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# L27, Automatics Street Light Using 555

A simple street light circuit diagram. When there is a need of light it automatically switches ON. When darkness rises to a certain level then sensor circuit gets activated and switches ON and when there is other source of light i.e. daytime, the street light gets OFF.

The sensitiveness of the street light can also be adjusted. In this project you can used four L.E.D as a symbol of street lamp, but for high power switching a can connect Relay (electromagnetic switch) at the output of pin 3 of I.C 555 that will make easy to turn ON/OFF any incandescent lamp that are connected through relay.

Principle-

This circuit uses a popular timer I.C 555. I.C 555 is connected as comparator with pin-6 connected with positive, the output goes high, when the trigger pin 2 is at lower then 1/3rd level of the supply voltage. So small change in the voltage of pin-2 is enough to change the level of output (pin-3) from high-to-low and low-to-high. It is powered by a 9V battery for portable use. Pin 4, 6 and 8 is connected to the positive supply and pin 1 is grounded. To detect the light (source) we have used LDR and a source of light.

LDR is a special type of resistance whose value depends on the brightness of the light which is falling on it. It has resistance of about 1 mega ohm when in total darkness, but a resistance of only about 5k ohms when brightness illuminated. It responds to a large part of light spectrum. We have made a potential divider circuit with LDR and 100K variable resistance connected in series. We know that voltage is directly proportional to conductance so more voltage we will get from this divider when LDR is getting light and low voltage in darkness. This divided voltage is given to pin 2 of IC 555. Variable resistance is so adjusted that it crosses potential of 1/3rd in brightness and fall below 1/3rd in darkness.

Sensitiveness can be adjusted by this variable resistance. As soon as LDR gets dark the voltage of pin 2 drops 1/3rd of the supply voltage and pin 3 gets high and LED or buzzer which is connected to the output gets activated.

Component used -

1. 9v Battery with strip

2. Switch

3. L.D.R (Light Depending Resistance)

4. I.C NE555 with Base

5. L.E.D (Light Emitting Diode) 3 to 6 pieces.

6. Variable Resistance of 47 KΩ

7. 330-ohm Resistor

Working:

When light falls on the LDR then its resistance decreases which results in increase of the voltage at pin 2 of the IC 555. When input falls below 1/3rd then output is set high otherwise it is set low. Since inbrightness, input voltage rises so weobtain no positive voltage at output of pin 3 to drive relay or LED, besides in poor light condition we get output to energize.

Precautions:

• Use a Sensitive LDR. You can test it using a multimeter.

• IC should not be heated too much while soldering, excess heat can destroy it. For safety and easy to replace, use of I.C base is suggested. While placing the I.C pin no 1 should be made sure at right hole.

• Please check the polarity of battery before switching ON the circuit. Opposite polarity can destroy I.C One should use diode in series with switch for safety since diode allows flowing current in one direction only.

• Output voltage of this project is 7.3 volt therefore 4 LED in series can be easily used without resistance.

• Each component should be soldered neat and clean.

• Check the polarity of LEDs and then connect.