Help with Switches

A little bit about switches in general, but particularly designed to help you connect a switch to your Arduino, for example when using the Arduino button tutorial.

Very simply, the function of a switch is to safely make or break an electrical connection. When switches are used in a circuit, the switch can control whether the circuit is opened or closed.

However, that simple function is often obscured by the huge range of switches available. For instance switches can be momentary, like a doorbell button or a car starter switch, or can be toggle like a light switch (it stays on without you having to keep pressing it).

The important part that makes the electrical connection is called a contact, and a simple switch consists of two contacts which are connected or disconnected as the switch is turned on or off, respectively. The contacts are inside the switch, and there will be one terminal, or lead protruding from the switch for each contact.

It turns out it is often useful to have multiple switches in one unit, all activated by the same lever. We refer to this as a single switch with multiple contacts. An example of this is the so-called 3 way switch you often find in hallways or staircases, where there are two switches at either end of the hall connected such that either one can turn the light on or off regardless of the position of the other switch.

Why should you worry about this complication, if all you need is a simple switch? It turns out that these multiple switches are actually more common than the single switch, so it is very likely that the switch in your kit is of the multiple switch variety. In this case your switch will have more than two terminals, and you need to know which terminals to use.

There are two fundamental ways multiple contacts can be configured:

  1. One main contact can be selectively connected to one of a number of secondary contacts. Think of this as a selector switch. The main contact is called the pole and the secondary contacts are called the throws. The single pole can be connected (thrown) to any one of the multiple throws.
  2. A single physical switch can contain two or more independent switches, all controlled by the same lever. In this case, the lever is attached to multiple main contacts, or multiple poles, each of which can be connected to their own independent contacts, or throws.

Each pole and throw will have its own terminal, which is what makes it difficult for you.

Confused by my description? Don’t worry. For some excellent pictures, examples, and  descriptions look here.

The important part is that every complicated switch (multiple poles and/or multiple throws) has in in at least one simple switch (single pole, single throw, or SPST). All you have to do is identify which of the terminals to use.

After you get some experience, you will learn to look at a switch and make a pretty good educated guess as to which of the terminals are poles, which are throws, and which poles connect to which throws.

But you don’t need to worry about that right now, because you have a multimeter in your kit, which can help you identify the two terminals of one of the internal SPST (Single Pole Single Throw) switches.

Some tutorials on how to use a multimeter to test a switch are here and here. Or ask me.

It’s just a process of elimination. Place your multimeter in the resistance or continuity mode. (Why resistance? Remember that a switch, when closed, represents a closed circuit, and so has zero resistance, while an open switch is an open circuit, and has extremely high, or infinite, resistance.)

Next, you will need to attach the multimeter probes to the switch. You might find it easier to use the jumper cables, because you will need your hands free to manipulate the switch.

Pick any two terminals, and attach the leads. Note whether you measure an open circuit or a closed circuit. Now, move the switch lever to the other position, and see if it changes. If it does, you have found one of the internal SPST switches. You are done.

If not, move one of the leads to another terminal, and try again. Keep going until you find one of the internal SPST switches.

The two terminals that are connected to the internal SPST switch are the ones you use. You can ignore all other terminals.

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