One of the most elementary and easy-to-overlook circuit component is the switch.
Switches don’t require any fancy equations to evaluate. All they do is select between an open circuit and a short circuit. Simple. But how could we live without buttons and switches!? What good is a blinky circuit with no user input? Or a deadly robot with no kill switch? What would our world be without with big red buttons you should never, ever press.
What is a Switch?
A switch is a component which controls the open-ness or closed-ness of an electric circuit. They allow control over current flow in a circuit (without having to actually get in there and manually cut or splice the wires). Switches are critical components in any circuit which requires user interaction or control.
A switch can only exist in one of two states: open (off) or closed (on). In the off state, a switch looks like an open gap in the circuit. This, in effect, looks like an open circuit, preventing current from flowing.
In the on state, a switch acts just like a piece of perfectly-conducting wire. A short. This closes the circuit, turning the system “on” and allowing current to flow unimpeded through the rest of the system.
A circuit diagram with an LED, resistor, and a switch. When the switch is closed, current flows and the LED can illuminate. Otherwise no current flows, and the LED receives no power.
There are tons and tons of switches out there: toggle, rotary, DIP, push-button, rocker, membrane, … the list just goes on and on. Each of those switch types has a set of unique characteristics to differentiate it from others. Characteristics like what action flips the switch, or how many circuits the switch can control. Next up, we’ll go over some of the more basic switch characteristics.
In order to change from one state to another, a switch must be actuated. That is, some sort of physical action must be performed to “flip” the switch’s state. The actuation-method of a switch is one of its more defining characteristics.
Switch actuation can come from pushing, sliding, rocking, rotating, throwing, pulling, key-turning, heating, magnetizing, kicking, snapping, licking,…any physical interaction which can cause the mechanical linkages inside the switch to come into, or go out of, contact.
Momentary vs. Maintained
All switches fall into one of two distinct categories: momentary or maintained.
Maintained switches – like the light switches on your wall – stay in one state until actuated into a new one, and then remain in that state until acted upon once again. These switches might also be called toggle or ON/OFF switches.
Momentary switches only remain active as long as they’re actuated. If they’re not being actuated, they remain in their “off” state. You’ve probably got a momentary switch (or 50) right in front of you…keys on a keyboard!
Semantic alert! Most of the switches we refer to as “buttons” fall in the momentary category. Activating a button usually means pressing down on it in some manner, which just feels like a momentary control. Here, when we talk about “buttons,” think “momentary push-down switch.”
As with most components, the termination style of a switch always comes down to either surface mount (SMD) or through-hole (PTH).
Through-hole switches are usually larger in size. Some might be designed to fit in a breadboard for easy prototyping.
These Tactile buttons are through-hole and fit perfectly in a breadboard. Great for prototyping!
SMD switches are smaller than their PTH counterparts. They sit flat, on top of a PCB. SMD switches usually require a gentle touch, they’re not built to sustain as much switching force as a through-hole switch.
Panel mount switches – designed to sit outside an enclosure – are a popular mounting style as well. It’s hard to flip a switch when it’s hidden inside an enclosure. Panel mount switches come in all sorts of termination styles: PTH, SMD, or heavy-duty solder lugs for soldering to wires.
A panel mounted illuminated toggle switch.
One more important switch characteristic, which really deserves a page of its own, is the internal circuit arrangement of a switch. Are you looking for an SPST? DPST? 4PDT?
Poles and Throws, Open and Closed
A switch must have at least two terminals, one for the current to (potentially) go in, another to (potentially) come out. That only describes the simplest version of a switch though. More often than not, a switch has more than two pins. So how do all of those terminals line up with the internal workings of the switch? This is where knowing how many poles and throws a switch has is essential.
The number of poles on a switch defines how many separate circuits the switch can control. So a switch with one pole, can only influence one single circuit. A four-pole switch can separately control four different circuits.
A switch’s throw-count defines how many positions each of the switch’s poles can be connected to. For example, if a switch has two throws, each circuit (pole) in the switch can be connected to one of two terminals.
Knowing how many poles and throws a switch has, it can be more specifically classified. Commonly you’ll see switches defined as “single-pole, single-throw”, “single-pole, double-throw”, “double-pole, double-throw”, which are more often abbreviated down to SPST, SPDT, and DPDT, respectively.
A single-pole, single-throw (SPST) switch is as simple as it gets. It has one output and one input. The switch will either be closed or completely disconnected. SPSTs are perfect for on-off switching. They’re also a very common form of momentary switches. SPST switches should only require two terminals.
The circuit symbol for an SPST switch in the off position and a through-hole, right-angle, maintained, SPST, rocker switch.
