Electronic Components – Switches

The switch is perhaps the simplest and probably the dumbest component in all of electronics, not dumb in the sense that it is worthless or not useful, dumb in the sense that it is simple and doesn’t do anything complicated. All it does is open and close the circuit, which you and I understand as turning something on and off. The switch can only be opened or closed, when the circuit is open, something is off so when the circuit is closed (ie you have closed the open gap in the circuit) you have turned it on.

That’s really all there is to switches, see you next time!

Just kidding, there is a bit more. There is a good deal of subtlety and variation in what goes into how a switch is operated as well as how many terminals/poles it has. These elements are really the only thing that adds complexity to a switch and even then it’s still relatively basic. You’re still opening and closing the circuit even if you’re switching from one circuit to another, you’re just opening one and closing the other. Pretty easy.

The most common way to actuate a switch in a circuit is with a push button or a switch. In general terms, these are two different types of switches referred to as momentary and maintained. Maintained switches stay in their present state – whatever that is – until they are changed, ie the switch is turned on or off. Types of maintained switches include things like light switches, toggle switches, and slide switches, all of which stay in one state until moved. Momentary switches are activated “momentarily” while you are pushing the button and often have some sort of internal mechanism that automatically returns them to the original position. The windows in your car, the keys on your keyboard and the majority of the buttons in the Portal videogame that require the weighted companion cube are excellent examples of momentary switches. They do whatever they do while pressed and stop when released. When you’re designing or making your circuit, you’ll know which kind of switch best suits your need.

Where switches get a bit confusing, at least at first glance, is when you start talking about poles and throws. Poles are basically how many circuits a given switch can have control of. The number of poles is the number of circuits. The number of throws in a switch is how many places the switch can be connected. Sparkfun describes this quite simply by stating “if a switch has two throws, each circuit (pole) in the switch can be connected to one of two terminals.” This actually makes it relatively easy to classify switches and figure out how they will be used. The most common are:  single pole single throw (SPST), single pole double throw (SPDT), double pole single throw (DPST), and lastly double pole double throw (DPDT). So how are they different?

The SPST switch is the most easily recognized, it’s your basic light switch and it is either on or off. Pretty easy. Next up, the SPDT has three terminals, one of which is input and the other two are output. These are generally used for selecting between one of two power sources (finally I know the significance of locking in the auxiliary in Star Wars) or they can be used to change inputs for your circuit. So it seems that when one circuit is open, the other is closed. Double pole single throw (DPST) is generally used in regulating appliances that run on 240V. This set up is used to turn two separate circuits on or off simultaneously. Finally, DPDT, double pole double throw, operates two separate circuits and acts basically like two SPDT switches and they should all have six terminals. Essentially you have two inputs and four outputs.

While there are references to three and four pole single throw switches, that is three and four circuits on a switch, but examples of them seem to be in short supply suggesting they are uncommon and probably specialized.

Switches are the relatively easy part of an electronics project and designing a circuit. All you really need is to know how many circuits you’re working with (the pole) and the number of outputs (throws) you’re going to need. With that information you’ll know exactly what kind of switch is the right one. Here at EIO we carry many different switches, most if not all made by NTE Electronics, a manufacturer of high quality electronics components and many other related things. Take a look and let us know if you have any questions or would like to share some of your builds that incorporate switches!