In electronics, a switch is a device that is used to change the flow of a circuit. Switches can be activated by any number of inputs, depending on the nature of the switch. A limit switch changes the flow of a circuit based on physical contact with an object that switches the circuit between an ON and OFF position.

A limit switch is one of the simplest kinds of switches. An actuator is attached to a set of contacts on the circuit. When something comes into contact with the actuator, it activates the switch and either activates or deactivates the circuit. This mechanical stimulation can be as simple as touching a switch or pressing a button, but also has other applications.

You can set up limit switches to enable activation and deactivation of a circuit based on the presence or absence of a certain stimulus. Limit switches are commonly found in heady-duty drills, lathes, and boring machines. In these devices, the limit switch functions as a safety device, so that the machine will only work when in contact with something to drill--if removed from its contact, it will shut off to prevent accidental injuries.

Limit switches can be used in devices as complex as these power tools or in applications as benign as changing the course of a model train. A limit switch can detect the weight of the train, and activate a circuit to change the tracks, allowing more complex patterns of miniature locomotion than previously possible. Limit switches are available in many sizes and to differing degrees of sensitivity, so experiment until you find the part that's right for your complicated circuit.