Arduino Irrigation Control



Arduino Based Irrigation Control

College Logo




3Block diagram9
4Component list11


Sri Sukhmani Institute of Engg. & Technology, Derabassi

This is certify that

Mr.Harpreet Singh(Univ. R.No.-                 )

Have satisfactorily completed their Project on


In partial fulfillment for the Award of

 B.Tech. (Electronics & Communication)

During the Acadmic Year(2005)

Guide                                                                                 H.O.D   


I take this opportunity to congratulate my students Mr. Harpreet Singh & Mrs.            For successfully completeting the project on “ IR COMMUNICATION IN DUPLEX MODE”  as partial fulfillment B.Tech.( Electronics & Communication) Course

It was great pleasure to work with them as a guide .They have really work hard for the completetion of the project.

I wish them “All THE BEST” for the future and hope they will work with same sincerity.       


Many individual have proudly influenced us during our undergraduate studies (B.Tech.) at SSIET, Derabassi and it is pleasurevto acknowledge thiesr guidance and support.In SSIET, I learned many things, like, the project training is mainly aimed at enabling the student to apply their theoretical knowledge to practical as “The theory is to know how and practical to do how” and to appreciate the limitation of knowledge gained in the class room to practical situation and to appreciate the importance of discipline, punctuality, team work, sense of responsibility, money, value of time, dignity of labour.

I will like to express my gratitude towards Miss Vimmy Bhatia who took keen interest in our project,Who helped me in every possible way and is source of inspiration for all the group members.

I would also like to thank Mr. Sanjeev Chopra(hod, Electronics & Communication) who motivated me to complete our  project with enthusiasm and hard work. He helped every time when I need

Text Box:        LCD 16x2

Block Diagram:

Text Box: Mositure sensor Plates

Text Box:        Relay Drive
Text Box:        Arduino Uno 3


Text Box:       Relay


Text Box:        Water Pump

Circuit Diagram:


in this project we will make a arduino based irrigation control system. Arduino is used as a mail control system. Metal plates will sens sand moisyiure. When it is more than a limit then out water pump will automatically switched off.

We will use arduino because it latest technology for electronics design and development. We selected irrigation based project because we want to launch some application which can be possible for agriculture work. We will show all values on HT44780 standard LCD 16×2.


In this project we will use arduino uno3 . Moisture plat will give input to Arduino at pin no A0 analog pin. We will get output from pin 5,4,3,2 for lcd data pins . We are connecting lcd with arduino in 4 bit mode. RS pin of lcd connected at pin 12 and en at pin 11 of enable. LCD 16×2 work on 5v DC . So we may need seperate supply for lcd or we can use from arduino module.

Relay will be connected at pin 13 of arduino. We cant connect relay directly with arduino. Because relay consumes more current. So we need a amplifier circuit. We normally call it relay drive circuit.

For relay drive circuit we will use BC npn 548 transistor.

A  1k ohm resistor will be used  also

Analog value will be monitored. We will show that analog value on lcd.

Arduino is an open source computer hardware and software company, project, and user community that designs and manufactures single-board microcontrollers and microcontroller kits for building digital devices and interactive objects that can sense and control objects in the physical world. The project’s products are distributed as open-source hardware and software, which are licensed under the GNU Lesser General Public License (LGPL) or the GNU General Public License (GPL),[1] permitting the manufacture of Arduino boards and software distribution by anyone. Arduino boards are available commercially in preassembled form, or as do-it-yourself kits.

Arduino board designs use a variety of microprocessors and controllers. The boards are equipped with sets of digital and analog input/output (I/O) pins that may be interfaced to various expansion boards (shields) and other circuits. The boards feature serial communications interfaces, including Universal Serial Bus(USB) on some models, which are also used for loading programs from personal computers. The microcontrollers are typically programmed using a dialect of features from the programming languages C and C++. In addition to using traditional compiler toolchains, the Arduino project provides an integrated development environment (IDE) based on the Processing language project.

The Arduino project started in 2003 as a program for students at the Interaction Design Institute Ivrea in Ivrea, Italy,[2] aiming to provide a low-cost and easy way for novices and professionals to create devices that interact with their environment using sensors and actuators. Common examples of such devices intended for beginner hobbyists include simple robots, thermostats, and motion detectors.

The name Arduino comes from a bar in Ivrea, Italy, where some of the founders of the project used to meet. The bar was named after Arduin of Ivrea, who was the margrave of the March of Ivrea and King of Italy from 1002 to 1014.


  • Moisture detection
  • moisture value on LCD
  • control on limit value

Hardware required-

Arduino board


Digital multimeter

Software required:

Arduino IDE 1.61

ORCAD for PCB design

Proteus for simulation


Arduino UNO3

LCD 16x 2

Relay Cube 12v 10A

Transistor npn BC548

Resistor 10k ohm ¼ watt, 1k 1/4watt, 470 ohm, 4.7k ohm

Capacitor 1000μ

Connecting wires

soldering Iron 25 watt

Soldering Iron

Arduino Uno:

Arduino is an open-source platform used for building electronics projects. Arduino consists of both a physical programmable circuit board (often referred to as a microcontroller) and a piece of software, or IDE (Integrated Development Environment) that runs on your computer, used to write and upload computer code to the physical board.

The Arduino platform has become quite popular with people just starting out with electronics, and for good reason. Unlike most previous programmable circuit boards, the Arduino does not need a separate piece of hardware (called a programmer) in order to load new code onto the board – you can simply use a USB cable. Additionally, the Arduino IDE uses a simplified version of C++, making it easier to learn to program. Finally, Arduino provides a standard form factor that breaks out the functions of the micro-controller into a more accessible package

Power (USB / Barrel Jack)

Every Arduino board needs a way to be connected to a power source. The Arduino UNO can be powered from a USB cable coming from your computer or a wall power supply (like this) that is terminated in a barrel jack. In the picture above the USB connection is labeled (1) and the barrel jack is labeled (2).

