Introduction

In many circuits, a button is used to allow one to control a voltage (or, correspondingly, the flow of current). This voltage might be used to directly power something, for example, an LED. Alternatively, this voltage might serve as an input signal that can be read by a chipKIT™ (digital) I/O pin (and then the chipKIT board can take an action based on this signal).

Here we want to consider a particular kind of button, a push-to-close button. There are many different configurations for push-to-close buttons. The one we will consider here is shown in Fig. 1. There are four distinct legs, or “terminals,” on the device. The terminals are grouped so that two are together on one side of the button and two are on the opposite side. In Fig. 1, you can see two of these terminals. As shown, the left side has two terminals, as does the right side (although the left-side terminals are not visible in the figure).

The terminals on the same side of the button are not connected unless the button is pressed. However, the terminals on the left and right sides that are directly across from each other are always electrically connected, regardless of whether or not the button is pressed. The internal layout of the button is depicted in the schematic representation shown in Fig. 2. Keep in mind that different symbols are sometimes used to represent the same physical device. A common schematic representation of this type of button is shown in Fig. 3.

An alternate symbol, shown in Fig. 4, appears in some of these pages and is slightly more representative of the underlying physical device. Note that when the button is pressed, all the terminals are electrically connected. For the purpose of our analysis, the pair of terminals that are always connected (regardless of whether or not the button is pressed) can be thought of as just a single connection, as shown in Fig. 4.

In contrast to different symbols being used to represent the same physical device, sometime different physical devices that have the same electrical characteristics can be represented with the same schematic symbol. For example, the symbol in Fig. 3 might represent a button or it might be used to represent a switch, i.e., a device that is either electrically open or closed but doesn't require continuous physical input to maintain one position or the other.

In contrast to push-to-close buttons, there are also push-to-open buttons. These two types of buttons may appear physically similar, but for a push-to-open button the terminals are electrically connected unless the button is pressed. When the button is pressed, an open circuit is formed.

In summary, when a push-to-close button is pressed, a small piece of conductive material makes a physical connection between the “top” and “bottom” terminals. This connection is what allows current to flow between the terminals of the device. When the button is not pressed, no current can flow.