LAMPS CONTROLLED FROM TWO POINTS: THE SPDT SWITCH (PRACTICE)

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Welcome to the home’s electrical systems course organized by NPR Online Technologies!

Before starting the lesson, we recommend that you subscribe to our youtube channel and visit the main webpage of the course.

In this lesson we will see how to control a group of lamps from two distinct points by using a pair of single-pole double-throw switches, just as it has been illustrated in the theoretical lesson to which we refer for further information.

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In this lesson we will connect the two single-pole double-throw switches according to the diagram illustrated in the theoretical lesson.

In particular, we must connect the hot wire to the permanent contact of the first switch.

Then we connect the two SPDT switches with two wires.

We then connect the permanent contact of the second switch to the light bulb.

The other terminal of the light bulb must finally be connected to the neutral wire.

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As seen in the theoretical lesson, the installation of the circuit requires three wall chases in order to embed the electric conduits.

Let us now consider another possible configuration. In particular, let’s suppose that we have a room with the two entrances indicated in the figure.

In this case, we can create the circuit with a different configuration of the three electric conduits.

In particular, it is observed that the configuration of the electric conduits has changed but the electrical circuit has not changed.

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We have, in fact, the hot wire connected to the permanent contact of the first switch.

Then we have the connection between the two switches by using two distinct wires,

and finally, we have the permanent contact of the second switch connected to the light bulb.

The other terminal of the light bulb is connected to the neutral wire.

Furthermore, we observe that with the configuration illustrated in the figure, the light bulb is off,

 because the hot wire continuity is interrupted at the second switch.

By flipping one of the two switches,

we create electrical continuity between the hot wire and the light bulb, and consequently the light bulb will turn on.

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Well, the goal of this lesson is to build this circuit.

In the following screen, we show how the circuit will appear at the end of this lesson.

It can be noted, that the arrangement of the elements reflects the connection diagram illustrated in the previous figures.

We have the junction box at the top, the first switch on the right, the second switch at the bottom, and the light bulb in the center.

Let’s proceed step by step in the realization of this circuit.

We assume to start from the point where we concluded the previous practical lesson, where we had a junction box at the top, and a switch positioned to the right of the work table.

Starting from this configuration, we take a second electric box from and fix it at the bottom on the work table.

We make a hole on the right side of the electric box. In this way, we are preparing this box for the necessary connections.

In particular, in this box we will install one of the two single-pole double-pole switches.

We have seen that these switches have three terminals, and we have also seen that it is possible (or in some countries it could be mandatory) to use different colors for the wires used to make the connections of the switches.

In particular, in our circuit diagram, we have used a brown color for the hot wire connected to the first switch, then we have used a black wire for the two wires that connect the two switches, and finally we have used a gray color for the wire that connects the second switch to the lamp.

Note that brown, black and gray are colors allowed in Europe for the hot wire.

The use of different colors for the wires could be a good practice because it allows us to easily identify the various wires inside the system. However, from a conceptual point of view, nobody prevent us from using a single color.

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The use of a single color allows us to buy only one color and for these tutorials we believe it is the most convenient solution.

Let’s see now how to use the three wires shown in the following figure.

We have prepared these wires with an appropriate length. The longest one will be used for the connection with the light bulb. The other two wires will be connected to the second single-pole double-throw switch that will be installed in the electric box placed on the right of the work table.

First of all, we want to observe that the two wires that connect the two single-pole double-throw switches are typically known as traveler wires,

and consequently the two terminals on the switch body are known as the traveler terminals.

The third terminal of the single-pole double-throw switch, is connected to the permanent contact of the switch and is generally known as common terminal.

We will see soon that it is a useful terminology for our installation.

For the moment, let’s continue with the installation procedure.

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In particular, we have to cut a piece of  flexible PVC tube and insert the three brown wires that we have prepared in the previous step.

After this step, we must insert the single-pole double-throw switch in the appropriate plastic support.

We have seen in the previous practical lesson, that, when we are using a single device, its correct position in the plastic support, is the central one.