The USB connection is also how you will load code onto your Arduino board. More on how to program with Arduino can be found in our Installing and Programming Arduino tutorial.

Pins (5V, 3.3V, GND, Analog, Digital, PWM, AREF)

The pins on your Arduino are the places where you connect wires to construct a circuit (probably in conjuction with a breadboard and some wire. They usually have black plastic ‘headers’ that allow you to just plug a wire right into the board. The Arduino has several different kinds of pins, each of which is labeled on the board and used for different functions.

GND (3): Short for ‘Ground’. There are several GND pins on the Arduino, any of which can be used to ground your circuit.

5V (4) & 3.3V (5): As you might guess, the 5V pin supplies 5 volts of power, and the 3.3V pin supplies 3.3 volts of power. Most of the simple components used with the Arduino run happily off of 5 or 3.3 volts.

Analog (6): The area of pins under the ‘Analog In’ label (A0 through A5 on the UNO) are Analog In pins. These pins can read the signal from an analog sensor (like a temperature sensor) and convert it into a digital value that we can read.

Digital (7): Across from the analog pins are the digital pins (0 through 13 on the UNO). These pins can be used for both digital input (like telling if a button is pushed) and digital output (like powering an LED).

PWM (8): You may have noticed the tilde (~) next to some of the digital pins (3, 5, 6, 9, 10, and 11 on the UNO). These pins act as normal digital pins, but can also be used for something called Pulse-Width Modulation (PWM). We have a tutorial on PWM, but for now, think of these pins as being able to simulate analog output (like fading an LED in and out).

AREF (9): Stands for Analog Reference. Most of the time you can leave this pin alone. It is sometimes used to set an external reference voltage (between 0 and 5 Volts) as the upper limit for the analog input pins.

Reset Button

Just like the original Nintendo, the Arduino has a reset button (10). Pushing it will temporarily connect the reset pin to ground and restart any code that is loaded on the Arduino. This can be very useful if your code doesn’t repeat, but you want to test it multiple times. Unlike the original Nintendo however, blowing on the Arduino doesn’t usually fix any problems.

Power LED Indicator

Just beneath and to the right of the word “UNO” on your circuit board, there’s a tiny LED next to the word ‘ON’ (11). This LED should light up whenever you plug your Arduino into a power source. If this light doesn’t turn on, there’s a good chance something is wrong. Time to re-check your circuit!


TX is short for transmit, RX is short for receive. These markings appear quite a bit in electronics to indicate the pins responsible for serial communication. In our case, there are two places on the Arduino UNO where TX and RX appear – once by digital pins 0 and 1, and a second time next to the TX and RX indicator LEDs (12). These LEDs will give us some nice visual indications whenever our Arduino is receiving or transmitting data (like when we’re loading a new program onto the board).

Main IC

The black thing with all the metal legs is an IC, or Integrated Circuit (13). Think of it as the brains of our Arduino. The main IC on the Arduino is slightly different from board type to board type, but is usually from the ATmega line of IC’s from the ATMEL company. This can be important, as you may need to know the IC type (along with your board type) before loading up a new program from the Arduino software. This information can usually be found in writing on the top side of the IC. If you want to know more about the difference between various IC’s, reading the datasheets is often a good idea.

Voltage Regulator

The voltage regulator (14) is not actually something you can (or should) interact with on the Arduino. But it is potentially useful to know that it is there and what it’s for. The voltage regulator does exactly what it says – it controls the amount of voltage that is let into the Arduino board. Think of it as a kind of gatekeeper; it will turn away an extra voltage that might harm the circuit. Of course, it has its limits, so don’t hook up your Arduino to anything greater than 20 volts.

Component Details:


In this project firstly we use one step down transformer. Step down transformer step down the voltage from 220 volt Ac to  12 volt Ac. This Ac voltage is further converted into DC with the help of rectifier circuit. In rectifier circuit we use four diode. All the diodes  are arranges as a bridge rectifier circuit. Output of this rectifier is pulsating Dc. To convert this pulsating DC into smooth dc we use one capacitor as a filter components. Capacitor converts the pulsating Dc into smooth DC with the help of its charging and discharging effect.

Output of the rectifier is now regulated with the help of  IC regulator circuit. In this project we use positive voltage regulator circuit. Here we use three pin regulator. Output of this regulator is regulated voltage. If we use 7805 regulator then its means its is 5 volt regulator and if we use 7808 regulator then its means that it is 8 volt regulator circuit. In this project we use 5 volt dc regulated power supply for the complete circuit.

Rectifier Circuit:


Diodesare being used as the rectifier circuit. In electronics, a diode is a two-terminal electronic component that conducts primarily in one direction. It has low resistance to the flow of current in one direction, and high  resistance in the other. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. A vacuum tube diode has two electrodes, a plate (anode) and a heated cathode. Semiconductor diodes were the first semiconductor electronic devices. The basic function of diode is to convert AC into DC.

A full wave rectifier is consist of two diodes. It is a circuit arrangement which makes use of both half cycles of input alternating current (AC) and convert them to direct current (DC). This process of converting both half cycles of the input supply (alternating current) to direct current (DC) is termed full wave rectification.


It is consist of π Filter which is made of two capacitors. It is used to Produce a Stable regulated Dc supply which will be used to give power supply to the digital circuit.


A capacitor is a passive two-terminal electrical component that stores electrical energy in an electric field. The effect of a capacitor is known as capacitance. While capacitance exists between any two electrical conductors of a circuit in sufficiently close proximity, a capacitor is specifically designed to provide and enhance this effect for a variety of practical applications by consideration of size, shape, and positioning of closely spaced conductors, and the intervening dielectric material. A capacitor was therefore historically first known as an electric condenser. The physical form and construction of practical capacitors vary widely and many capacitor types are in common use. Most capacitors contain at least two electrical conductors often in the form of metallic plates or surfaces separated by a dielectric medium. A conductor may be a foil, thin film, sintered bead of metal, or an electrolyte. The nonconducting dielectric acts to increase the capacitor’s charge capacity. Materials commonly used as dielectrics include glass, ceramic, plastic film, paper, mica, and oxide layers.


