EE40 Final Project 2 Light Sensor Name SID 1 Name SID 2 Introduction In 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 Diagram Figure 1 Block diagram of the light sensing circuit Block 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 the detection 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 or output 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 K 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 voltage Square Wave Generator Figure 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 amp will be determined through a voltage division V 3 X R4 R3 R4 V Because the negative terminal of the op amp is greater than the positive terminal of the opamp 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 potential of the other side of the resistor R5 This charging happens with the time constant of 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 toward the 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 3 X R4 R3 R4 and 3 X R4 2R3 R3 R4 with the time constant of R5 C3 and the output 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 Vout Light Detection R6 R7 Figure 5 Light generation and detection circuitry Light 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 Flash light Photoresistor Room light Photoresistor IR LED IR detector Phototransistor R6 50 R7 10K 1K 100K Figure 6 Light generation and detection phototransistor FPT100 pinout Output V 0 5 2 4 0 4 Comparator Figure 7 Comparator Schematic An 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 comparator output will be low and when there is no light it will be high 6V AND The easiest way to implement an AND function is via two diodes and a …
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