Single-Pole, Single-Throw (SPST)
The single-pole, single-throw (SPST) switch allows a connection between two contacts. One of two conditions exist. Either the circuit is open in one position or closed in the other position. [Figure 12-71]
Single-Pole, Double-Throw (SPDT)
The single-pole, double-throw (SPDT) switch is shown in Figure 12-72. With this switch, contact between one contact can be made between one contact and the other.
Double-Pole, Single-Throw (DPST)
The double-pole, single-throw (DPST) switch connection can be made between one set of contacts and either of two other sets of contacts. [Figure 12-73]
Double-Pole, Double-Throw (DPDT)
The schematic symbol for the double-pole, double-throw (DPDT) switch is shown in Figure 12-74. This type of switch makes a connection from one set of contacts to either of two other sets of contacts.
A toggle switch that is spring-loaded to the OFF position and must be held in the ON position to complete the circuit is a momentary contact two-position switch. One that comes to rest at either of two positions, opening the circuit in one position and closing it in another, is a two-position switch. A toggle switch that comes to rest at any one of three positions is a three-position switch.
A switch that stays open, except when it is held in the closed position, is a normally open switch (usually identified as NO). One that stays closed, except when it is held in the open position is a normally closed switch (NC). Both kinds are spring loaded to their normal position and return to that position as soon as they are released.
Locking toggles require the operator to pull out on the switch toggle before moving it in to another position. Once in the new position, the switch toggle is release back into a lock, which then prevents the switch from inadvertently being moved.
A microswitch opens or closes a circuit with a very small movement of the tripping device (1⁄16 inch or less). This is what gives the switch its name, since micro means small. Microswitches are usually pushbutton switches. They are used primarily as limit switches to provide automatic control of landing gears, actuator motors, and the like. Figure 12-75 shows a normally closed microswitch in cross-section and illustrates how these switches operate.
When the operating plunger is pressed in, the spring and the movable contact are pushed, opening the contacts and the circuit. Figure 12-76 shows a push button microswitch.
Rotary Selector Switches
A rotary selector switch takes the place of several switches. When the knob of the switch is rotated, the switch opens one circuit and closes another. Ignition switches and voltmeter selector switches are typical examples of this kind of switch. [Figure 12-77]
Pushbutton switches have one stationary contact and one movable contact. The movable contact is attached to the pushbutton. The pushbutton is either an insulator itself or is insulated from the contact. This switch is spring loaded and designed for momentary contact.
Lighted Pushbutton Switches
Another more common switch found in today’s aircraft is the lighted pushbutton switch. This type of switch takes the form of a 5⁄8-inch to 1-inch cube with incandescent or LED lights to indicate the function of the switch. Switch designs come in a number of configurations; the two most common are the alternate action and momentary action and usually have a two-pole or four-pole switch body. Other less common switch actions are the alternate and momentary holding coil configurations. The less known holding or latching coil switch bodies are designed to have a magnetic coil inside the switch body that is energized through two contacts in the base of the switch. When the coil is energized and the switch is pressed, the switch contacts remain latched until power is removed from the coil. This type of design allows for some degree of remote control over the switch body. [Figure 12-78]
The display optics of the lighted pushbutton switch provide the crew with a clear message that is visible under a wide range of lighting conditions with very high luminance and wide viewing angles. While some displays are simply a transparent screen that is backlit by an incandescent light, the higher quality and more reliable switches are available in sunlight readable displays and night vision (NVIS) versions. Due to the sunlight environment of the flight deck, displays utilizing standard lighting techniques “washout” when viewed in direct sunlight. Sunlight readable displays are designed to minimize this effect.
Lighted pushbutton switches can also be used in applications where a switch is not required and the optics are only for indications. This type of an indicator is commonly called an annunciator.
Dual In-Line Parallel (DIP) Switches
The acronym “DIP” switch is defined as Dual In-Line Parallel switch in reference to the physical layout. DIP switches are commonly found in card cages, and line replaceable units (LRU) and are used in most cases to adjust gains, control configurations, and so forth. Each one of the switches is generally an SPST slide or rocker switch. The technician may find this switch in packages ranging in size from DIP2 through DIP32. Some of the more common sizes are DIP4 and DIP8.
Switch guards are covers that protect a switch from unintended operation. Prior to the operation of the switch, the guard is usually lifted. Switch guards are commonly found on systems such as fire suppression and override logics for various systems. Figure 12-79A shows a traditional switch with a guard while Figure 12-79B shows a push button switch with a guard. The guard needs to be moved before the switch can be pushed.
A relay is simply an electromechanical switch where a small amount of current can control a large amount of current. [Figure 12-80] When a voltage is applied to the coil of the relay, the electromagnet is energized due to the current. When energized, an electromagnetic field pulls the common (C) or arm of the relay down. When the arm or common is pulled down, the circuit between the arm and the normally closed (NC) contacts is opened and the circuit between the arm and the normally open (NO) contacts are closed. When the energizing voltage is removed, the spring returns the arm contacts back to the normally closed (NC) contacts. The relay usually has two connections for the coil. The (+) side is designated as X1 and the ground-side of the coil is designated as X2.