A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical activity. Resistors are common elements of electrical networks and electronic circuits and are ubiquitous in electronic equipment. Practical resistors as discrete components can be composed of various compounds and forms. Resistors are also implemented within integrated circuits. The electrical function of a resistor is specified by its resistance: common commercial resistors are manufactured over a range of more than nine orders of magnitude. The nominal value of the resistance falls within the manufacturing tolerance, indicated on the component.


A light-emitting diode (LED) is a two-lead semiconductor light source. It is a p–n junction diode, which emits light when activated. When a suitable voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescent, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. LED’s are typically small (less than 1 mm2 ) and integrated optical components may be used to shape the radiation pattern.

Voltage Regulator:

A voltage regulator is designed to automatically maintain a constant voltage level. A voltage regulator may be a simple “feed-forward” design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

Electronic voltage regulators are found in devices such as computer power supplies where they stabilize the DC voltages used by the processor and other elements. In automobile alternators and central power station generator plants, voltage regulators control the output of the plant. In an electric power distribution system, voltage regulators may be installed at a substation or along distribution lines so that all customers receive steady voltage independent of how much power is drawn from the line.


Relays are electromechanical devices that use an electromagnet to operate a pair of movable contacts from an open position to a closed position. The advantage of relays is that it takes a relatively small amount of power to operate the relay coil, but the relay itself can be used to control motors, heaters, lamps or AC circuits which themselves can draw a lot more electrical power. The elector-mechanical relay is an output device (actuator) which come in a whole host of shapes, sizes and designs, and have many uses and applications in electronic circuits. But while electrical relays can be used to allow low power electronic or computer type circuits to switch relatively high currents or voltages both “ON” or “OFF”, some form of relay switch circuit is required to control it. The design and types of relay switching circuits is huge, but many small electronic projects use transistors and MOSFET’s as their main switching device as the transistor can provide fast DC switching (ON-OFF) control of the relay coil from a variety of input sources so here is a small collection of some of the more common ways of switching relays.

Relay Drive Circuit:

Relay is basically known as electrically controlled switch. It is used to switch the devices turn ON and OFF in a circuit where we are dealing with high voltage ratings. It has 5 pins, Pin number 1st and 3rd is internally connected through a coil and pin number 2 is known as common terminal. Other pins are 4th and 5th one is known as Normally closed and other one is Normally open. In normal mode, when we are not providing any kind of voltage to the Relay. The common pin is connected with the normally closed pin. In operating mode, when we are providing 12 volts DC voltage to the 1st pin of relay, which is connected with one of the terminal of the inbuilt coil and we get -ve voltage at the 3rd pin of the relay which is connected with the other end of the inbuilt coil through the relay driving circuit. It switches the common terminal of the relay from normally closed state to the normally open state. We can get the output from any of the terminal NC and NO to switch the circuit on and off.

The driver circuit is consist of two 1k ohm resistors and two transistors BC548 NPN and BC557 PNP. We are providing 5 volts dc supply at the emitter of the PNP transistor and ground at the emitter of the NPN transistor. The collector of the NPN transistor is connected to the relay coil of the relay. The collector of the PNP transistor is connected with the base of the NPN transistor Through 1k ohm resistor. When we give a low pulse at the base of the PNP transistor through 1K ohm transistor it gets triggered and start conducting and gives high pulse at the base of the NPN transistor which causes the NPN transistor to start conducting and its gives Low signal at the collector output which is attached with the coil of the relay. On the other side of the Relay we are giving 12 volts to DC which tends the common terminal of the Relay to switch from Normally Closed to Normally Open.

Details of RESISTORS

The flow of charge (or current) through any material, encounters an opposing force similar in many respect to mechanical friction. This opposing force is called resistance of the material. It is measured in ohms.  In some electric circuits resistance is deliberately introduced in the form of the resistor.

Resistors are of following types:

  1. Wire wound resistors.
  2. Carbon resistors.
  3. Metal film resistors.

Wire Wound Resistors:

Wire wound resistors are made from a long (usually Ni-Chromium) wound on a ceramic core. Longer the length of the wire, higher is the resistance. So depending on the value of resistor required in a circuit, the wire is cut and wound on a ceramic core. This entire assembly is coated with a ceramic metal. Such resistors are generally available in power of 2 watts to several hundred watts and resistance values from 1ohm to 100k ohms. Thus wire wound resistors are used for high currents.

Carbon Resistors:

Carbon resistors are divided into three types:

  1. Carbon composition resistors are made by mixing carbon grains with  

      binding material (glue) and moduled in the form of rods. Wire leads

      are inserted at the two ends. After this an insulating material seals the

      resistor. Resistors are available in power ratings of 1/10, 1/8, 1/4 ,

      1/2 , 1.2 watts and values from 1 ohm to 20 ohms.

  • Carbon film resistors are made by deposition carbon film on a ceramic         

rod. They are cheaper than carbon composition resistors.

  • Cement film resistors are made of thin carbon coating fired onto a

      solid ceramic substrate. The main purpose is to have more precise

      resistance values and greater stability with heat. They are made in a

      small square with leads.

Metal Film Resistors:

They are also called thin film resistors. They are made of a thin metal   coating deposited on a cylindrical insulating support. The high resistance values are not precise in value; however, such resistors are free of inductance effect that is common in wire wound resistors at high frequency.

Variable Resistors:

Potentiometer is a resistor where values can be set depending on the requirement. Potentiometer is widely used in electronics systems. Examples are volume control, tons control, brightness and contrast control of radio or T.V. sets.

