07-05-2012, 03:31 PM
LIGHT SENSOR
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INTRODUCTION
Light sensor circuit is widely used in projects for security systems and light control. In our project we have performed sensing operation using a light dependent resistor (LDR). This circuit will activate a relay when light falls to a preset level. This kind of circuit is basically used in automatic turn on and off of street light, security alarm etc.
For this project various components like transistor, relay, potentiometer, resistor, capacitor, diode and 741 IC are used. Components and there datasheets and working is being described in the report.
CIRCUIT DESCRIPTION
The light sensor used is the ORP12 photocell. According to this circuit, a relay will be activated when light falls to a preset level. Light level can be adjusted with potentiometer and the relay contacts may be used to operate an external light or buzzer.
Transistor is being used here as a switch. A diode is also connected which is used for the protection of relay. LM 741 works as a comparator IC. LED is being used for indication switching circuit.
By swapping the positions of the 10K resistor (R1) and the LDR (LDR1), the relay will be closed when the LDR is under light rather than under darkness. Therefore a device can automatically be switched off at night time.
Since this circuit still contains a relay we need to make some changes to reduce the amount of power to make it more suitable for renewable resource powered low-current applications.
WORKING
In our project “LIGHT SENSOR” the main sensing component used is LDR (light dependent resistor). It follows the principle that intensity of light falling is inversely proportional to resistance i.e. negative temperature coefficient.
The light sensor used is the ORP12 photocell. In bright light the resistance of the ORP12 can be as low as 80 ohm and at 50 lux(darkness) the resistance increases to over 1Mohm. The 1M control should provide a wide range for light intensities, if not its value may be increased.
The op-amp senses the voltage difference between pins 2 and 3. The control VR1 is adjusted so that the relay is off, the output of the op-amp will be around 2 volts. When light falls, the resistance of the photocell increases and the difference in input voltage is amplified by the op-amp, the output will swing towards full supply and drive the transistor and relay.
LIGHT DEPENDENT RESISTOR
A photoresistor or light dependent resistor is a component that is sensitive to light. When light falls upon it then the resistance changes. Values of the resistance of the LDR may change over many orders of magnitude the value of the resistance falling as the level of light increases.
It is not uncommon for the values of resistance of an LDR or photoresistor to be several megohms in darkness and then to fall to a few hundred ohms in bright light. With such a wide variation in resistance, LDRs are easy to use and there are many LDR circuits available.