Another common switch-type is the single-pole, double-throw (SPDT). SPDTs have three terminals: one common pin and two pins which vie for connection to the common. SPDTs are great for selecting between two power sources, swapping inputs, or whatever it is you do with two circuits trying to go one place. Most simple slide switches are of the SPDT variety. SPDT switches should usually have three terminals. (Sidenote: in a pinch an SPDT can actually be made into an SPST by just leaving one of the switch throws unconnected).
An SPDT switch circuit symbol, and an SPDT slide switch.
Adding another pole to the SPDT creates a double-pole, double-throw (DPDT) switch. Basically two SPDT switches, which can control two separate circuits, but are always switched together by a single actuator. DPDTs should have six terminals.
A DPDT circuit symbol, and a 6-terminal DPDT Rocker Switch
Switches with more than two poles or throws are not too common, but they’re out there (in all their oddly-shaped, difficult-to-connect-to glory). Once we get past one or two poles/throws, we just start sticking numbers in the abbreviation. Here’s a 4PDT switch, for example, it can control four separate circuits, 2 positions per circuit:
A massive 4PDT circuit symbol, and an physically massive 4PDT Toggle Switch.
When a momentary switch is not actuated, it’s in a “normal” state. Depending on how the button is constructed, its normal state can be either an open circuit or a short circuit. When a button is open until actuated, it’s said to be normally open (abbreviated NO). When you actuate a NO switch, you’re closing the circuit, which is why these are also called “push-to-make” switches.
Conversely, if a button usually acts like a short circuit unless actuated, it’s called a normally closed (NC) switch. NC switches are “push-to-break”; actuating the switch creates an open circuit.
Among the two types, you’re probably much more likely to encounter a normally open momentary switch.
Momentary switches are switches which only remain in their on state as long as they’re being actuated (pressed, held, magnetized, etc.). Most often momentary switches are best used for intermittent user-input cases; stuff like reset or keypad buttons.
Examples of Momentary Switches
Push-button switches are the classic momentary switch. Typically these switches have a really nice, tactile, “clicky” feedback when you press them. They come in all sorts of flavors: big, small, colorful, illuminated (when an LED shines up through the button). They might be terminated as through-hole, surface-mount, or even panel-mount.
An assortment of tactile push-button switches. Starting top-left, clockwise: blue and red arcade buttons, 12mm push button, white capped button, orange illuminated, right-angle, panel-mount, and a mini push button.
Large arrays of momentary buttons, like your keyboard or even smaller groupings like a keypad, usually arrange all of their switches into a big matrix. Every button on the pad is assigned a row and column. This requires some extra button-press-processing on the microcontroller end, but frees up a big chunk of I/O pins.
Momentary switches don’t always have to be actuated by a pushdown. It could be push-sideways, like the movement action in a handful of joysticks.
An arcade joystick uses four microswitches to sense up, down, left and right movements. The tiny little surface-mount 5-way tactile switch is an SP5T directional switch (up, down, left, right, and press-down).
On the other end of the spectrum, reed switches open or close when exposed to the presence of a magnetic field. These are great for making a non-contact switch.
A maintained switch retains its state until it’s actuated into a new one. Just look to the nearest wall for an example of a maintained switch – the thing controlling your lights! Maintained switches are great for set-it-and-leave it applications like turning power on and off.
Examples of Maintained Switches
Need a really basic, no-frills ON/OFF or selector switch. Slide switches might be for you! These switches have a tiny little nub which protrudes from the switch, and it slides across the body into one of two (or more) positions.
You’ll usually find slide switches in SPDT or DPDT configurations. The common terminal is usually in the middle, and the two select positions are on the outside.
Toggle switches have a long lever, which moves in a rocking motion. As they move to a new position, toggle switches make a really satisfying “snap”.
Toggle switches are commonly SPST (two terminals) or SPDT (three terminals). As usual, you can find them in through-hole, surface-mount, or – probably most commonly – as panel-mountable.
DIP switches are through-hole switches designed in the same mold as a through-hole DIP IC. They can be placed in a breadboard, in the same manner a through-hole IC might, by straddling the center area.
An 8-position DIP switch, handy for configuring 8 somethings.
These switches often come in arrays of eight or more separate SPST switches, with tiny little sliding levers. They were widely used in the olden days of computing, but they’re still useful for configuring a devices via hardware.
Resources and Going Further
Well, that about covers the basics of switches.
Check out some of our project tutorials for some inspiration for your own projects. Switches are so widely used, we could probably link all of our projects. But here are a couple which make really ingenious use of switches to do their thing:
- MP3 Player Shield Music Box - This project (bigger on the inside) uses one of those non-traditional switches – a reed switch – to trigger it’s action.
- Reaction Timer - Use giant dome push buttons to create a fun reaction game.
This article was originally published on learn.sparkfun.com on 5/7/13 and authored by Jim Lindblom.