Fusible Resistors:

These resistors are wire wound type and are used in T.V. circuits for protection. They have resistance of less than 15 ohms. Their function is similar to a fuse made to blow off whenever current in the circuit exceeds the limit.

Resistance of a wire is directly proportional to its length and inversely proportional to its thickness.

             R     L   

             R     1/A 


Example:   1k or 1000 ohms

                                                                1st     2nd     3rd                   4th



     Band 2

    Band 3

     Band 4


Black                                 Brown Red Orange Yellow Green Blue Violet Grey White Gold Silver      0       1       2       3       4       5       6       7       8       9         100         101         102             103         104         105         106         107         108         109         10-1         10-2     Gold Silver No colour      5%        10%        20%


A capacitor can store charge, and its capacity to store charge is called capacitance. Capacitors consist of two conducting plates, separated by an insulating material (known as dielectric). The two plates are joined with two leads. The dielectric could be air, mica, paper, ceramic, polyester, polystyrene, etc.  This dielectric gives name to the capacitor. Like paper capacitor, mica capacitor etc.

Types of capacitors:

Fixed capacitor  Variable capacitor  
Electrolytic  Gang condenser  Trimmer  
Mica  Paper  Ceramic  

Capacitors can be broadly classified in two categories, i.e., Electrolytic capacitors and Non-Electrolytic capacitors as shown if the figure above.

Electrolytic Capacitor:

Electrolytic capacitors have an electrolyte as a dielectric. When such an electrolyte is charged, chemical changes takes place in the electrolyte. If its one plate is charged positively, same plate must be charged positively in future. We call such capacitors as polarized. Normally we see electrolytic capacitor as polarized capacitors and the leads are marked with positive or negative on the can. Non-electrolyte capacitors have dielectric material such as paper, mica or ceramic. Therefore, depending upon the dielectric, these capacitors are classified.

Mica Capacitor:

It is sandwich of several thin metal plates separated by thin sheets of mica. Alternate plates are connected together and leads attached for outside connections. The total assembly is encased in a plastic capsule or Bakelite case. Such capacitors have small capacitance value (50 to 500pf) and high working voltage (500V and above). The mica capacitors have excellent characteristics under stress of temperature variation and high voltage application. These capacitors are now replaced by ceramic capacitors.

Ceramic Capacitor:

Such capacitors have disc or hollow tabular shaped dielectric made of ceramic material such as titanium dioxide and barium titanate. Thin coating of silver compounds is deposited on both sides of dielectric disc, which acts as capacitor plates. Leads are attached to each sides of the  dielectric disc and whole unit is encapsulated in a moisture proof coating. Disc type capacitors have very high value up to 0.001uf. Their working voltages range from 3V to 60000V. These capacitors have very low leakage current. Breakdown voltage is very high.

Paper Capacitor:

It consists of thin foils, which are separated by thin paper or waxed paper. The sandwich of foil and paper is then rolled into a cylindrical shape and enclosed in a paper tube or encased in a plastic capsules. The lead at each end of the capacitor is internally attached to the metal foil. Paper capacitors have capacitance ranging from 0.0001uf to 2.0uf and working voltage rating as high as 2000V.


Diodes are polarized, which means that they must be inserted into the PCB the correct way round. This is because an electric current will only flow through them in one direction (like air will only flow one way trough a tyre valve). Diodes have two connections, an anode and a cathode. The cathode is always identified by a dot, ring or some other mark.


The PCB is often marked with a +sign for the cathode end. Diodes come in all shapes and sizes. They are often marked with a type number. Detailed characteristics of a diode can be found by looking up the type number in a data book. If you know how to measure resistance with a meter then test some diodes. A good one has low resistance in one direction and high in other. They are specialized types of diode available such as the zener and light emitting diode (LED).


               anode                        cathode

                          simple diode                                      zener diode  


IC (Integrated Circuit) means that all the components of the circuit are fabricated on same chip. Digital ICs are a collection of resistors, diodes, and transistors fabricated on a single piece of semiconductor, usually silicon called a substrate, which is commonly referred to as ‘wafer’. The chip is enclosed in a protective plastic or ceramic package from which pins extend out connecting the IC to other device. Suffix N or P stands for dual-in-line (plastic package (DIP)) while suffix J or I stands for dual-in-lime ceramic package. Also the suffix for W stands for flat ceramic package.

The pins are numbered counter clockwise when viewed from the top of the package with respect to an identity notch or dot at one end of the chip.The manufacturer’s name can usually be guessed from its logo that is printed on the IC. The IC type number also indicates the manufacturer’s code. For e.g. DM 408 N SN 7404 indicates National Semiconductor and Texas Instruments.

Other examples are:

          Fair Child                       : UA, UAF

          National Semiconductor : DM, LM, LH, LF, and TA.

          Motorola                        : MC, MFC.

          Sprague                          : UKN, ULS, ULX.

          Signetic                          : N/s, NE/SE, and SU.

          Burr-Brown                    : BB.

          Texas Instruments           : SN.

The middle portion i.e. the IC type number tells about the IC function and also the family, which the particular IC belongs to.IC’s that belongs to standard TTL series have an identification number that starts with 74; for e.g. 7402, 74LS04, 74S04 etc. IC’s that belongs to standard CMOS family their number starts with 4, like 4000, 451B, 4724B, 1400. The 74C, 74HC, 74AC & 74ACT series are newer CMOS series.

Various series with TTL logic family are:-

          Standard TTL 74.

          Schottky TTL 74s.

          Low power Schottky 74LS.

          Advance Schottky 74AS.

          Advanced Low Power Schottky 74ALs.

Also there are various series with CMOS logic family as metal state CMOS 40 or 140.


Power Supply

For TTL circuits, the power supply pin is labeled Vcc and its nominal value.

