final report activate switch

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Acknowledgements I would like to take this time to express our greatest thanks to my Project supervisor, Mrs.Ajita Selvapandi for her time to give her invaluable advice and help during the course of the project. I would like to thank my school for providing me with the equipment and other material, which helped me in my project. To all those who have helped in one way or another during the course of the project, Thank you.


Table of Contents TitleAbstract1 Introduction 1.1 Back ground 1.2 Specification 1.3 Statement of problem 1.4 Functional block diagram 6 1.41 Description of the block 7 1.42 Overall operation of the block 8 1.5 Objectives 9 1.6 Justification of the project 10 1.7 Scope and Limitation

Page45 5 5

2 Theory of components and diagram 11 2.1 Light 11 2.2 Optical detectors 12 2.21 Design of sensory unit 19 2.3 Voltage comparator 21 2.4 Power supply 27 2.41 Transformer 30 2.42 Rectifier 34


2.43 Filters and regulation 41 2.44 Voltage regulator 43 2.45 Design of power supply 45 2.5 48 2.51 Design of the output section 53 3 Complete circuit 55 Relays


List of Figures Title1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 55 Functional block diagram Light spectrum Photo transistor Photo diode Photo conductor cell Photo valtic cell Photo multiplier Design of sensory unit Hex inverter Schmitt trigger Inverting Schmitt trigger design of control unit Transformer principle operation Half wave rectifier Positive half cycle Negative half cycle Bridge full wave rectifier Negative cycle Bridge output voltage Fixed positive linear voltage regulator Adjustable positive linear regulator Fixed negative linear regulator Stepdown transformer Rectification and filtering Voltage regulator Relay connection Design of output section Complete circuit diagram

Page6 11 12 14 17 17 18 19 23 25 27 31 35 36 37 38 39 39 43 44 45 45 47 48 50 53


1.0 INTRODUCTION The automatic light activated switch refers to a circuit that employs a photo detector that senses the amount of light intensity. It will automatically switch ON or OFF the lamps depending with the amount of light intensity. A light activate switch circuit is an electronic circuit that using light to control the relay for open or close circuit home appliances. When photo transistor detects light, relay is working by starting those connected electric home appliance. When photo transistor does not detect light, the relay is not working and immediately stopping those electric appliances. Light activate switch is Simple and inexpensive circuit, suitable for many applications like the automatic switching of the lights in a shop-window or a room, according to the ambient light level. Suitable for alarm systems, production control, remote controls etc. The circuit uses a light dependent resistor that changes its value according to the amount of light it receives and three transistors which are used to5

amplify the signal from the LDR and operate a mains rated relay.

1.1 BACKGROUND Whenever the sun goes down, many people are gripped with fear. The fear of being robbed even in the early hours of the night will make people leave their work stations early just to save their lives and property. Thus in the major busy towns and other small trading centers upcountry, the need for security lights is crucial. Such security lights will be installed in residential areas for safety purposes. Therefore vices and other incidents that occur at night are curbed completely from the society. The automatic light activated switch refers to a circuit that employs a photo detector that senses the amount of light intensity. It will automatically switch ON or OFF the lamps depending with the amount of light intensity.


1.2 SPECIFICATIONS Supply voltage Vcc is 12V DC Mains supply voltage is 240V to 250V, 50Hz frequency Vref is 5V DC 1.3 STATEMENT OF THE PROBLEM Due to the rising cases of insecurity, it is paramount that people take security measures to protect themselves from crime. One of these measures is installation of security lights of which this project is meant to achieve. 1.4 FUNCTIONAL BLOCK DIAGRAM240V A.C. INPUT RELAY SECURITY LIGHT






Fig1: Functional block diagram of automatic light activated switch

1.41 DESCRIPTION OF THE BLOCKS AC input power from the mains supply is of about 220V to 240V. It supplies power to the powering unit of the circuit and the security lights. Powering unit It consists of a 240V to 12V step down transformer, bridge rectifier, filter capacitor and regulator. It provides the necessary positive voltage Vcc to power the Schmitt trigger, voltage regulator and the relay.8

Sensory unit It comprises of the voltage divider and photo resistor. The photo resistor senses the light or darkness falling on it. The voltage divider that has two resistors will set the sensitivity of the sensory unit. Control unit The control unit comprises of the Schmitt trigger and a PNP transistor. The Schmitt trigger is an electronic circuit whose output switches suddenly to either positive saturation or negative saturation depending on the input voltage.

