25-08-2017, 09:32 PM
Light meter design project
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ABSTRACT
This project use a light dependent resistor (LDR) as a sensor of the light ,to
build a lightdensity meter and calibrate it against the lux meter
By using the Lux and multimeter the relationship between the light and the
resistance is found also the maximum and the minimum values of resistance are
taken then the DC bridge circuit is built which used to found the relationship
between the resistance(or the light) and the output voltage .
Anumber of reading for the output voltage is taken with the corresponding
lux values and a relation ship with the least resolution between them was found
using excel program
A 16f876 pic microcontroller is used and the code was written using c
program to take two analogue inputs relates to the output voltage and convert it
to digital then substitute it in the relation ship and display the value of the lux
on the LCD
Finally the simple circuit which contain the LDR and other resistors can
give the value of LUX at any light density
INTRODUCTION:
LDRs or light dipendant resistors are very very useful sensor especially on
light/ dark sensor . normally the resistance of an LDR is very high , but they are
illuminated with light resistance drop dramatically .
Everything has an electrical resistance, some more than others. An LDR
will have a resistance that varies according to the amount of visible light that
falls on it. A close up of an LDR is shown below:
The light falling on the brown zigzag lines on the sensor, causes the
resistance of the device to fall. This is known as a negative co-efficient. There
are some LDRs that work in the opposite way i.e. their resistance increases
with light (called positive co-efficient).
THEORY:
The internal components of a photoelectric control for a typical American
streetlight. The photoresistor is facing rightwards, and controls whether current
flows through the heater which opens the main power contacts. At night, the
heater cools, closing the power contacts, energizing the street light. The
heater/bimetal mechanism provides a built-in time-delay.
A photoresistor or LDR is an electronic component whose resistance
decreases with increasing incident light intensity. It can also be referred to as a
light-dependent resistor (LDR), photoconductor, or photocell.
A photoresistor is made of a high-resistance semiconductor. If light falling
on the device is of high enough frequency, photons absorbed by the
semiconductor give bound electrons enough energy to jump into the conduction
band. The resulting free electron (and its hole partner) conduct electricity,
thereby lowering resistance.
A photoelectric device 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.
Wheatstone Bridge
The Wheatstone bridge is a type of d.c. bridge that is used for precision
measurement of resistance from approximately 1 ohm to the low mega-ohm
rangerange. A typical Wheatstone null type bridge is shown in Figure 1 below.
An excitation voltage source is used to operate the bridge (Vi). A
galvanometer is used to connect the mid-points of the right hand side voltage
divider (made up of R2 and R4) and the left hand side voltage divider (made
up of R1 and R3). The galvanometer connection between the two mid-points
form a bridge between the two sides, hence the name of the device.
When used as a null type device, it can produce an accurate
measurement of resistance. When used in the deflection type mode, it can
produce a change in an output voltage that is proportional to a change in the
sensor under question (a resistor). The change in the resistance of the sensor
is representative of a change in the value of an external variable (e.g., stress,
force, temperature). The null type mode is more accurate than the deflection
type mode, as the error in the former case will be in the mV or μV compared
to fraction of a volt in the latter case