For CMOS ICs, the power supply pin is labeled as VDD & its nominal value range from T3 to 18V.

Unconnected Inputs

An unconnected input is called “floating input”. The floating TTL input acts as logic 1. High level is applied to it. This characteristic is often used when

testing a TTL circuit. A floating TTL input will measure a DC level between 1.4V to 1.8V when checked with VOM as oscilloscope. If a CMOS input is left floating, it may have disastrous results. The IC may become overheated and eventually destroy itself. For this reason, all inputs to CMOS circuit must be connected to a LOW or HIGH level or to the output of another IC.


A relay is an electrically operated switch. The relay contacts can be made to operate in the pre-arranged fashion. For instance, normally open contacts close and normally closed contacts open. In electromagnetic relays, the contacts however complex they might be, they have only two position i.e. OPEN and CLOSED, whereas in case of electromagnetic switches, the contacts can have multiple positions.


The reason behind using relay for switching loads is to provide complete electrical isolation. The means that there is no electrical connection between the driving circuits and the driven circuits. The driving circuit may be low voltage operated low power circuits that control several kilowatts of power. In our circuit where a high fan could be switched on or off depending upon the output from the telephone.

Since the relay circuit operated on a low voltage, the controlling circuit is quite safe. In an electromagnetic relay the armature is pulled by a magnetic force only. There is no electrical connection between the coil of a relay and the switching contacts of the relay. If there are more than one contact they all are electrically isolated from each other by mounting them on insulating plates and washers. Hence they can be wired to control different circuits independently.

Some of the popular contacts forms are described below:

     1.  Electromagnetic relay

  • Power Relay.
  • Time Delay Relay.
  • Latching Relay.
  • Crystal Can Relay.
  • Co-axial Relay.

1. Electromagnetic relay:

An electromagnetic relay in its simplest form consists of a coil, a DC current passing through which produces a magnetic field. This magnetic field attracts an armature, which in turn operates the contacts. Normally open contacts close and normally closed contacts open. Electromagnetic relays are made in a large variety of contacts forms.

2. Power relays:

Power relays are multi-pole heavy duty lapper type relays that are capable of switching resistive loads of upto 25amp.. These relays are widely used for a variety of industrial application like control of fractional horse power motors, solenoids, heating elements and so on. These relays usually have button like silver alloy contacts and the contact welding due to heavy in rush current is avoided by wiping action of the contacts to quench the arc during high voltage DC switching thus avoiding the contact welding.

3. Time Delay Relay:

A time delay relay is the one in which there is a desired amount of time delay between the application of the actuating signal and operation of the load switching devices.

4.Latching Relay:

 In a Latching Relay, the relay contacts remain in the last energized position even after removal of signal in the relay control circuit. The contacts are held in the last relay-energized position after removal of energisation either electrically or magnetically. The contacts can be released to the normal position electrically or mechanically.

5. Crystal Can Relay:

They are so called, as they resemble quartz crystal in external shapes. These are high performance hermetically sealed miniature or sub-miniature relay widely used in aerospace and military application. These relays usually have gold plated contacts and thus have extremely low contact resistance. Due to low moment of inertia of the armature and also due to statically and dynamically balanced nature of armature, these relays switch quite reliably even under extreme condition of shock and vibration.

6. Co-axial Relay:

A Co-axial Relay has two basic parts, an actuator which is nothing but some kind of a coil and a cavity, housing the relay contacts. The co-axial relay are extensively used for radio frequency switching operations of equipment


                         Collector                                         Collector

_ _ _ _  _ _ _ _ _ _ + + + + + _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _  + + + + + + + + + +  + + +  + + + + + + + + + +       
+ + + + + +  
— — — — — —  

Base                                                 Base     

                             Emitter                                                  Emitter


                                       C                                                          C                                                                                                        


                      B                                                          B                                      


                                       E                                                           E                                                                                                                                                                                                                                                                             

                                     NPN                                                         PNP

Junction transistors consists of two junctions made from N-type and P-type semiconductor materials and are called bipolar transistors (two polarities). They have three connections emitter, base, and collector.

The forward biased base/emitter junction causes electrons to be attracted from the emitter area towards the base. Arriving in the base area, most of the negative electrons come under the influence of the more positive collector and are attracted by it. This is shown in the left hand drawing, where the base current plus collector current equals the emitter current. Alpha gain is collector current divided by emitter current, and is always less than 1. Beta gain is collector current divided by base current and can be fairly high number. Therefore, causing a small base current to flow makes a much larger collector current to flow. A small base current controls a large collector current. There is 0.6 volts across the base\emitter junction, where it is forward biased (0.3 volts for germanium).

How to control sensors

What is a voltage divider?

You are going to find out but don’t be in too much of a hurry. Work through the Chapter and allow the explanation to develop.

The diagram below shows a light dependent resistor, or LDR, together with its circuit symbol:

The light-sensitive part of the LDR is a wavy track of cadmium sulphide. Light energy triggers the release of extra charge carriers in this material, so that its resistance falls as the level of illumination increases.

A light sensor uses an LDR as part of a voltage divider.

The essential circuit of a voltage divider, also called a potential divider, is:

What happens if one of the resistors in the voltage divider is replaced by an LDR? In the circuit below, Rtop is a 10 resistor, and an LDR is used as Rbottom :

Suppose the LDR has a resistance of 500 , 0.5 , in bright light, and 200 in the shade (these values are reasonable).

When the LDR is in the light, Vout will be:

In the shade, Vout will be:

In other words, this circuit gives a LOW voltage when the LDR is in the light, and a HIGH voltage when the LDR is in the shade. The voltage divider circuit gives an output voltage which changes with illumination.

A sensor subsystem which functions like this could be thought of as a ‘dark sensor‘ and could be used to control lighting circuits which are switched on automatically in the evening.

