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Introduction:
Automatic Street Light Control System is a simple and powerful concept, which uses transistor as a switch to switch ON and OFF the street light automatically. By using this system manual works are removed. It automatically switches ON lights when the sunlight goes below the visible region of our eyes. It automatically switches OFF lights under illumination by sunlight. This is done by a sensor called Light Dependant Resistor (LDR) which senses the light actually like our eyes.
By using this system energy consumption is also reduced because now-a-days the manually operated street lights are not switched off properly even the sunlight comes and also not switched on earlier before sunset. In sunny and rainy days, ON time and OFF time differ significantly which is one of the major disadvantage of using timer circuits or manual operation.
This project exploits the working of a transistor in saturation region and cut-off region to switch ON and switch OFF the lights at appropriate time with the help of an electromagnetically operated switch.
Automatic Streetlight needs no manual operation of switching ON and OFF. The system itself detects whether there is need for light or not. When darkness rises to a certain value then automatically streetlight is switched ON and when there is other source of light, the street light gets OFF. The extent of darkness at which the street light to be switched on can also be tailored using the potentiometer provided in the circuit.
Moreover, the circuit is carefully designed to avoid common problems like overload, relay chattering and inductive kick back in relay.
2. Principle
The automatic streetlight control system operates on 12 V DC supply. The automatic streetlight controller has a photoconductive device whose resistance changes proportional to the extent of illumination, which switches ON or OFF the LED with the use of transistor as a switch.
Light dependent resistor, a photoconductive device has been used as the transducer to convert light energy into electrical energy. The central dogma of the circuit is that the change in voltage drop across the light dependent resistor on illumination or darkness switches the transistor between cut-off region or saturation region and switches OFF or ON the LED.
3. Block Diagram & Circuit Diagram
3.1 Block Diagram
3.1.1 Individual Block Explanation
Power supply: AC power supply is stepped down, rectified and filtered to get almost ripple-free DC output for the operation of the circuit.
Light dependent resistor: LDR senses the illumination level and gives the input signal as voltage drop.
Amplifier: Darlington circuit amplifies the input current to get maximum current gain.
Switch: Relay switch closes or opens electrically and automatically, which is energized or de energized by the Darlington pair.
Street light: Street light is the output of the circuit. In this circuit, it has been replaced by LED
3.1.2 Amplification Unit
Darlington pair
In the Darlington configuration, the emitter current of one transistor becomes the base current of the second, so that the amplified current from the first is amplified further by the second transistor. This gives the Darlington pair a very high current gain such as 10000, since the Darlington configuration acts like one transistor with a beta which is the product of the betas of the two transistors. Darlington configuration can be used where high output currents are needed. The Darlington configuration has quite high input impedance.
A Darlington pair can be sensitive enough to respond to the current passed by skin contact even at safe voltages. Thus it can form the input stage of a touch-sensitive switch.
DC Current gain hFE = hFE1 X hFE2
3.1.3 ON OFF control
The circuit is switched ON or OFF by the transistor in saturation region or cut off region respectively, which is controlled by the signal from LDR. The collector current from the transistor toggle between ON or OFF modes.
3.2 Circuit Diagram
The circuit diagram of automatic street light controller is given below:
The description of all the components used in this circuit is given in chapter 5.
4. Component Description
4.1 Diode
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. A vacuum tube diode is a vacuum tube with two electrodes: a plate and a cathode.
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. Thus, the diode can be thought of as an electronic version of a check valve. This unidirectional behavior is called rectification, and is used to convert alternating current to direct current and to extract modulation from radio signals in radio receivers.
When p-type and n-type materials are placed in contact with each other, the junction is depleted of charge carriers and behaves very differently than either type of material. The electrons in n-type material diffuse across the junction and combines with holes in p-type material. The region of the p-type material near the junction takes on a net negative charge because of the electrons attracted. Since electrons departed the N-type region, it takes on a localized positive charge. The thin layer of the crystal lattice between these charges has been depleted of majority carriers, thus, is known as the depletion region. It becomes nonconductive intrinsic semiconductor material. This separation of charges at the p-n junction constitutes a potential barrier, which must be overcome by an external voltage source to make the junction conduct.
The electric field created by the space charge region opposes the diffusion process for both electrons and holes. There are two concurrent phenomena: the diffusion process that tends to generate more space charge and the electric field generated by the space charge that tends to counteract the diffusion