# Automatic Street Light

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## Introduction: Automatic Street Light

A simple project yet effective in terms of power save. Many time it happens during day time street lights are kept ON till someone notice thus leading to huge amount of energy loss.

List of Hardware Components:

1) Light Dependent Resistor (LDR) - 8mm

2) 2N2222 Transistor - Metal Package

3) 2 Pin Screw Connectors (PCB)

4) DC connector Female

5) 40 Pin Male to Male Jumper Wires (2.54 mm)

6) 12V Power Supply

7) Resistor 100K

8) 8mm 0.75W Super Bright StrawHat White LED

9) Slide Switch - PCB Mount (Pitch 0.1 inch)

OR

General purpose Dotted PCB

Tools (only needed If making circuit on dotted PCB instead of breadboard):

1) Soldron - Soldering Iron 25W 230V

2) Solder Wire

3) Wire Stripper & Cutter

Software Used:

1. Proteus – for circuit simulation

2. Fritzing – for breadboard circuit design

### Teacher Notes

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## Step 1: Photoresistor or Light Dependent Resistor LDR

A photoresistor or light dependent resistor LDR is a component that is sensitive to light. When light falls upon it then the resistance changes.

Values of resistance of an LDR or photoresistor change to several Megaohms(MΩ) in darkness and then fall to a few hundred ohms in bright light. With such a wide variation in resistance, LDRs are easy to use in many application circuits. Here we will be using LDR to automatically control the demo Street Lights.

## Step 2: Transistors

Unlike resistors, which enforce a linear relationship between voltage and current, transistors are non-linear devices. They have four distinct modes of operation, which describe the current flowing through them. (When we talk about current flow through a transistor, we usually mean current flowing from collector to emitter of an NPN.)

The four transistor operation modes are: Saturation – The transistor acts like a short circuit or closed switch. Current freely flows from collector to emitter. Cut-off – The transistor acts like an open circuit or open switch. No current flows from collector to emitter. Active – The current from collector to emitter is proportional to the current flowing into the base. Reverse-Active – Like active mode, the current is proportional to the base current, but it flows in reverse. Current flows from emitter to collector (not, exactly, the purpose transistors were designed for).

Here in this application NPN transistor 2n2222 will be operated in Saturation (closed switch) and Cut-off (open switch) modes. There are variants available of 2n2222 as plastic(TO-92) and metal(TO-18) form. I have used metal one since more current handling capacity from collector to emitter(max. 800 mA).

## Step 4: During Presence of Light

When there is Light during day time then LDR resistance decreases. This makes voltage at base less than 0.6V and so, transistor moves in Cut-off mode – no current flows from Collector to Emitter acting as open switch.

## Step 5: During Absence of Light

When Light intensity starts to decrease than LDR resistance increases. This makes voltage at base greater than 0.6V and so, transistor moves in Saturation mode – current flows from Collector to Emitter acting as closed switch.

## Step 6: Simulation

You can download ldr_streetLight.DSN provided here and open in proteus software to simulate.

### Attachments

Implement circuit on breadboard to test or you build circuit on dotted PCB

## Recommendations

Large Motors Class

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