2. ABSTRACT
A streetlight that automatically switchesON when the night falls and turns OFF
when the sun rises.
In fact we can use this circuit for implementing any type of automatic night light.
The circuit uses a LDR to sense the light .
When thereis light the resistanceof LDRwill be low.
So the voltage drop across POT R2 will be high.Thiskeepsthe transistor Q1
ON.The collector of Q1(BC107) is coupled to base of Q2(SL100).
So Q2 will be OFF and so do the relay.The bulb will remain OFF.
When night falls the resistanceof LDR increasesto make the voltage across the
POT R2 to decreasebelow 0.6V
.
This makes transistor Q1 OFF whichin turn makes Q2 ON.The relay will be
energizedand the bulb will glow.
4. Relay is an electromagnetic device which is used to isolate two circuits electrically and
connect them magnetically.
They are very useful devices and allow one circuit to switch another one while they are
completely separate.
They are often used to interface an electronic circuit (working at a low voltage) to an
electrical circuit which works at very high voltage.
For example, a relay can make a 5V DC battery circuit to switch a 230VAC mains
circuit.Thus a small sensor circuit can drive, say, a fan or an electric bulb.
A relay switch can be divided into two parts:input and output.
Relay
5. VOLTAGE REGULATOR
It is a voltage regulator (7805) integrated circuit. It is a member of 78xx series of
fixed linear voltage regulator ICs.The voltage source in a circuit may have fluctuations
and wouldnot give the fixed voltage output.
The voltage regulator IC maintains the output voltage at a constant value.The xx in
78xx indicates the fixed output voltage it is designed to provide. It provides +9V
regulated power supply.
Capacitors of suitable values can be connected at input and output pins depending upon
the respective voltage levels.
6. FUSE[1A]
Fuse is an electronics safety device placed in a circuit consisting of a replaceable
plug or tube containing wire or metal that will melt and break the circuit if the
currentexceeds a specified amperage.
Blow a fuse: to cause an electrical fuse to melt.
7. LIGHT DEPENDENT RESISTOR
An LDR (Light dependent resistor),as its name suggests,offers resistance in
responseto the ambient light.
The resistance decreases as the intensity of incident light increases,and vice
versa.
In the absence of light, LDR exhibits a resistance of the order of mega-ohms
which decreases to few hundred ohms in the presence of light.
It can act as a sensor,since a varying voltage drop can be obtained in accordance
with the varying light.
It is made up of cadmium sulphide (CdS).
An LDR has a zigzag cadmium sulphide track. It is a bilateral device, i.e.,
conducts in both directions in same fashion.
8. RESISTOR
Resistor is a passive component used to control current in a circuit.
Its resistance is given by the ratio of voltage applied across its
terminals to the current passing through it.
Thus a particular value of resistor, for fixed voltage, limits the current
through it.
They are omnipresent in electronic circuits.
Resistors can be either fixed or variable.
The low power resistors are comparatively smaller in size than high
power resistors.
9. TRANSISTOR
BC107 is an NPN bi-polar junction transistor.A transistor,stands for transfer of
resistance, is commonly used to amplify current.
A small currentat its base controlsa larger currentat collector & emitter terminals.
BC107 is mainly used for amplification and switching purposes.It has a maximum
currentgain of 800.
The transistor terminals require a fixed DC voltage to operate in the desired region of
its characteristic curves.This is known as the biasing.
The voltage divider is the commonly used biasing mode.
For switching applications,transistor is biased so that it remains fully on if there is a
signal at its base.
In the absence of base signal,it gets completely off.
10. TRANSISTOR[SL100]
SL100 is a general purpose,medium power NPN transistor.It is mostly used as switch
in common emitter configuration.
The transistor terminals require a fixed DC voltage to operate in the desired region of
its characteristic curves.This is known as the biasing.
For switching applications,SL100 is biased in such a way that it remains fully on if
there is a signal at its base. In the absence of base signal,it gets turned off completely.
The emitter leg of SL100 is indicated by a protruding edge in the transistor case.The
base is nearest to the emitter while collector lies at other extreme of the casing.
11. BATTERY
It is a collection of one or more electrochemical cells in which stored chemical energy
is converted into electrical energy.
One half cell houses the Anode to which the positive ions migrate from the
Electrolyte and the other houses the Cathodeto which the negative ones drift.
The two cells are may be connected via a semi permeablemembranous structure
allowing ions to flow but not the mixing of electrolytes as in the case of most primary
cells or in the same solution as in secondary cells.
The performance of the cell continues to dip gradually as the concentrationof ions in
the solutions decrease, marked by an increase in internal resistance eventually leading to
the exhaustion of the battery.
12. Applications
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Photo resistors are applied, most of which involve recognizing
the presence of light.
Photo resistors are also used in digital cameras to perceive how
much light camera sees and adjust the picture quality
accordingly.
They are also used in some clocks, alarms, and other electronic
devices that are semi-dependent on sunlight.
Smoke detection.
Automatic lighting control.
Burglar alarm systems.
Camera (electronic shutter).
Strobe (color temperature reading).
13. Disadvantages
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Can be more intricate to align detector pairs.
Is sensitive to ambient light and require careful shielding.
Photo resistors are only sensitive to light and no other force
can power it without risking damage. Also, they are unable to
detect low light levels and may take a few seconds to deliver
a charge while their electrons build up momentum.
