Name & SID 1 :Name & SID 2 :IntroductionBlock DiagramSupply CircuitSquare Wave GeneratorLight DetectionComparatorANDComponent listEE40 Final Project-2 Light SensorName & SID 1 :Name & SID 2 :IntroductionIn this Project you will implement a light sensing circuit. When there is no light shining on the detection part of the circuit, the green LED will be flashing. At the presence of light the red LED will start flashing instead of the green one.Block DiagramFigure 1 . Block diagram of the light sensing circuitBlock diagram of the light sensor is depicted in Figure 1. We are using a single 6V battery for this project, but for different building blocks we need both 3V and 6V as supply voltages. Therefore through the “Supply Circuit” block we generate this additional 3V supply voltage.A “Square Wave Oscillator” generates a 0-6V square wave signal required for making LEDs flash.A “Light Detection” block detects if there is any light being shined on the detection part or no. When there is no light the output of the detection circuit is 0 and in the presence of light it goes up, close to 3V. A comparator compares the output of detection block with a threshold. When thedetection signal is higher than the threshold (there is light) the output of comparator is low and when the detection output is lower than the threshold (no light) the output of comparator is high.The comparator output needs to be connected to a MOSFET to be able to drive (provide enough current) for subsequent blocks and LEDs. When comparator output is high(6V), drain of the MOSFET is low(0V) and when the comparator output is low, MOSFET’s drain is set to 6V.LEDs will emit light when their anodes (longer leads) are connected to higher voltages with respect to their cathodes(shorter leads). Therefore to make the green LED flash when there is no light at the input, its anode should be connected to square wave generator output while its cathode being connected to the drain of the MOSFET. Anode of the red LED can be connected to ANDed signal of 1-square wave generator output and 2- drain of the MOSFET while its cathode being grounded, hence when there is light the red LED flashes instead of the green one. LEDs never should be directly connected to a voltage source oroutput of a logic circuit(or output of any other circuit that can be approximated as a voltage source in series with a low impedance). Because if a voltage is directly applied to anode-cathode of the LED, it draws a huge current which burns the LED. So LEDs should always be in series with a resistor (in the range of KΩ) to limit the current.Supply Circuit The circuit will be powered by a 6V battery. But lower DC voltages are also needed for other building blocks. This additional voltage is generated through the circuit which its schematic is depicted in Figure-2.Since 3V is half of the 6V supply, R1 and R2 are equal. C1 and C2 are filtering capacitors which are usually placed in parallel with the DC power supplies to remove any AC signal and high-frequency noise.Figure 2. Builing block for generating additional 3V DC supply voltageSquare Wave GeneratorFigure 3. Square wave generator The op-amp feedback is positive, which means that the output voltage is either Vss=0 or Vcc=6V. The output voltage toggles between these two values through the charge and discharge of the capacitor (C3). Initially there is no charge on the capacitor (Q=CV) hence the voltage drop across the cap is zero and the negative terminal of the op-amp is at 3V (V_=3V). Since Vout is at zero potential at the beginning, the positive terminal of the op-ampwill be determined through a voltage division. V+=3 X R4 / (R3+R4) < V_ . Becausethe negative terminal of the op-amp is greater than the positive terminal of the op-amp the output of the ap-amp will go down to Vss (zero potential in this case).Now capacitor C3 starts to be charged, So there will be a positive voltage difference between top and bottom plates of the capacitor. The top-plate stays at 3V and bottom plate(V_) moves down to reach to the zero potential (the potentialof the other side of the resistor R5). This charging happens with the time-constantof (R5 C3). Whenever the bottom plate(V_) that is moving down towards zero reaches and slightly goes below the voltage level of the (V+=3 X R4 / (R3+R4) then the condition will be (V+ > V_) and the output toggles to VCC(6V).When the output is at 6V, we use the superposition to find the V+ :V+ = 3 X R4 / (R3+R4) + 6 X R3/ (R3+R4) = 3 X (R4+2R3)/(R3+R4)Now the capacitor C3 will be discharged so that the bottom plate (V_) rises towardthe voltage value of the other side of the resistor (6V). In this scenario when V_ is moving up from 3 X R4 / (R3+R4) towards 6V with time constant of (R5 C3) at some point it reaches and goes slightly above V+= 3 X (R4+2R3)/(R3+R4) and that’s the point where again (V+ < V_) and the output toggles to 0. Hence the negative terminal of op-amp charges and discharges between 3X R4 / (R3+R4) and 3 X (R4+2R3)/(R3+R4) with the time constant of (R5 C3) and theoutput toggles between 0 and 6V, generating a square wave.Human eyes can differentiate frequencies below 30Hz. Based on the explanation above calculate the frequency of the square wave based on R and C values and choose R and C values in such a way that the frequency of the square wave is almost 4Hz.Figure 4. Plot of V_ ,V+ and VoutLight DetectionFigure 5. Light generation and detection circuitryLight generation can be done via visible LED, room light or infrared(IR) LED. Light source intensity is determined by the sensitivity of the detector. Photo transistor can be used as detector. Light exposure causes an increase in the current flow of phototransistor and pulls up the output voltage of the detection block. Here are the list of choices that you have.Source/Detector R6 (Ω) R7(Ω) Output(V)Flash light/Photoresistor - 10K 0-5Room light/Photoresistor - 1K 2-4IR LED/IR detector (Phototransistor) 50 100K 0-4Figure 6. Light generation and detection, phototransistor FPT100 pinoutR6R7ComparatorFigure 7. Comparator SchematicAn open loop op-amp as shown above can be used as a comparator. Since the input (V_) is the output of the detection circuit and varies from close to 0 to a voltage close to 3V, by connecting V+ to the middle point of a potentiometer this V+(in other words the threshold voltage) will be set in the mid-range of 0-3V. therefore in the presence of light the
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