Electrical switches and relays are used in model train layout design for both manual and automatic control of a large variety of devices. Simply stated, a switch is a device to allow, stop, or direct the flow of current through an electric circuit. If the switch is manually controlled (like a light switch), it is called a switch. If it is electrically operated (like an overload relay), it is called a relay. Both devices have sets of contacts which are described as closed (allowing electrical current flow), or open (not allowing current flow). Contacts are rated for the maximum allowable current flow, i.e. - 1 amp. Switch contact variations are normally described by the number of poles and throws it contains. The number of poles defines the number of separate contacts for a switch position. The number of throws is the number of switch positions available. A double-pole, double-throw switch (DPDT) means it has two contacts (poles) at each of the two switch (throw) positions.
Switches come in a variety of configurations and sizes, but the most common types for model railroaders are toggle switches, rotary switches, momentary contact, and micro switches. Toggle switches are used to control current flow in multiple applications. They are commonly used on control panels to switch power off or on to track sections, turnout control, reversing, or many applications where turning current off and on are required. Rotary switches are used to select power routing to any device or use that requires more than the 1 or 2 contacts provided by standard switch types. A good example would be the power routing to a number of turntable tracks. Momentary contact switches are usually pushbutton types and are specified as normally open (N.O.) or normally closed (N.C.). Micro switches are devices which can be actuated by a small amount of pressure to open or close contacts and are commonly used as limit switches.
A switch which is activated by an electric current is called a relay. Most relays use coils and electromagnetism to open or close switch contacts but there are transistor devices which contain no moving parts and are defined as solid-state relays. The most common use of a relay is to control high power electrical devices by the use of a low-power signal or open or close switch contacts upon detecting a fault in a circuit. The following illustrations show the circuit symbols for some relays.