used to start and stop the process.
- When the start button is pressed, solenoid A energizes to start filling the tank.
- As the tank fills, the empty level sensor switch closes.
- When the tank is full the full level sensor switch closes.
- Solenoid A is de-energized.
- The agitate motor starts automatically and runs for 3 min to mix the liquid.
- When the agitate motor stops, solenoid B is energized to empty the tank.
- When the tank is completely empty, the empty sensor switch opens to de-energize solenoid B.
- The start button is pressed to repeat the sequence.
Ladder Logic
Timing Diagram
Problem 9 - Multi Input Handling
Develop a ladder logic program that will latch on an output B 20 sec after input A has been turned ON. After A is pushed, there will be a 10 sec delay until A can have any effect again. After A has been pushed 3 times, B will be turned OFF
Ladder Logic
Timing Diagram
Problem 10 - Dual Conveyor Belt
Develop a ladder logic program for a motor dual conveyor belt carrying cement bags. Every time a start is pushed, run the conveyor to deliver 10 cement bags at the other end. Glow a green light when the conveyor is running otherwise red light will glow
Ladder Logic
Timing Diagram
Problem 11 - Process Implementation
Develop ladder logic for the given process. A motor will be controlled by two switches. The GO switch will start the motor and the STOP switch will stop it. If the STOP switch was used to stop the motor, the GO switch must be thrown twice to start the motor. When the motor is active a light should be turned ON. The STOP switch will be wired as normally closed.
Ladder Diagram
Timing Diagram
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