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Optically Operated Circuit to Sense the Intensity of the Surrounding Light

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Optically Operated Circuit to Sense the Intensity of the Surrounding Light 1 Optically Operated Circuit to Sense the Intensity of the surrounding Light PRUTHVI.K.S ,, ABHIRAM.RDr.AIT, Bangalore and , TEJUS. P , Dr.AIT [email protected] Abstract—The transistor is the key active component in II. BIPOLAR TRANSISTOR BIASING practically all modern electronics, and is considered by many to be A very proper biasing of transistor is a very crucial measure one of the greatest inventions of the twentieth century. Transistors are invariably used in tremendous applications but its use is mostly for any transistor operation that decides the performance and as integrated form. The essential usefulness of a transistor comes efficiency of the device. There are many ways of biasing a from its ability to use a small signal applied between one pair of its BJT but the very stable way of biasing is the voltage divider terminals to control a much larger signal at another pair of configuration and hence it is been implemented in our light terminals. This property is called gain. This is the only basic intensity sensing circuit. In this section we discuss on the principle used to make an Optically Operated Circuit. An LED various ways of biasing a BJT and finally conclude that the (Light Emitting Diode) connected at the output pair of terminals voltage divider way of biasing is the best. light whose intensity is proportional to the surrounding light intensity. Case 1(Using two supplies) Keywords—Transistor biasing, LDR, phototransistors, 555 timer circuit, relay. I. INTRODUCTION he transistor is the fundamental building block of modern T electronic devices, and is ubiquitous in modern electronic systems. Following its release in the early 1950s the transistor revolutionized the field of electronics, and paved the way for smaller and cheaper radios, calculators, and computers, among other things. Nowadays the modern computers have built in over a billion transistors in its embedded processors, GPUs(Graphical Processing Unit) etc. The bipolar junction transistor, or BJT, was the most commonly used transistor in the 1960s and 70s. Even after MOSFETs became widely available, the BJT remained the Fig 1. Biasing using two separate supplies transistor of choice for many analog circuits such as simple amplifiers because of their greater linearity and ease of We use two supplies for separately biasing the base-emitter manufacture. Unlike the FET, the BJT is a low–input- and collector-base section in the transistor. But here the impedance device. Also, as the base–emitter voltage (Vbe) is problem is exact value of Vbe is to be known i.e. to get a increased the base–emitter current and hence the collector– certain fixed Ie (say Ieq) so that the exact Vbe is fixed. Even if emitter current (Ice) increase exponentially according to the there is a little variation in Vbe, the current will vary by large Shockley diode model and the Ebers-Moll model. Because of amount. this exponential relationship, the BJT has a higher transconductance than the FET. WKT, Bipolar transistors can be made to conduct by exposure to Ie = (β+1)Ib light, since absorption of photons in the base region generates =>Ib=Ie/(β+1) a photocurrent that acts as a base current; the collector current It is easier to fix I than setting up V because if the current is approximately β times the photocurrent. This is the part by b beq is fixed then automatically V adjusts itself. which we mainly take advantage of using a BJT. beq Therefore, I =(V -V )/Rb b cc beq =>I = I b e/(β+1) Optically Operated Circuit to Sense the Intensity of the surrounding Light 2 But , Ic=βIb =>Ic=β{(Vcc-Vbeq)/Rb} But this is sensitive to variation of the current gain i.e. β. Hence, it is not a stable way of biasing. Case 2 (Using dual supply) Fig 4. Hence make a potential divider(Fig 4) circuit which uses only single supply (Vcc) and 2 resistors R1 & R2. By suitably choosing the values of R1 & R2 we can get any voltage which is less than Vcc-IcRc . This is the most stable way of biasing a BJT because it uses a single supply and also the merits of 1st and 2nd ways of biasing Fig 2. Biasing using dual supply are also included in it. For the light intensity indication circuit we hence use the The relationship between collector current and emitter current voltage divider configuration. We replace R1 by an LDR and is, R2 replaced by potentiometer (preferably 100k) so that the Ic=αIe sensitivity of the LDR can be altered as per requirements. Ic=α{(Vee-Vbeq)/Re} Here the collector current Ic does not vary that much due