Resistors are the electronic components that resist the flow of electrons in a medium. In simple words, they can be thought of as simple “road blocks” to the flow of electrons. In any electronic circuit, every component may individually require different input value. One solution can be providing this input using different value of batteries. A more sophisticated solution is using resistors before each component to drop some voltage so that input to the component matches its specifications. This is the most important application of resistor. Further, it also saves the circuit element from receiving extra power that can cause damage to the component.

In short, they are used to limit the current to an adjoining component, reduce the voltage in some part of circuit, used in series with a sensitive component to protect it from burn-out. In your workshop, you will use the resistor to limit the current to the LEDs and photo-diode.

The colour code table is given as follows:


Variable resistor: A variable resistor, called a potentiometer, allows for the continual adjustment from virtually no (zero) ohms of maximum value. The potentiometer usually has the maximum value printed on it somewhere. It is also called like 10k, preset which means it can have resistance from 0 Ohm to 10 Kohm



Capacitor: Capacitor is a device that stores charge. We will learn capacitors with the help of water-tank analogy. Charging of capacitor is similar to collection of water in the tank,

Whereas discharging of capacitor is similar to the leakage of water through the point hole as shown in figure

The 2 types of capacitors we frequently use in circuits are ceramic and electrolytic capacitors. While ceramic capacitors do not have a fixed polarity; electrolytic capacitors should be connected in their specified polarities only else they might blow off! This polarity is usually provided on the side of the capacitors ‘corresponding leg.


Electrolytic cap with –ve polarity leg seen                      Ceramic cap with value 15×104 pF


In our workshop, we have used the capacitor for filtering purposes. It has been used to filter ripples from the signal. As you can see from the figure, after the signal is passed through the capacitor, it has fewer ripples. So, a capacitor here helps us get a signal that is less fluctuating in nature, so that output is a steady dc voltage. We are using ceramic capacitors as they come in low values as in pF range.



Breadboard is a base that is used for making the circuits. It’s easy to use, durable and reliable. The metal strip inside the breadboard makes it pretty easy to use. They allow us to take multiple connections from a single point without any need of soldering as in PCBs. So, it is advisable to always check the circuit on a breadboard before making it on a PCB.

According to the fig. which is shown at the bottom of this topic, breadboard is divided equal side from the middle of the white strip, Inner both part is respectively horizontally connected through the copper wire and the outer both part is respectively vertically connected through copper wire.



You must have heard about photo-synthesis. Photo means light. In photo-synthesis, light is absorbed by plants to synthesize it into their foods. Similarly, photo-diode absorbs light energy and converts it into current.

A photodiode is a semiconductor device that converts light into current. The current is generated when photons are absorbed in the photodiode. A small amount of current is also produced when no light is present. Photodiodes usually have a slower response time as its surface area increases. The common, traditional solar cell used to generate electric solar power is a large area photodiode, most of the time the connections is reverse biased (Anode to negative terminal and cathode to positive). Now the question is why we reverse bias the photodiode?

The Answer is to get light even if the intensity of light is very less or the precise answer is to make photodiode more sensitive or to increasing its sensitivity.

We use this photodiode in our line follower project to make our robot eye by using photodiode and IR LED(infrared light emitting diode) so that our line follower can see and differentiate the BLACK line and WHITE background.IR LED will be discussed later in this book.



LED stands for “Light-Emitting Diode.” An LED is an electronic device that emits light when an electrical current is passed through it. It works on low current supply that is why it is energy efficient and emitting lights same as compared to our regular light source. Early LEDs produced only red light, but modern LEDs can produce several different colors, including red, green, and blue (RGB) light. Recent advances in LED technology have made it possible for LEDs to produce white light as well.

LEDs are commonly used for indicator lights (such as power on/off lights) on electronic devices. They also have several other applications, including electronic signs, clock displays, and flashlights. Since LEDs are energy efficient and have a long lifespan (often more than 100,000 hours), they have begun to replace traditional light bulbs in several areas. Some examples include street lights, the red lights on cars, and various types of decorative lighting. You can typically identify LEDs by a series of small lights that make up a larger display. For example, if you look closely at a street light, you can tell it is an LED light if each circle is comprised of a series of dots.

The energy efficient nature of LEDs allows them to produce brighter light than other types of bulbs while using less energy. For this reason, traditional flat screen LCD displays have started to be replaced by LED displays, which use LEDs for the backlight. LED TVs and computer monitors are typically brighter and thinner than their LCD counterparts.

We use LEDs in our line following project to indicate that our robot’s eye is properly working or not.

If any object comes across the eye of our robot we will see that LED will glow up, if it is not glowing we have to see our circuit again because it may have wrong or loose connections.

The connection of LED is forward bias which means that ANODE part of the LED is connected to the positive terminal of the battery (but before putting 9 volt battery positive terminal to the ANODE part of LED, be aware that our LED can’t take so much load from the 9 volt battery, so we need to connect any one side of 1 kohm of resistor to the positive terminal of  battery and ANODE part of the LED  and the sequence will be :- positive terminal of 9 volt battery connected to any one side of the 1 kohm resistor and then the free side of the resistor is connected to the positive side of the LED or ANODE part). At last negative part or CATHODE of the LED will be grounded.





photoresistor or light-dependent resistor (LDR) or photocell is a light-controlled variable resistor. The resistance of a photo resistor decreases with increasing incident light intensity. In other words, it exhibits photoconductivity. A photoresistor can be applied in light-sensitive detector circuits, and light- and dark-activated switching circuits.

A photoresistor is made of a high resistance semiconductor. In the dark, a photoresistor can have a resistance as high as a few megaohms (MΩ), while in the light, a photoresistor can have a resistance as low as a few hundred ohms. If incident light on a photoresistor exceeds a certain frequencyphotons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electrons (and their hole partners) conduct electricity, thereby lowering resistance. The resistance range and sensitivity of a photoresistor can substantially differ among dissimilar devices. Moreover, unique photoresistors may react substantially differently to photons within certain wavelength bands.