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Light consists of discrete packets of energy called photons thus the energy contained in a photon depends on the frequency of light. Essentially, optical detectors are devices used to convert optical energy or light into electrical energy. The automatic light activated switch will use a photoconductive cell as its optical detector. Its resistance will decrease when light falls on it and increase with darkness. It is applicable in light night control, street light control etc.

language english
wordcount 7243 (cca 20 pages)
contextual quality N/A
language level N/A
price free
sources 5
Table of contents


1.0 Introduction 1
1.1 Background 1
1.2 Specifications 1
1.3 Statement of the problem 1
1.4 Block diagram 2
            1.41 Description of the blocks 3
            1.42 Overall operation of the block diagram 4
1.5 Objectives 5
1.6 Justification of the project 5
1.7 Scope and Limitations 5
             1.71 Scope 5
             1.72 Limitations 5

2.0 Theory of components and Design   6
2.1 Light theory   6
2.2 Optical detectors   7
2.21 Design of the Sensory unit 12
2.3 Voltage comparators 14
2.31 Design of the Control unit 18
2.4 Power supply 21
2.41 Transformer 21
2.42 Rectifiers 24
2.43 Filters and regulators 28
2.44 Voltage regulators 30
2.45 Design of the Power supply 32
2.5 Relays 35
2.51 Design of the Output section 39


3.0 Complete circuit diagram showing test points 40


CHAPTER ONE 1.0 INTRODUCTION 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 ...

... the voltage across LDR1 reduces to a value below the Lower Threshold Voltage VLT, the output of the Schmitt Trigger switches to positive saturation. The output of the Schmitt Trigger is connected to the base of the PNP transistor, Q1. This drives it to cut-off.

With transistor Q1 into cut-off, no current flows through it and in effect the relay winding. This makes the relay, RLY1 contacts remain open. The mains power supply to the security lights is connected through the relay contacts which act as a switch.
Thus, with the contacts open, no power flows to the security lights and they remain off during the day.

During the night, the voltage across LDR1 increases due to the increase in its resistance. Any moment this voltage increases to a value greater than the Upper Threshold Voltage VUT, the Schmitt Trigger switches to negative saturation. Transistor Q1 is driven into near-saturation and current flows through the relay winding making it close its contacts. This allows current to flow to the security lights and they light up.
Essay is in categories


Electrical Engineering
Electronic Devices
pius n.

this is great and i want to try and make mine to see if it can work.

Princess R.

I commed the writer for a job well-done in this paper. Very instructive and elucidating.

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