A collection of electronic components that have been assembled and interconnected to perform a given function is commonly referred to as a circuit. Electronic circuits can be divided into two broad categories: digital and analog. We will be focusing on digital but will touch on analog because many applications require a knowledge of both. The word circuit derives from the fact that electric power must flow from the positive terminal of a power source through one or more electronic devices and back to the negative terminal of a power source, thereby forming a circuit. If the connections between an electronic device and either the positive or negative terminals of a power supply are interrupted, the circuit will be broken and the device will not function.
Many different types of components and devices can be found in modern circuits including; resistors, capacitors, inductors, semiconductor devices like diodes, transistors, integrated circuits, transducers like microphones, light sensors, motions sensors, actuators like motors, solenoids, and various other devices like heating and lighting elements. Devices in a circuit are connected to one another by means of electrical conductors, or wires. These wires can move electric currents between various points in a circuit. Once a wire connects two or more devices, the wire and all attached device connectors are said to form a single circuit node, or net. Any electrical activity on a given net is communicated to all devices attached to the net. Certain nets provide electric power to devices, and other nets carry information between devices. Nets that carry information are called signals, and signals transport information encoded as voltage levels around a circuit. Signal nets typically use smaller conductors, and transport very small currents. Nets that carry power are called supply rails (or just rails) and supply rails transport electric power around a circuit. Rails typically use much larger conductors that signal nets, because they must transport larger currents.
Electric circuits use electric power to perform some function like, energize a heating or lighting element, turn a motor, or create an electromagnetic filed. Electronic circuits differ from electric circuits in that they use devices that can themselves be controlled by other electric signals. Restated, electronic circuits are built from devices that use electricity to control electricity. Most electronic circuits use signals that are within 5 to 10 volts of ground; in recent years, circuit signals are within 3 to 5 volts of ground. Some electronic circuits represent information encoded as continuous voltage levels that can wander between the high and low voltage supply rails—these are called analog circuits. As an example, a sound pressure level transducer (i.e., a microphone) might drive a signal between 0V and 3.3V in direct proportion to the detected sound pressure level. In this case, the voltage signal output from the microphone is said to be an analog of the sound pressure wave itself. An analog quantity is one having continuous values. Digital circuits use only two distinct voltage levels to represent information. Most often, these two voltage levels use the same voltages supplied by the power rails. In these circuits all information must be represented as binary numbers (explained later), with a signal at 0V (or ground) representing one kind of information, and a signal at 3.3V (or whatever the upper voltage supply rail provides) representing the other kind of information. In this series of modules, we will confine our discussions to digital circuits.