 A selector switch is a device with an operator that is manually rotated to activate electrical contacts. They are normally used to select either two or three different circuit conditions. Contact position on a selector switch can easily be illustrated using truth tables. This truth table illustrates the manual or automatic contact positions of two contact selector switch. Each contact on the line diagram is marked A or B. The positions of the selector switch are marked MAN for manual or AUTO for automatic. The truth table can be used to easily interpret what contacts are closed for a particular position. An X is placed in the table if a contact is closed in any position. An O is used to depict that a contact is open. Thus, if the selector switch is in the MAN position, A is X or closed and B is 0 or open. When the switch position is set to AUTO, A is 0 or open and B is X or closed. In this illustration of a three-position selector switch, a third row is added to the truth table. So we now have a manual position, an off position, and an automatic position. Thus, in the MAN position, A is X or closed and B is 0 or open. At the OFF position, both A and B are 0 or open. With the selector switch set to AUTO, contact A is 0 or open and B is X or closed. Another type of switch used in electrical mechanics is the limit switch. A limit switch uses contact with an outside force or stationary object to physically open or close the switch. In this schematic diagram, the direction from which a force must contact the switch's actuator is indicated by the triangle below or above the switch. A normally open limit switch will remain open until the actuator is acted upon by an outside force or object, thus holding it closed. A normally closed limit switch will remain closed until an outside force or object holds it open. A good example of limit switch application is a garage door circuit. A garage door circuit includes a limit switch that will remain closed until the actuator comes into contact with the floor. When the garage door motor is activated, it will continue to close the door until the lower edge of the door hits the floor. When opposing pressure is applied to the normally closed limit switch, current to the motor is interrupted, stopping the motor and bringing the door to rest in a down position. In this way, the limit switch defines the limit of movement of an object.