All About Electrical Switches

Updated 18 December 2016

On and Off. It’s the basis for all electronics, from the most sophisticated computer network to a simple circuit. Electronics is all about using one thing, to control another, input and output, on and off. Even the transistors inside modern computer chips, measuring mere nanometres in size and acting as switches to control the flow of current, allowing a system to make binary decisions. Understanding how switches work is a core element of any electronic design, no matter or big or small. So today we’re going to take a look at a wide range of mechanical switches, simple and complex, understand how they work, and the various applications they are suited to.

Characteristics and Terms

Before we take a look at any switches, we need to go understand some basics terms that apply to different types of switches:

  • Momentary or Latching: Switches usually come in two main varieties, momentary switches, or latching switches. Momentary switches have a ‘default state’ and one or more ‘active states’. When they aren’t pressed, they revert to a default state which may disconnect the contacts, or connect them in a certain way.
  • Poles: The number of poles a switch has is the number of individual, isolated channels that single physical actuator can switch. A single pole switch has one pin which can connect to other pins depending on the position, whereas a double pole switch has two pins, both electrically isolated from each other, with its own set of contacts that it can connect to.
  • Throws: A throw is an individual contact that can be made from the signal pin. For example, a switch with one throw means that in one state, the signal pin is connected to another pin, closing the switch, and in the other state, it is disconnected from the pin, leaving the switch open and nothing connected. A switch with two throws, however, means that there are two possible positions of contact the switch can be in. So in one state the signal pin is connected to one contact, and in another state, it’s connected to the other. Switches with multiple throws are used to direct signals from one place to another, not just make or break a single circuit.
  • Electrical Rating: The electrical rating of a switch is important to be aware of, especially when switching large loads. A switch has conductive contacts inside it which are rated to a certain amount of current and voltage (power), which should not be exceeded for safety reasons. You can destroy a switch by melting the internal mechanisms if it isn’t adequately rated. Generally, switches fall into 3 categories for rating: small signal, medium power, and mains power. Small switches such as tactile toggle switches, are only rated for milliamps and low voltage (usually under 24V), medium power switches are usually designed for high current DC applications like robotics where you will need to control the supply of 12V-50V batteries and supplies pulling several amps, and large power switches are usually rated for mains voltages and AC current draws (240V, 10A).

To give you a bit of an idea on how the switches work here's some schematic images of how the poles are arranged for double throw switches:

Single Pole Double Throw (SPDT):

SPDT switch schematic

Double Pole Double Throw (DPDT):

DPDT switch schematic

Triple Pole Double Throw (3PDT):

3PDT switch schematic

Different Types of Switches

So now that we’ve got a grasp of what’s important when looking for a switch, let’s take a look at some of the different types of switch available. When referring to switches, they are usually notated by the poles are throws such as ‘xPxT’, so a switch with a single throw and two poles would be SPDT (Single Pole, Double Throw) or 3PDT (Triple Pole, Double Throw). That’s how switches are identified, not let’s look at a variety of the different switches that we carry:

As you can see, each of these switches is different, with a different number of poles, throws, ratings, actuators, and can look physically different. For example, the rocker switch is rated to carry mains loads and is a good choice for the main power switch on something using a 240V input, whereas the humble tactile switch is the mainstay of breadboards and prototyping. Even the arcade style joystick is simply made up of four switches, one in each direction, when by pushing the joystick, you are making/breaking contact with those switches.

Something to consider when choosing a switch for your project is the jitter of the switch. When you press/pull/move a switch, the contacts don’t just touch and then stop, like any mechanical object, they have inertia. This means that there is actually a slight ‘bouncing’ of the contacts before they finally settle. This bouncing usually occurs over a matter of microseconds but is still present, and digital system which uses switches for input, run fast enough to detect this bounces and cause false-positive triggers. For this reason, de-bouncing is utilized, either using hardware components (RC filters) or software, but that’s another article in itself. Just bear in mind that the high rating a switch carries, the heavier the contacts in it are, so the greater their momentum, and bouncing. For example, a common issue in audio circuits such as guitar effects pedals is the ‘pop’ which occurs when stepping on the heavy duty footswitch used to activate the effect. Whilst there are a few different causes for this (such as capacitor leakage), while the contacts are bouncing around, they create a capacitance (a capacitor is just two conductors separated by an insulator), which can introduce stray voltages into the signal.

Debouncing however, is a whole topic in itself, so we'll be taking a look at that in another tutorial, but something to be aware of when designing something that uses switches. Hopefully this article has given you a good overview of different types of switches, and how they can be used in electronics projects. Happy making!

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