PCB Friday: Fundamental Switch and Button Concepts (NO, NC, SPST, SPDT, xPxT, Latching...)

Today I was researching guitar pedals, specifically how the foot pedal switch works to activate or bypass the effect. What I came across was the exotic-to-me 3PDT foot switch. After a bit of skimming/looking at pictures/ChatGPTing that now passes for "research," I now understand how the switch works — it's basically just three independent switches chained together.

If this switch concept was confusing to me, then certain switch fundamentals may also be confusing to you. So, without further ado, here's the Hackster guide to button and switch definitions that you need to know but were afraid to ask. Or whatever shortened version makes it into the title!

First fundamental: the terms switch and button are sometimes used interchangeably, but technically a button is a type of switch. Also — not fundamental — but if you're into guitars check out my Hackstercaster 3D-printed guitar. It has both a wild-looking body and LED lighting!

Switches: Normally open (NO), normally closed (NC)

Unlike how doors work in the non-electrical world, circuits must be closed (connected) to pass electrons. Open in this context is the same as disconnected, i.e. there's an opening between conductors. Normally means the default button state if you're not pressing or otherwise actuating it.

So if you have an NO switch/button, it normally does not let electricity flow. When activated/pushed, its conductors are closed, turning the circuit on.

If you have an NC switch/button, it normally lets electricity flow. When activated/pushed, it becomes open, turning the circuit off. You can also have both NO and NC contacts on a single switch, discussed below.

SPST, SPDT, xPxT

Beyond NO/NC, switches come in a range of input/output conductor configurations. These are indicated by arcane abbreviations like SPST, SPDT (image below), SP3T, DPDT, xPxT.

So what do these abbreviations mean?

• The first and third letters indicate a numerical value, which may be S (Single), D (Double), 3 (three), 4 (four), or any other number.
• The second letter (P) stands for pole, the character before which indicates how many separate circuits the switch can control. Each pole can connect to one throw at a time.
• The fourth letter (T) stands for throw, the number before which indicates the number of conductors each pole can connect to.

As shown in the figure above, an SPDT switch has one pole, which can connect to two throws to control a single circuit.

The simplest type of switch, the SPST, has one pole, which can only connect to one throw. In other words, it is either connected or disconnected. A DPDT switch has two poles, each of which can be connected to two throws. If you need two circuits to be switched simultaneously — with NO and NC contacts on both — a DPDT is your answer.

Or you could go wild and go for a 3PDT, seen below and explained here in the context of guitar pedal switching. You could think of the 3PDT switch as three SPDTs activated together, while a DPDT is equivalent to two simultaneous SPDTs.

Momentary and latching switches

NO/NC configurations apply to momentary switch operation. When this type of device is activated by an operator, its output (or outputs) switch from the default state to the opposite state (i.e. NO becomes closed, NC becomes open). When the operator releases the switch, it reverts to its normal state. Thus it's momentarily in its alternate (non-normal) state.

Latching switches — button-style or otherwise — stay in whatever state the operator sets them. They may be closed when latched and open when unlatched or vice versa. So the NO/NC terminology doesn't apply in the same way, though xPxT can be used to define the contact operation.

Toggle switches — such as the SPDT device shown in the image above —latch in both positions. Note the "ON ON" markings imprinted on its housing (not to be confused with "NO NO").

Other switches

While understanding the definitions of NO/NC/momentary/latching/xPxT will help you understand the vast majority of the switches you see, there are many variations on the design. For example, the on-off-on rocker switch acts like a SPDT switch, but has a disconnected setting when the switch is in the middle — like a third blank throw.

Then there's the rotary switch, which is typically a single pole, multi throw (SPxT) device and connections that are made depending on how the dial is set. You might also encounter industrial-style buttons, which are configurable depending on the contact blocks applied. And there is a wide range of other switch variations that I haven't named here (or perhaps even heard of). In other words, we're just scratching the surface!

Know your switches

While the type of switches discussed here may seem like (and are) something you'd see inside an arcade machine or a... guitar pedal, the concepts are very much appropriate for PCB design as well. KiCad, as shown below, includes a wide range of symbols and schematics to accommodate your design needs, as do other EDA packages. As with everything in engineering, there's always something more to learn!

Thanks for reading!

Thanks for reading my biweekly PCB Friday column! I'm looking forward to sharing more PCB-related knowledge and insights here, and I hope you'll follow along for the journey. You can find me on YouTube @jeremycook, read my semi-technical musings at TechAdjacent.io, or email me at hi@jeremyscook.com if you have any comments or suggestions!

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