it is automatic street light system with LDR (light sensor)

2.  The main purpose of designing this project is to prevent
loss of current unnecessarily during day time.
 During night the bulb will automatically glow and day time
automatically switch OFF.
 No switch require to switch light ON.
 Free from human error .

3.  RELAY ( Sugar Cube 9Volt )
 VARIABLE RESISTOR ( 100K )
 DIODE ( 1N4007 )
 RESISTOR ( 1K )
 LIGHT DEPENDENT RESISTOR
 TRANSISTOR ( BC547 )
 BATTERY ( 9Volt )
 BULB ( 220Volt )
 LED
 PCB

5.  An LDR (Light dependent resistor), as its name suggests, offers resistance
in response to 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 cadmium sulphide track. It is a bilateral device, that conducts
in both directions in same fashion.

7.  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.

8.  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
230V AC 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.
Spring
To Control Circuit
Moveable Contact
Load
Power Supply
Electromagnet
Stationary Contact

9.  A diode is a two-terminal electronic component
that conducts electric current in only one direction.
 A semiconductor diode is a crystalline piece of semiconductor material
connected to two electrical terminals.
 The most common function of a diode is to allow an electric current to pass in
one direction while blocking current in the opposite direction.
 Diode has to type of biasing FORWARD Bias and REVRSE Bias.
 In this Project we used diode as a protection diode .

10.  BC547 is an NPN bi-polar junction transistor. A transistor, stands for
transfer of resistance, is commonly used to amplify current.
 A small current at its base controls a larger current at collector &
emitter terminals.
 BC547 is mainly used for amplification and switching purposes. It has a
maximum current gain 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.
 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.

11. R4

12.  A street light that automatically switches ON 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 there is light the resistance of LDR will be low.
 So the voltage drop across variable resistance R4 will be high. This keeps the
transistor T1 ON. The collector of T1(BC547) is coupled to base of T2(BC547).
 So T2 will be OFF and so do the relay. The bulb will remain OFF.
 When night falls the resistance of LDR increases to make the voltage across the
variable resistance R4 to decrease below 0.6V.
 This makes transistor T1 OFF which makes T2 ON. The relay will be energized
and the bulb will glow.