to the fact that α is not as sensitive as β. Hence, this method is more advantageous than that in case 1 , where it is highly sensitive to β variations. But the only penalty would be using dual supplies (i.e. Vcc & Vee) Case 3 (Voltage divider configuration) As in case 2 we have the following circuit, III. LIGHT INTENSITY SENSING ELEMENT The surrounding light can be sensed using devices like LDR (Light Dependent Resistor), phototransistor and few others. As the intensity varies the current output from these devices also varies. Here we use a simple LDR in the circuit because it offers high quality performance for applications that require quick response and good characteristic of spectrum. It is relatively easy to understand the basics of how an LDR Fig 3. Voltage divider configuration works without delving into complicated explanations. It is first In the shown configuration, in order to reverse bias the necessary to understand that an electrical current consists of transistor in the C-E section V < (V -I R ) ee cc c c the movement of electrons within a material. Good conductors Optically Operated Circuit to Sense the Intensity of the surrounding Light 3 have a large number of free electrons that can drift in a given monostable, astable and bistable(Shmitt trigger). For the direction under the action of a potential difference. Insulators purpose of blinking an LED the astble mode is used. with a high resistance have very few free electrons, and In the Astable mode, the 555 functions as an oscillator. therefore it is hard to make them move and hence a current to The 555 timer puts out a continuous stream of rectangular flow. pulses and the frequency of the pulses can be designed. An LDR or photoresistor can be made up of any semiconductor material with high resistance. It has a high resistance because there are very few electrons that are free and able to move - the vast majority of the electrons are locked into the crystal lattice and unable to move. Therefore in this state there is a high LDR resistance. As light falls on the semiconductor, the light photons are absorbed by the semiconductor lattice and some of their energy is transferred to the electrons. This gives some of them sufficient energy to break free from the crystal lattice so that they can then conduct electricity. This results in a lowering of the resistance of the semiconductor and hence the overall LDR Fig 6. Circuit in Astable resistance. The process is progressive, and as more light shines on the LDR semiconductor, so more electrons are released to conduct electricity and the resistance falls further. The Resistance of the LDR as a function of illumination is as shown:- Fig 7. Waveforms of Astable operation The resistors R1, R2 and the capacitor C is connected as in Fig 5. Resistance of LDR as a function of illumination Fig13. The output of the 555 which is a train of rectangular pulses(Fig 7) is given to the red LED which blinks. The There is an inverse linear relationship between the resistance frequency at which the LED blinks can be designed by fixing of the LDR and the illumination of the light incident on it. the values of R1, R2 and C. The ON time and OFF time of the Further, the current passing through the LDR varies directly in blinking LED is given by the following equations:- linear with the light illumination. Hence it can be clearly T =0.693(R1+R2)C justified that as the surrounding light intensity doubles, the on T =0.693R2C current through LDR also doubles giving a corresponding off variation in the intensity of the LED connected at the collector Frequency of blinking, f=1.44/((R1+2R2)C) side. Example:- If Ton= Toff, the output coming out of the multivibrator is a train of square pulses. IV. MODIFICATION TO THE MAIN CIRCUIT USING MULTIVIBRATOR We have designed the circuit such that when the surrounding By integrating a multivibrator circuit(a 555 timer IC), a light intensity goes low, the LED blinks for 0.5sec i.e blinking LED can be made which would indicate that the Ton=0.5sec and Toff=0.5sec. The design leads to having intensity is very low. Further we use a 6V relay to switch resistance values R1=0, R2=721.5K and C=1 micro farad. between the intensity indicating LED and the blinking LED. The modified circuit therefore is as shown, The 8-pin 555 timer must be one of the most useful IC’s ever made. With just a few external components it can be used to build many components not all of them involving timing. The timer circuit has three main operations namely Optically Operated Circuit to Sense the Intensity of the surrounding Light 4 . REFERENCES [1] Boylestad Nashelsky, “Electronic Devices and Circuit Theory,” in Plastics, 10th ed., Pearson 2010, pp.
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