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INTRODUCTION
This light meter is used measure the light in ranges from dark light to daylight in lux. In this meter I have used a light dependent resistor (LDR) as a sensor to measure light. The sensor changes its resistance according to intensity of light falling on it.
The LDR has maximum resistance when its exposed to dark light minimum for the day light. The minimum and maximum values of resistance are measured. This LDR is connected in the voltage divider circuit and the output voltage is noted down for the different light intensities. The voltage is measured in all the conditions in dark room and day light and the corresponding lux values for the light.
A relationship is found between the intensity of light(lux) and output voltage from the voltage divider circuit.
This output voltage is given to the LM324 which is used as a buffer by connecting it in a negative feedback circuit. The output from LM324 buffer is given to the one of the input port of microcontroller.
A ‘ADUC841’ microcontroller is used and the code is written using embedded C to get the analog input voltage and convert it to digital then substitute it in the relationship and display the value of LUX on the Liquid Cristal Display.
Finally the simple circuit which contain the LDR and 22kΩ resistor are connected in an voltage divided circuit which gives the value of LUX at any light density
SPECIFICATIONS:
GENERAL SPECIFICATIONS:
Plug and play- and ready for outdoor mounting
Voltage supply 5v
Max output Current 4.5mA
Length width height 100mm x 100mm x 15mm
Weight 167g
Measure range 0 to 255 lux
Detection limit 1 lux
Operating temperature –40°C to +55°C.
Storage temperature –40°C to +85°C
BACKGROUND INFORMATION ON SENSOR:
A Light Dependent Resistor (LDR) is an electronic component whose resistance decreases with increasing incident light intensity on it.
A Light Dependent resistor is made up of a cadmium sulphide containing no or all most negligible free electrons when not illuminated. LDR’s resistance is then quite high when its not illuminated. When its luminated, electrons are liberated it then has free electrons for the conductivity. Therefore the conductivity of the material increases. Due to which Cadmium sulphide (CdS) is also known as photoconductor.
The relationship between the resistance and illumination is given R = kE^-a where E is illumination in lux, R is resist¬ance in ohms, k and a are constants. These values depends on the cadium sulphide used and on the manufacturing process. Values around 0.4 to 0.7 are quite common If light falling on the device is of high enough frequency. When light falls on it photons are absorbed by the semiconductor which gives bound electrons sufficient energy to jump into the conduction band. The free electron (and hole partner) conduct electricity, thereby decrease in the resistance.
This devise can be can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, eg. silicon. In intrinsic devices, the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities added, which have a ground state energy closer to the conduction band. Since the electrons don't have as far to jump, lower energy photons (i.e. longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms(impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.