LIGHT CONTROLLED SYSTEMS1) Aim of the experimentHere we try to understand the meaning of reversing the polarity of comparator inputs and also the threshold level of the comparator on a light controlled system.2)Introduction A wide range of optical sensors are available , responding to variations of incident light. I will try to give general information about them ; Instrumentation for optical measurements generally consists of a light source, a number of opticalcomponents to generate a light beam with specific characteristics and to direct this light to somemodulating agent, and a photodetector for processing the optical signal.An optical sensor unit, free from electrical noise, includes a sensor case which has both lateral sides having high frequency connectors mounted thereon, and a metallic plate mounted inside the case. The metallic plate has both ends connected to inner conductors of the high frequency connectors, and its upper surface having a current and a voltage sensor unit mounted thereon for measuring the high frequency current and voltage, respectively. The current sensor unit includes total-reflection mirrors and a current sensor, and the voltage sensor unit includes further total-reflection mirrors and a voltage sensor. The voltage sensor has its upper surface having an electrode provided thereon to be connected to the sensor case. The laser light from outside is input to the current and voltage sensor units over optical fibers. The signal light output from the current and voltage sensor units is taken outside on output optical fibers.Recently, fibre optical sensors (FOS) have gained increased popularity and market acceptance. In comparison to conventional sensors they offer a number of distinct advantages which makes them unique for certain types of applications, mainly where conventional sensors are difficult or impossible to deploy or can not provide the same wealth of information.Aphotodiodeis a type ofphotodetectorcapable of convertinglightinto eithercurrentorvoltage, depending upon the mode of operation.The common, traditionalsolar cellused to generate electricsolar poweris a large area photodiode.Photodiodes are similar to regularsemiconductordiodesexcept that they may be either exposed (to detectvacuum UVorX-rays) or packaged with a window oroptical fiberconnection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode use aPIN junctionrather than ap-n junction, to increase the speed of response. A photodiode is designed to operate inreverse bias.

TYPES OF FIBRE OPTICAL SENSORSAccording to the spatial distribution of the measurand (the quantity to be measured), FOS can be classified as...Point sensors:the measurement is carried out at a single point in space, but possibly multiple channels for addressing multiple points. Examples are Fabry-Perot sensors and single Fibre Bragg Grating (FBG) sensors.

Integrated sensors:the measurement averages a physical parameter over a certain spatial section and provides a single value. An example is a deformation sensor measuring strain over a long base length.

Quasi-distributed or multiplexed sensors:the measurand is determined at a number of fixed, discrete points along a single fibre optical cable. The most common example are multiplexed FBG's.

Distributed sensor:the parameter of interest is measured with a certain spatial resolution at any point along a single optical cable. Examples include systems based on Rayleigh, Raman and Brillouinscattering.3 )EXPERMENTAL DATAComparator Threshold LevelChannel 1Channel 2

Max. / Min.Max. / Min.

1.52.58 V / 1.18 V8.27 V / - 11.96 V

2.52.56 V / 1.16 V8.27 V / - 4.90 V

3.52.55 V / 1.34 V8.27 V / -11.7 V

4) QUESTIONS

5 ) CONCLUSIONIn question one the audible alarm sounds when the light level is high ;Because when the light level is high , the channel 1 voltage exceeds the treshold value. Then it begans to sound.In question two ; alarm sounds for progressively longer periods because the time required for exceeding the threshold value is getting longer for increasing threshold values.In question three ; reversing of input makes vice verca effect to working principle of the circuit and it stops.(When the light level is high enough for the photocell output voltage to be below the comparator reference level , the output of the comparator flips negative.The output of the comparator is used here.)Any increase in threshold voltage level causes to increase the light intensity.