However, from a conceptual point of view, there is no difference if we place the single device in another position. For example, we can place the switch on the right side of the plastic support. Once again, it is not a good idea, but in our lesson we can do it!

We can then proceed with the connection of the wires to the three switch.

We know that the wire that will be connected to the light bulb must be connected to the common terminal of the switch,

while the other two wires must be connected to the other two terminals of the switch.

In this case, we can see how valuable it is to have cables with different colors.

Among the three cables available, we have to identify the one to connect to the common terminal and those to connect to the traveler terminals.

In this specific case, the problem has a simple solution because we know that the longest wire will be used for the connection with the light bulb.

For this reason, we can connect it to the common terminal of the switch,

and then we can proceed with the connection of the two traveler wires.

The identification of wires is very simple in our case because they are short, and a simple visual inspection on our work table allows to identify them.

Of course, this is not the typical case of real systems where we can have very long wires. In this case we can consider using different wire colors or alternatively we can mark the ends of the wires with colored insulating tape.

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Once the connections to the first single-pole double-throw switch have been completed, we can fix it to the corresponding electric box.

Let’s now consider the second single-pole double-throw switch, that must be installed in the electric box located to the right of the work table.

This electric box is available from the previous exercise and we therefore find installed in this box

the single-pole single-throw switch used in the previous practical lesson.

Of course, in its place we have to insert the second single-throw double-throw switch in order to proceed with the installation of the system of the present lesson.

For this purpose, we disconnect the wires from the single-pole single-throw switch,

and then remove the single-pole switch from the support holder.

This can be done with a screwdriver, by releasing the plastic tab that holds the switch locked.

Once this operation has been carried out, we can insert the single-pole double-throw switch in the same plastic support. We can easily recognize the switch by the presence of the three contacts.

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The connection of this second switch is very simple to make. As indicated in the circuit diagram, we must connect the traveler wires coming from the first switch to the two traveler terminals of the second switch,

while the hot wire coming from the junction box will be connected to the common terminal of the single-pole double-throw switch.

Finally, we have the neutral cable also coming from the junction box which is connected directly to the light bulb

but we need to pass it through the electric box of the second switch. In particular, we will see soon that the connection of the blue cable to the light bulb has already been made in the previous tutorial.

Let’s identify all the wires in our electric box. First of all it is interesting to note that even in this simple circuit, we have five wires in the electric box.

In particular, the wire labeled with the number one, comes from the common terminal of the first switch and must be connected to the light bulb, as indicated by the red connection in the following figure.

We can see that this connection, starts from the electric box of the first switch, then it passes through the electric box of the second switch,  and finally reaches the light bulb.

The wires labeled with numbers 2 and 3 are the traveler wires coming from the first switch, and must be connected to the traveler terminals of the second switch.

The wire labeled with the number 4, is the hot wire

which is coming from the junction box,

and must be connected to the common terminal of the single-pole double-throw switch.

Finally, we have the blue wire which is the neutral wire that is already connected to the light bulb. As already said, it is an operation carried out in the previous practical lesson.

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Well, we have identified all the wires in our electrical box and we can then proceed with the connection of the wires to the switch.

Finally, we can connect to the light bulb the wire coming from the common terminal of the first switch.

Once this last connection has been made, we just have to fix the plastic support to the electrical box using the appropriate screws.

If you have followed our instructions, the final circuit will look like the one shown in the figure, and you will then be able to control the light bulb by means of the two switches.

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Well, we have come to the end of this lesson. We have analyzed the circuit that allows to turn on and off a groups of lamps from two distinct places by using two single-pole double-throw switches.

We suggest you immediately consolidate the concepts you have just learned by carefully reviewing this lesson and above all by going through all the videos of our course, by accessing our playlists.

We remind you that in this lesson the concepts have been illustrated on a practical level. Do not forget to deepen the concepts learned, also looking at the theoretical lessons of the course.

We remind you that our website is active with numerous insights on these issues.

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Thanks for your attention and … see you next time!

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