I’m always amazed how one thing leads to another. As part of documenting our ongoing project to build a 4-bit computer from the ground up, for example, I realized that we would have to address a multitude of design considerations. One of these topics is that of switch bounce, which refers to the fact that when we activate or deactivate a switch, it may turn on and off multiple times very quickly.
We will discuss switch-related terminology in Part 2 of this mini-series, followed by bounce/debounce techniques (both hardware and software) in Part 3. In this column, we will commence by considering what we mean by the term “switch” at the most fundamental level. Also, we will take a high-level look at a few examples of the different types of switches that we may encounter as we meander our way through the world.
Topics covered in this column are as follows:
•What is a switch?
•Morse code keys
•Micro switches and limit switches
Note that I’ve augmented some of the topics below with comments that include terms like “make” and “break”; “open” and “closed”; “single-pole, single-throw” (SPST) and “single-pole, double-throw” (SPDT); “break-before-make” (BBM) and “make-before-break” (MBB); and “normally open” (NO) and “normally closed” (NC). For the purposes of this column, these comments are for reference only; we will explain what they all mean in Part 2 of this mini-series.
What is a switch?
In the context of electrical engineering, a switch is an electrical component that can “make” or “break” an electrical circuit, interrupting the current or diverting it from one conductor to another.
When most people hear the word “switch,” they instinctively think of a wall-mounted version used to control the lights. These devices are typically of a type known as a toggle switch. As we will see, however, there are many different types of switches that are intended to address different design requirements, deployment environments, and usage scenarios.
Morse code keys
One very early type of switch, called a Morse key, was (and still is) used to transmit messages in Morse code, which was invented by the American painter and inventor Samuel Finley Breese Morse in 1837.
The most common types of toggle switches are SPST and SPDT, but there are many more possibilities, including single-pole, center-off (SPCO), which is like an SPDT but with three positions for the lever.
The contact on an SPST toggle switch is typically NO, but NC versions are available. The contacts on SPDT toggle switches are typically BBM, but MBB versions are available.
To see the rest of this article and to follow the series, see EEWeb.