This document describes an automatic street light control circuit using an LDR (light dependent resistor). The circuit uses an LDR, resistors, capacitors, diodes, a transistor and relay to automatically turn on LED street lights when it gets dark. When the LDR detects a drop in light levels, it causes the transistor to switch on, powering the relay and turning on the lights. The circuit requires few components, consumes little power and provides an efficient automatic method for controlling street lights without manual operation.

1. Automatic Street Light control
using LDR
SABIR ALI MOLLAH
NAME-SABIR ALI MOLLAH
ALIAH UNIVERSITY
KOLKATA

2. Contents
 Introduction
 Circuit Diagram
 Hardware Requirement
 Description of Hardware
 Working
 Bread-board Mounting
 PCB Mounting
 Conclusion
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3. Introduction
Many of the people have a phobia of darkness, so
to assist them in such situation, we have explained
a simple circuit. It will automatically turn on street
light in the way of LEDs or bulb coupled with relay.
Working this circuit is very much easy and also the
power consumed by the circuit is very low because
of the very few components used in the circuit.
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4. Circuit diagram
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5. Components:
 Diodes - 4
 Resistor R1 (10 K) – 1
 Resistor R2 (3300 E) – 1
 Resistor R3 (1 K) – 1
 Variable Resistor VR1 (10 K) – 1
 Capacitor C1 (470uF) – 1
 Capacitor C2 (200uF) - 1
 LDR – 1
 Transistor T1 (BC548) – 1
 Relay-1
 LED-5SABIR ALI MOLLAH

6. Description of Hardware :
(1) LDR:
 Sensitivity depends on intensity of
light.
 Resistance of LDR decrease with
increase in intensity of light and
vice-versa.
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7. (2) Resistor:
 It is a passive component having two terminals that are used
to manage the current flow in the circuit.
 A current that flows via a resistor is directly proportional to
the voltage that appeared into resistor.
 Two types:
(1) Fixed value resistor
(2) Variable resistor
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8. (5) BC548 Transistor :
 It is a general purpose silicon, NPN, bipolar
junction transistor. It is used for amplification &
switching purpose.
 Here, for switching purpose it is
biased so that it remains on if
there is signal at its base , other-
wise it is completely off.
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9. Bridge rectifier
 A diode bridge is an arrangement of four (or
more) diodes in a bridge circuit configuration that provides
the same polarity of output for either polarity of input.
 When used in its most common application, for conversion
of an alternating current (AC) input into a direct
current (DC) output, it is known as a bridge rectifier. A
bridge rectifier provides full wave rectification from a two-
wire AC input, resulting in lower cost and weight as
compared to a rectifier with a 3-wire input from
a transformer with a center tapped secondary winding.
 The essential feature of a diode bridge is that the polarity
of the output is the same regardless of the polarity at the
input.
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10. Capacitors
 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.
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11. Working of Automatic Street Light Controller
Switch Circuit:
 The working of circuit is very much easy to
understand. In this circuit, we used transformer
which will basically convert the high voltage at
input into small voltage. After it we put bridge
rectifier which will convert ac into dc current and
provide pure dc current.
 In this LDR and VR1 form one potential divider .
Here transistor is also placed which will basically
act as switch. Transistor will be operating in
cutoff and saturation mode to act as switch.
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14. Advantages:
 No man power required.
 Simple Construction.
 Efficient method.
 Less Consumption of electrical energy.
 Less maintenance.
 Cheap and economical.
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15. Conclusion:
In coming days, this will prove a great
boon to the world, since it will save a
lot of electricity of power plants that
gets wasted in illuminating the street
lights. As the conventional sources are
depleting very fast, then it`s time to
think of alternatives.
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