Perhaps this does not seem terribly exciting, but almost every sensor circuit you can think of uses a voltage divider. There’s just no other way to make sensor subsystems work.

Here is the voltage divider built with the LDR in place of Rtop :

Temperature sensors

A temperature-sensitive resistor is called a thermistor. There are several different types:

The resistance of most common types of thermistor decreases as the temperature rises. They are called negative temperature coefficient, or ntc, thermistors. Note the -t° next to the circuit symbol. A typical ntc thermistor is made using semiconductor metal oxide materials. (Semiconductors have resistance properties midway between those of conductors and insulators.) As the temperature rises, more charge carriers become available and the resistance falls.

Although less often used, it is possible to manufacture positive temperature coefficient, or ptc, thermistors. These are made of different materials and show an increase in resistance with temperature.

How could you make a sensor circuit for use in a fire alarm? You want a circuit which will deliver a HIGH voltage when hot conditions are detected. You need a voltage divider with the ntc thermistor in the Rtop position:

How could you make a sensor circuit to detect temperatures less than 4°C to warn motorists that there may be ice on the road? You want a circuit which will give a HIGH voltage in cold conditions. You need a voltage divider with the thermistor in place of Rbottom :

This last application raises an important question: How do you know what value of Vout you are going to get at 4°C?


Key point: The biggest change in Vout from a voltage divider is obtained when Rtop and Rbottom are equal in value

Sound sensors

Another name for a sound sensor is a microphone. The diagram shows a cermet microphone:

Cermet’ stands for ‘ceramic’ and ‘metal’. A mixture of these materials is used in making the sound-sensitive part of the microphone. To make them work properly, cermet microphones need a voltage, usually around 1.5 V across them. A suitable circuit for use with a 9 V supply is:

The 4.7 and the 1 resistors make a voltage divider which provides 1.6 V across the microphone. Sound waves generate small changes in voltage, usually in the range 10-20 mV. To isolate these small signals from the steady 1.6 V, a capacitor is used.

Signals from switches

When a switch is used to provide an input to a circuit, pressing the switch usually generates a voltage signal. It is the voltage signal which triggers the circuit into action. What do you need to get the switch to generate a voltage signal? . . . You need a voltage divider. The circuit can be built in either of two ways:

The pull down resistor in the first circuit forces Vout to become LOW except when the push button switch is operated. This circuit delivers a HIGH voltage when the switch is pressed. A resistor value of 10 is often used.

In the second circuit, the pull up resistor forces Vout to become HIGH except when the switch is operated. Pressing the switch connects Vout directly to 0 V. In other words, this circuit delivers a LOW voltage when the switch is pressed.

In circuits which process logic signals, a LOW voltage is called ‘logic 0’ or just ‘0’, while a HIGH voltage is called ‘logic1’ or ‘1’. These voltage divider circuits are perfect for providing input signals for logic systems.

What kinds of switches could you use. One variety of push button switch is called a miniature tactile switch. These are small switches which work well with prototype board:

As you can see, the switch has four pins which are linked in pairs by internal metal strips. Pressing the button bridges the contacts and closes the switch. The extra pins are useful in designing printed circuit boards for keyboard input and also stop the switch from being moved about or bent once soldered into position.

There are lots of other switches which you might want to use in a voltage divider configuration. These include magnetically-operated reed switches, tilt switches and pressure pads, all with burglar alarm applications.

Transistor Circuits

This page explains the operation of transistors in circuits. Practical matters such as testing, precautions when soldering and identifying leads are covered by the Transistors page.

General:    Types | Currents | Functional model | Darlington pair
Switching:  Introduction | Use relay? | IC output | for NPN | and PNP | Sensors | Inverter

Next Page: Analogue and Digital Systems
Also See: Transistors (soldering, lead identification)

Types of transistor

Text Box:  	
Transistor circuit symbols
There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. This page is mostly about NPN transistors and if you are new to electronics it is best to start by learning how to use these first.

The leads are labelled base (B), collector (C) and emitter (E).
These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels!

A Darlington pair is two transistors connected together to give a very high current gain.

In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. They have different circuit symbols and properties and they are not (yet) covered by this page.

Transistor currents

The diagram shows the two current paths through a transistor. You can build this circuit with two standard 5mm red LEDs and any general purpose low power NPN transistor (BC108, BC182 or BC548 for example).

The small base current controls the larger collector current.

When the switch is closed a small current flows into the base (B) of the transistor. It is just enough to make LED B glow dimly. The transistor amplifies this small current to allow a larger current to flow through from its collector (C) to its emitter (E). This collector current is large enough to make LED C light brightly.

When the switch is open no base current flows, so the transistor switches off the collector current. Both LEDs are off.

A transistor amplifies current and can be used as a switch.

This arrangement where the emitter (E) is in the controlling circuit (base current) and in the controlled circuit (collector current) is called common emitter mode. It is the most widely used arrangement for transistors so it is the one to learn first.

Functional model of an NPN transistor

The operation of a transistor is difficult to explain and understand in terms of its internal structure. It is more helpful to use this functional model:

  • The base-emitter junction behaves like a diode.
  • A base current IB flows only when the voltage VBE across the base-emitter junction is 0.7V or more.
  • The small base current IB controls the large collector current Ic.
  • Ic = hFE × IB   (unless the transistor is full on and saturated)
    hFE is the current gain (strictly the DC current gain), a typical value for hFE is 100 (it has no units because it is a ratio)
  • The collector-emitter resistance RCE is controlled by the base current IB:
    • IB = 0   RCE = infinity   transistor off
    • IB small   RCE reduced   transistor partly on
    • IB increased   RCE = 0   transistor full on (‘saturated’)

Additional notes:

  • A resistor is often needed in series with the base connection to limit the base current IB and prevent the transistor being damaged.
  • Transistors have a maximum collector current Ic rating.
  • The current gain hFE can vary widely, even for transistors of the same type!
  • A transistor that is full on (with RCE = 0) is said to be ‘saturated‘.
  • When a transistor is saturated the collector-emitter voltage VCE is reduced to almost 0V.
  • When a transistor is saturated the collector current Ic is determined by the supply voltage and the external resistance in the collector circuit, not by the transistor’s current gain. As a result the ratio Ic/IB for a saturated transistor is less than the current gain hFE.
  • The emitter current IE = Ic + IB, but Ic is much larger than IB, so roughly IE = Ic.

