Switches are an important electrical component, because switches are where we interface and control electrical devices. There are a variety of different ways that switches open and close circuits. Switches are symbolised by breaks in the wiring with a short line segment nearby that looks like it could be pushed into place to repair the break.
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Poles and Throws
Switches are described in terms of poles and throws. Poles list how many switches are combined in one mechanism. Throws indicate how many of the positions the switch can be in are active circuits. A typical wall switch is single pole, single throw. It controls one circuit, and only one of the positions controls an active circuit. In symbols for switches, there will be short lines that looks like it could be moved to make contact with any number of different lines. The number of short movable lines indicates the number of poles, and the number of circuits these short movable lines can be moved to indicate the throws.
Push button switches are pushed with the finger tip. They can be normally closed--pushing the switch breaks the circuit--or they can be normally open--pushing the switch creates the circuit. Pushbutton switches are symbolises by a gap that has a short parallel line the size of the gap. If the short line is touching the line it shows a "push to open" switch. If the short parallel line is not touching the line, it symbolises a "push to close" switch.
Microswitches usually have a long lever that activates the switch. They are activated by physical movement in the environment. Burglar alarms often have microswitches so a window or door opening activates the alarm. Backups on a conveyor belt can activate a microswitch that closes down the conveyor belt. Microswitches inside a vending machine monitors product movement. Microswitches are usually used when the switches are activated by something other than the human hand. Microswitches can be normally open or normally closed, single or double pole, and single or double throw. Microswitches do not have a special symbol. They have the symbol that shows the poles and throws along with the part number for the microswitch.
Some switches are activated electronically--one circuit controls another. The oldest of these are relays. These are electromagnets that open or close switches. Relays can be normally open or normally closed, single or double pole, and single or double throw. By making one of the controlled circuits run through the coil of the electromagnet, you can have a "latching relay." These stay on once activated and the only way to unlatch the relay is to momentary break the circuit. The symbol for a relay is the collection of symbols that show the poles and throws along with a coil of several loops that symbolises the electromagnet. The two wires leading to the electromagnet are shown as the end of coil of loops.
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