Alternate (2-way) Switch

The alternate or 2-way switch is basically a configuration of two switches which turn on or off the same load. This configuration is useful in many practice cases. The switches are placed on different physical locations, but they are both connected to the same load. A common use of this configuration can be found in our homes or buildings, mostly for switching on and off the light in some room, halls, stairs, etc.
For example, when we approach to the stairs for the next floor in our home, we want to turn on the light at the beginning of the stairs, and then to turn off the same light at the end, when we arrived at the top of them, and vice versa. In order to achieve this, we need to connect two switches which will power on/off the light. One switch will be placed at the bottom of the stairs, and the other at the top. These switches should be connected to each other in a way that when we turn on/off each one of them, they will bring the load to the opposite state of its current state. In other words, if the light is turned on, no matter which switch we will press now, the switch should turn off the light. After the light is turned off, again, no matter which switch we will press, it should turn on the light again, and so on.

Picture 1: Alternate 2-way Switch configuration diagram


So, in order to achieve this functionality, we need to connect the switches in a proper way. One configuration diagram for alternate switch is shown on Picture 1. As we can see from the picture, we have two switches SW1 and SW2 connected to each other and to the load, the light bulb. The switches in this configuration are 2-way switches, which have 3 connection terminals, marked as 1, 0 and 2. So, when the switch is in position 1, it has connection between the terminals 0 and 1, and when is in position 2, it has connection between the terminals 0 and 2. As shown on the diagram, the hot line from the power supply, the phase R, is connected to the terminal 2 of the switch SW1. Then, the terminals of the both switches are connected to each other: terminal 0 of SW1 is connected to the terminal 0 of SW2, terminal 1 of SW1 to the terminal 1 of SW2, and terminal 2 of SW1 to the terminal 2 of SW2. The hot terminal of the light bulb is connected to the terminal 1 of the second switch SW2, and the neutral line is connected directly to the light. The currently shown positions of the switches on Picture 1, provide the hot line to come to the bulb, so the light is turned on. In this state, the switch SW1 is in position 2 and the switch SW2 is in position 1. At this state, if we change the position of one of the switches, no matter which of them, we will cut the hot line path, and the light will turn off. After that, it's easy to see, that each next change in the position of the switches, no matter which of them, will altere the state of the light, turning it on and off, alternating. That's why we call this switch configuration an alternating switch. The configuration of the switches and connections seems to be simple, but the functionality is very practical and effective.