There is a table showing technical data for some popular transistors on the transistors page.

Text Box:  
Touch switch circuit
Darlington pair

This is two transistors connected together so that the current amplified by the first is amplified further by the second transistor. The overall current gain is equal to the two individual gains multiplied together:

Darlington pair current gain, hFE = hFE1 × hFE2
(hFE1 and hFE2 are the gains of the individual transistors)

This gives the Darlington pair a very high current gain, such as 10000, so that only a tiny base current is required to make the pair switch on.

A Darlington pair behaves like a single transistor with a very high current gain. It has three leads (BC and E) which are equivalent to the leads of a standard individual transistor. To turn on there must be 0.7V across both the base-emitter junctions which are connected in series inside the Darlington pair, therefore it requires 1.4V to turn on.

Darlington pairs are available as complete packages but you can make up your own from two transistors; TR1 can be a low power type, but normally TR2 will need to be high power. The maximum collector current Ic(max) for the pair is the same as Ic(max) for TR2.

A Darlington pair is sufficiently sensitive to respond to the small current passed by your skin and it can be used to make a touch-switch as shown in the diagram. For this circuit which just lights an LED the two transistors can be any general purpose low power transistors. The 100k resistor protects the transistors if the contacts are linked with a piece of wire.

Using a transistor as a switch

When a transistor is used as a switch it must be either OFF or fully ON. In the fully ON state the voltage VCE across the transistor is almost zero and the transistor is said to be saturated because it cannot pass any more collector current Ic. The output device switched by the transistor is usually called the ‘load’.

The power developed in a switching transistor is very small:

  • In the OFF state: power = Ic × VCE, but Ic = 0, so the power is zero.
  • In the full ON state: power = Ic × VCE, but VCE = 0 (almost), so the power is very small.

This means that the transistor should not become hot in use and you do not need to consider its maximum power rating. The important ratings in switching circuits are the maximum collector current Ic(max) and the minimum current gain hFE(min). The transistor’s voltage ratings may be ignored unless you are using a supply voltage of more than about 15V. There is a table showing technical data for some popular transistors on the transistors page.

For information about the operation of a transistor please see the functional model above.

Protection diode

If the load is a motor, relay or solenoid (or any other device with a coil) a diode must be connected across the load to protect the transistor from the brief high voltage produced when the load is switched off. The diagram shows how a protection diode is connected ‘backwards’ across the load, in this case a relay coil.

Current flowing through a coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.

When to use a relay

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 Photographs © Rapid Electronics 

Transistors cannot switch AC or high voltages (such as mains electricity) and they are not usually a good choice for switching large currents (> 5A). In these cases a relay will be needed, but note that a low power transistor may still be needed to switch the current for the relay’s coil!

Advantages of relays:

  • Relays can switch AC and DC, transistors can only switch DC.
  • Relays can switch high voltages, transistors cannot.
  • Relays are a better choice for switching large currents (> 5A).
  • Relays can switch many contacts at once.

Disadvantages of relays:

  • Relays are bulkier than transistors for switching small currents.
  • Relays cannot switch rapidly, transistors can switch many times per second.
  • Relays use more power due to the current flowing through their coil.
  • Relays require more current than many ICs can provide, so a low power transistor may be needed to switch the current for the relay’s coil.

Connecting a transistor to the output from an IC

Most ICs cannot supply large output currents so it may be necessary to use a transistor to switch the larger current required for output devices such as lamps, motors and relays. The 555 timer IC is unusual because it can supply a relatively large current of up to 200mA which is sufficient for some output devices such as low current lamps, buzzers and many relay coils without needing to use a transistor.

A transistor can also be used to enable an IC connected to a low voltage supply (such as 5V) to switch the current for an output device with a separate higher voltage supply (such as 12V). The two power supplies must be linked, normally this is done by linking their 0V connections. In this case you should use an NPN transistor.

A resistor RB is required to limit the current flowing into the base of the transistor and prevent it being damaged. However, RB must be sufficiently low to ensure that the transistor is thoroughly saturated to prevent it overheating, this is particularly important if the transistor is switching a large current (> 100mA). A safe rule is to make the base current IB about five times larger than the value which should just saturate the transistor.

Choosing a suitable NPN transistor

The circuit diagram shows how to connect an NPN transistor, this will switch on the load when the IC output is high. If you need the opposite action, with the load switched on when the IC output is low (0V) please see the circuit for a PNP transistor below.

The procedure below explains how to choose a suitable switching transistor.

  1. Text Box:  
NPN transistor switch
(load is on when IC output is high) 

Using units in calculations
Remember to use V, A and  or
V, mA and k . For more details
please see the Ohm's Law page.

The transistor’s maximum collector current Ic(max) must be greater than the load current Ic.
load current Ic =  supply voltage Vs
load resistance RL
  • The transistor’s minimum current gain hFE(min) must be at least five times the load current Ic divided by the maximum output current from the IC.
hFE(min)  >   5 ×    load current Ic  
max. IC current
  • Choose a transistor which meets these requirements and make a note of its properties: Ic(max) and hFE(min).
    There is a table showing technical data for some popular transistors on the transistors page.
  • Calculate an approximate value for the base resistor:
RB =  Vc × hFE   where Vc = IC supply voltage
  (in a simple circuit with one supply this is Vs)
5 × Ic
  • For a simple circuit where the IC and the load share the same power supply (Vc = Vs) you may prefer to use: RB = 0.2 × RL × hFE
  • Then choose the nearest standard value for the base resistor.
  • Finally, remember that if the load is a motor or relay coil a protection diode is required.

