Problems and Solutions of PLC Ladder Logic

Problem: Try to develop a relay based controller and ladder logic that will allow three switches in a room to control a single light.

Ladder Logic Solution 1

There are two possible approaches to this problem. The first assumes that any one of the switches on will turn on the light, but all three switches must be off for the light to be off.

Ladder Logic Solution 2

The second solution assumes that each switch can turn the light on or off, regardless of the states of the other switches.

This method is more complex and involves thinking through all of the possible combinations of switch positions.

You might recognize this problem as an exclusive or problem.

Note: It is important to get a clear understanding of how the controls are expected to work. In this example two radically different solutions were obtained based upon a simple difference in the operation.