Output section It comprises of the relay and the security lights. The relay is an electronic circuit that opens and closes under the control of the Schmitt trigger. If there is flow of current through the relay windings then its contacts close hence lights ON the security lamps and the lights will be OFF if theres no current flow.


1.42 OVERALL OPERATION OF THE BLOCK The input section supplies power to the powering unit of the circuit and the security lights which is around 220-240V. This is stepped down by a transformer to 12V i.e. a low AC voltage that is rectified to DC by the bridge rectifier. Its then filtered by a filter capacitor and regulated by a 3 terminal voltage regulator. The powering unit provides the necessary positive Vcc to power the Schmitt trigger, voltage regulator and the relay. This then is inputted to the sensory unit that has a photo detector and voltage divider R1 and R2. The photo resistor senses either light or darkness and falling on it. When light falls on it, its resistance decreases and will increase with darkness. Resistors R1 and R2 set the sensitivity of the sensory circuit. Resistor R2 is a preset resistor that is used to just turn OFF the security lights at dawn or turn them OFF at sunset by adjusting its movable arm. At the control unit and output section, Schmitt trigger produces two voltages; upper threshold voltage VUT and lower threshold voltage VLT. The output of the10

Schmitt trigger switches to positive saturation when its voltage at its input is greater than VUT and switches negatively when its voltage at its input is less than VLT. Output of the Schmitt trigger is connected to the base of a PNP transistor driving it to cut off thereby cutting off current and making the relay contacts remain open. This makes the security lights remain OFF during the day. During the night, the Schmitt trigger switches to negative saturation driving transistor to near saturation. Current therefore flows through the relay winding closing its contacts and allowing current to flow hence lighting ON the security lights.

1.5 OBJECTIVES The broad objective is to design and construct an automatic light activated switch that controls the switching of on and off the security lights. The specific objectives will be; 1. To design the various blocks an come up with a schematic diagram 2. To construct the automatic light activated switch


3. To test the working of the blocks at each input and output and the overall operation of the block

1.6 JUSTIFICATION OF THE PROJECT In order to curb the rising insecurity in our society, it is necessary that people take precautions of their own safety. This can be achieved by installing such security measures as security lights in their homes which is what the project is meant to achieve.

1.7 SCOPE AND LIMITATIONS 1.71 SCOPE The main aim of designing this project is to come up with a workable project employing a photo detector that will sense the amount of light intensity and automatically switch on or off the security lights. 1.72 LIMITATIONS 1) Lack of some components used in the design 2) High cost of some of the components 3) Time allocated was minimal


2.0 THEORY OF COMPONENTS AND DESIGN 2.1 LIGHT The Quantum theory states that light consists of discrete packets of energy called photons. The energy contained in a photon depends on the frequency of light and is expressed as = hf where = energy h = planks constant i.e. 6.624 10-34 Joules/ second f = frequency As frequency increases so does the energy and vice versa. Light is usually referred in terms of wavelength.

=V F

Any light source emits light over a limited range of wavelength. When the amount of energy is plotted


against a wavelength, the graph is the emission spectrum.

Invisible light

Invisible light

Visible light Decreasing wavelength Green Blue Indigo Violet





Ultra Violet

Fig 2: Light spectrum

2.2 OPTICAL DETECTORS Optical detectors are devices that convert optical energy or light into electrical energy. There are two types of conversions: Photo voltaic effect This is where optical energy is converted to electrical voltage Photoconductive effect This is where light is converted into an electrical voltage. a) Photo transistor The photo transistor consists of a normal bi-polar transistor that is packaged in a transparent casing to14

enable light reach the base of the transis