Updated: Apr 25, 2022
If you're anything like us, you've probably got a bunch of switches tucked away in the components bin. When project time comes around you grab one out, and maybe check with a multimeter which contacts conduct for each switch position, throw it into your circuit, and call it a day. After all, a switch is just on or off, how much theory can there possibly be?
When you look a bit deeper, you may be confronted by unfamiliar terminology, acronyms (DPDT, NC whaaat??) and a whole world of problems you hadn't considered. Let's take a dive into the details of switches, and a practical circuit that uses a switch in an interesting way. Note that a lot of this background is applicable to buttons as well.
There are two main operating mechanisms behind switches and buttons, known as alternate action, and momentary.
An alternate action switch keeps the switch (or button) in a continuous state, i.e. flip the switch to on and it stays there. Flip it again, and it stays off. Pretty typical and expected behaviour for a switch, and certain buttons may show this behaviour as well (push-push style). Contrast this to a momentary mechanism, where the state only changes while engaging the switch (push for on, release for off). Most buttons are like this, though you can get momentary switches too.
Poles and Throws
The mechanical action of the switch makes and breaks electrical connections, and the exact configuration is determined by the number of poles and the number of throws. The number of poles can be thought of as the number of parallel "internal switches" that are actually controlled by the mechanical switching action. The number of throws tells us how many different contacts or wiring paths there are for each of these internal switches.
See below for the schematic of some common switch configurations.
Another marking you may see on a switches/button contact might be "NC" or "NO". These stand for "Normally Closed" and "Normally Open", and they tell you what that contact "normally" does when you aren't interacting with the device.
For example, a momentary button might have 3 contacts, labelled COM, NO and NC. When you aren't pressing the button (the normal state), COM is electrically connected to NC, and not connected to NO. Hopefully, this makes sense, normally, the NO contact is open, and the NC contact is closed. However, when you press the button (the non-normal state), the connection is closed between NO and opened between NC.
Let's explore a simple example by using a switch to control motor direction - we have a DC voltage source, a DC motor, and our choice of switch. Any ideas on what type of switch we should reach for?
We want to switch the two source polarities (+ and -) to two different states. Let's try a DPDT toggle switch. Start by hooking up the motor through the switch to run in one direction. Use the poles to connect the source, and a parallel set of throws through to the motor.
This should drive the motor in one direction. Can you see what we do next to make it run the other direction when the switch is toggled? The polarities need to be reversed - what was positive must become negative, and vice versa. Like so!
You should be relatively confident integrating the right switch into your next project, or at least have some idea what on earth a momentary SPDT switch with NC and NO connections is all about. Don't forget about switch bounce, or whether you need to worry about make-break patterns though! Happy switching!