Who discovered voltage? Okay, that's not important now, let's understand it first. The basic concept follows the physics behind the flow of water. Water can flow in two ways; one is a waterfall (for example, from a mountain top to the ground) and the second is forceful flow using a water pump. The concept behind understanding voltage is similar. Voltage is the potential difference between two points, which means that a voltage difference allows the flow of charges (electrons) from the higher potential to the lower potential. To understand the preceding example, consider lightning, which can be compared to a waterfall, and batteries, which can be compared to a water pump. When batteries are connected to a circuit, chemical reactions within them pump the flow of charges from the positive terminal to the negative terminal. Voltage is always mentioned in volts (V). The AA battery cell usually supplies 3V. By the way, the term voltage was named after the great scientist Alessandro Volta, who invented the voltaic cell, which was then known as a battery cell.

Current is the flow of charges (electrons). Whenever a voltage difference is created, it causes current to flow in a fixed direction from the positive (higher) terminal to the negative (lower) terminal (known as conventional current). Current is measured in amperes (A). The electron current flows from the negative terminal of the battery to the positive terminal. To prevent confusion, we will follow the conventional current, which is from the positive terminal to the negative terminal of the battery or the source.

The meaning of the word "resist" in the Oxford dictionary is "to try to stop or to prevent." As the definition says, a resistor simply prevents the flow of current. When current flows through a resistor, there is a voltage drop in it. This drop directly depends on the amount of current flowing through resistor and value of the resistance. There is a formula used to calculate the amount of voltage drop across the resistor (or in the circuit), which is also called as the Ohm's law (V = I * R). Resistance is measured in ohms (Ω). Let's see how resistance is calculated with this example: if the resistance is 10Ω and the current flowing from the resistor is 1A, then the voltage drop across the resistor is 10V. Here is another example: when we connect LEDs on a 5V supply, we connect a 330Ω resistor in series with the LEDs to prevent blow-off of the LEDs due to excessive current. The resistor drops some voltage in it and safeguards the LEDs. We will extensively use resistors to develop our projects.

A resistor dissipates energy in the form of heat. In contrast to that, a capacitor stores energy between its two conductive plates. Often, capacitors are used to filter voltage supplied in filter circuits and to generate clear voice in amplifier circuits. Explaining the concept of capacitance will be too hefty for this book, so let me come to the main point: when we have batteries to store energy, why do we need to use capacitors in our circuits? There are several benefits of using a capacitor in a circuit. Many books will tell you that it acts as a filter or a surge suppressor, and they will use terms such as power smoothing, decoupling, DC blocking, and so on. In our applications, when we use capacitors with sensors, they hold the voltage level for some time so that the microprocessor has enough time to read that voltage value. The sensor's data varies a lot. It needs to be stable as long as a microprocessor is reading that value to avoid erroneous calculations. The holding time of a capacitor depends on an RC time constant, which will be explained when we will actually use it.

Now, there is an interesting point to note: when there is voltage available on the terminal but no components are connected across the terminals, there is no current flow, which is often called an open circuit. In contrast, when two terminals are connected, with or without a component, and charge is allowed to flow, it's called a short circuit, connected circuit, or closed circuit.

In electrical theory, when the current flowing through a component does not divide into paths, it's a series connection. Also, if the current flowing through each component is the same then those components are said to be in series. If the voltage across all the components is the same, then the connection is said to be in parallel. In a circuit, there can be combination of series and parallel connections. Therefore, a circuit may not be purely a series or a parallel circuit. Let's study the circuits shown in the following diagram:

Series and parallel connections

At the first glance, this figure looks complex with many notations, but let's look at each component separately. The figure on the left is a series connection of components. The battery supplies voltage (V) and current (I). The direction of the current flow is shown as clockwise. As explained, in a series connection, the current flowing through every component is the same, but the voltage values across all the components are different. Hence, V = V1 + V2 + V3. For example, if the battery supplies 12V, then the voltage across each resistor is 4V. The current flowing through each resistor is 4 mA (because V = IR and R = R1 + R2 + R3 = 3K).

The figure on the right represents a parallel connection. Here, each of the components gets the same voltage but the current is divided into different paths. The current flowing from the positive terminal of the battery is I, which is divided into I1 and I2. When I1 flows to the next node, it is again divided into two parts and flown through R5 and R6. Therefore, in a parallel circuit, I = I1 + I2. The voltage remains the same across all the resistors. For example, if the battery supplies 12V, the voltage across all the resistors is 12V but the current through all the resistors will be different. In the parallel connection example, the current flown through each circuit can be calculated by applying the equations of current division. Give it a try to calculate!

When there is a combination of series and parallel circuits, it needs more calculations and analysis. Kirchhoff's laws, nodes, and mesh equations can be used to solve such kinds of circuits. All of that is too complex to explain in this book; you can refer any standard circuits-theory-related books and gain expertise in it.

Pull-up and pull-down resistors are one of the important terminologies in electronic systems design. As the title of this section says, there are two types of pulling resistors: pull-up and pull-down. Both have the same functionality, but the difference is that pull-up resistor pulls the terminal to the voltage supplied and the pull-down resistor pulls the terminal to the ground or the common line. The significance of connecting a pulling resistor to a node or terminal is to bring back the logic level to the default value when no input is present on that particular terminal. The benefit of including a pull-up or pull-down resistor is that it makes the circuitry susceptible to noise, and the logic level (1 or 0) cannot be changed from a small variation in terms of voltages (due to noise) on the terminal. Let's take a look at the example shown in the following figure. It shows a pull-up example with a NOT gate (a NOT gate gives inverted output in its OUT terminal; therefore, if logic one is the input, the output is logic zero). We will consider the effects with and without the pull-up resistor. The same is true for the pull-down resistor.

Connection with and without pull-up resistors

In general, logic gates have high impedance at their input terminal, so when there is no connection on the input terminal, it is termed as floating. Now, in the preceding figure, the leftmost connection is not recommended because when the switch is open (OFF state), it leaves the input terminal floating and any noise can change the input state of the NOT gate. The reason of the noise can be any. Even the open terminals can act as an antenna and can create noise on the pin of the NOT gate. The circuit shown in the middle is a pull-up circuit without a resistor and it is highly recommended not to use it. This kind of connection can be called a pull-up but should never be used. When the switch is closed (ON state), the VCC gets a direct path to the ground, which is the same as a short circuit. A large amount of current will flow from VCC to ground, and this can damage your circuit.

The rightmost figure shows the best way to pull up because there is a resistor in which some voltage drop will occur. When the switch is open, the terminal of the NOT gate will be floated to the VCC (pulled up), which is the default. When the switch is closed, the input terminal of the NOT gate will be connected to the ground and it will experience the logic zero state. The current flowing through the resistor will be nominal this time. For example, if VCC = 5V, R7 = 1K, and I = V/R, then I = 5mA, which is in the safe region. For the pull-down circuit example, there can be an interchange between the switch and a resistor. The resistor will be connected between the ground and the input terminal of the NOT gate. When using sensors and ICs, keep in mind that if there is a notation of using pull-ups or pull-downs in datasheets or technical manuals, it is recommended to use them wherever needed. In the next section, we will use the pull-up resistor in one of the communication protocols.