The output from a 4000 series CMOS IC is required to operate a relay with a 100 coil.
The supply voltage is 6V for both the IC and load. The IC can supply a maximum current of 5mA.

  • Load current = Vs/RL = 6/100 = 0.06A = 60mA, so transistor must have Ic(max) > 60mA.
  • The maximum current from the IC is 5mA, so transistor must have hFE(min) > 60 (5 × 60mA/5mA).
  • Choose general purpose low power transistor BC182 with Ic(max) = 100mA and hFE(min) = 100.
  • RB = 0.2 × RL × hFE = 0.2 × 100 × 100 = 2000. so choose RB = 1k8 or 2k2.
  • The relay coil requires a protection diode.

Text Box:  
PNP transistor switch
(load is on when IC output is low)
Choosing a suitable PNP transistor

The circuit diagram shows how to connect a PNP transistor, this will switch on the load when the IC output is low (0V). If you need the opposite action, with the load switched on when the IC output is high please see the circuit for an NPN transistor above.

The procedure for choosing a suitable PNP transistor is exactly the same as that for an NPN transistor described above.

Using a transistor switch with sensors

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LED lights when the LDR is dark
LED lights when the LDR is bright
The top circuit diagram shows an LDR (light sensor) connected so that the LED lights when the LDR is in darkness. The variable resistor adjusts the brightness at which the transistor switches on and off. Any general purpose low power transistor can be used in this circuit.

The 10k fixed resistor protects the transistor from excessive base current (which will destroy it) when the variable resistor is reduced to zero. To make this circuit switch at a suitable brightness you may need to experiment with different values for the fixed resistor, but it must not be less than 1k.

If the transistor is switching a load with a coil, such as a motor or relay, remember to add a protection diode across the load.

The switching action can be inverted, so the LED lights when the LDR is brightly lit, by swapping the LDR and variable resistor. In this case the fixed resistor can be omitted because the LDR resistance cannot be reduced to zero.

Note that the switching action of this circuit is not particularly good because there will be an intermediate brightness when the transistor will be partly on (not saturated). In this state the transistor is in danger of overheating unless it is switching a small current. There is no problem with the small LED current, but the larger current for a lamp, motor or relay is likely to cause overheating.

Other sensors, such as a thermistor, can be used with this circuit, but they may require a different variable resistor. You can calculate an approximate value for the variable resistor (Rv) by using a multimeter to find the minimum and maximum values of the sensor’s resistance (Rmin and Rmax):

Variable resistor, Rv = square root of (Rmin × Rmax)

For example an LDR: Rmin = 100, Rmax = 1M, so Rv = square root of (100 × 1M) = 10k.

You can make a much better switching circuit with sensors connected to a suitable IC (chip). The switching action will be much sharper with no partly on state.

A transistor inverter (NOT gate)

Inverters (NOT gates) are available on logic ICs but if you only require one inverter it is usually better to use this circuit. The output signal (voltage) is the inverse of the input signal:

  • When the input is high (+Vs) the output is low (0V).
  • When the input is low (0V) the output is high (+Vs).

Any general purpose low power NPN transistor can be used. For general use RB = 10k and RC = 1k, then the inverter output can be connected to a device with an input impedance (resistance) of at least 10k such as a logic IC or a 555 timer (trigger and reset inputs).

If you are connecting the inverter to a CMOS logic IC input (very high impedance) you can increase RB to 100k and RC to 10k, this will reduce the current used by the inverter.

Soil Moisture Sensor:

In the soil moisture sensor we check conductivity of the soil . for this purpose we insert two probes in the field. If the field is wet then conductivity is more and resistance is less. If the field is dry then conductivity is less and  resistance is high. To measure the conductivity we use one NPN transistor  circuit. Emitter of the NPN transistor is connected to the input of adc and collector of the transistor is connected to the positive supply 5volt. Base is biased through positive voltage through 100 ohm resistor in series with the conductivity probe. Emitter voltage is also set by the one variable resistor 10 k. One point of the 10 k ohm resistor is connected to the positive point and  third point of the 10 k ohm resistor is grounded. Center point of the 10 k ohm resistor is connected to the emitter of the transistor and go though the  input of ADC IN2. As the base voltage is change according the  resistance of the field. ADC input is also change.

Water Pump:

A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action. Pumps can be classified into three major groups according to the method they use to move the fluid: direct lift, displacement, and gravity pumps.[1]

Pumps operate by some mechanism (typically reciprocating or rotary), and consume energy to perform mechanical work by moving the fluid. Pumps operate via many energy sources, including manual operation, electricity, engines, or wind power, come in many sizes, from microscopic for use in medical applications to large industrial pumps.

Mechanical pumps serve in a wide range of applications such as pumping water from wells, aquarium filtering, pond filtering and aeration, in the car industry for water-cooling and fuel injection, in the energy industry for pumping oil and natural gas or for operating cooling towers. In the medical industry, pumps are used for biochemical processes in developing and manufacturing medicine, and as artificial replacements for body parts, in particular the artificial heart and penile prosthesis.

Single stage pump – When in a casing only one impeller is revolving then it is called single stage pump.

Double/multi-stage pump – When in a casing two or more than two impellers are revolving then it is called double/multi-stage pump.

In biology, many different types of chemical and bio-mechanical pumps have evolved, and biomimicry is sometimes used in developing new types of mechanical pumps.


Low cost



Easy to use- system is very easy to understand

It will also help in irrigation department for farmers to for automatic water supply.


We can use this project as a natural and automatic water resource for the farm system.

We can stop the wastage of the water resources as well by using the required level of water consumption.


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