EE 40 Spring 2008 Final Project EE 40 Final Project Part I General instructions 1 The final project will count for 7 5 of your grade 2 This project is composed of a basic part and a creation part Circuit diagram will be provided for the basic part while no circuit diagram will be given for the creation part 3 Every effort must be made to team up as a team of 1 or 2 NO EXCEPTIONS One of your team members will keep the board and components during the project period Try to protect your progress and keep it in good condition 4 Every group will get a project kit with a piece of breadboard and all the components Please check with the component list in lab guide part V to see if the kit is complete Keep it in a good condition during the whole project We don t have many extra parts to supply if you lose any of them 5 The grading will be mainly based on the lab report and the circuit functionality Typed lab report is preferred Hand written is also fine but has to be neat and clear Please see part IV for more information on the lab report 6 Lab report is due on May 12th last day of instruction 5 00 pm PST in the EECS 40 Spring 2008 Homework box in 240 Cory 7 Final contest is on May 12th last day of instruction 6 00 pm 8 00 pm PST in 306 Soda hall Prizes TBA Please email Chaohao wang chaohaowang gmail com if you intend to take part in the contest Make sure your email includes name s of the group members and what your project demo will be about 1 EE 40 Spring 2008 Final Project Part II Basic Circuit Light Sensor 1 Function analysis Figure 1 Basic circuit diagram 2 EE 40 Spring 2008 Final Project This is a light sensing circuit When there is no light shinning on the circuit mainly the detection part the green LED will be flashing When light is shinning on the circuit mainly the detection part the red LED will start flashing instead of the green one The whole circuit can be torn down into 5 basic building blocks labeled A E in the diagram We will analyze them one by one in order to understand the functions A 4 5 V DC power supply The circuit is going to be powered by a 9 V battery but lower DC power is also needed in the circuit Part A is using an op amp to build up a simple voltage follower which gives 4 5 V output Since 4 5 V is half of 9 V resistors R1 and R2 are equal C1 and C2 are filtering capacitors which are usually connected in parallel with the DC output in order to filter out any high frequency AC signal coming from the noise B Square wave oscillator Let s analyze it in a time sequence and use 4 5 V as the voltage reference The circuit diagram can be redrawn as shown in Figure 2 b a b Figure 2 Square wave oscillator a with reference at 0 V b with reference at 4 5 V Initially Vout 0 and there is no charge on the capacitor However to the reference 4 5 V Vout 4 5 V and V 4 5 V Since this circuit has a positive feedback loop any slight difference between V and V may make Vout high or low clamped to the power supply value In this initial condition V 4 5 R3 V 4 5 V so the output R3 R4 goes high and get clamped to 4 5 V immediately after the circuit is on Now the circuit is in state 1 Vout 4 5 V V 4 5 R3 and capacitor C3 is being charged from R3 R4 3 EE 40 Spring 2008 Final Project Vout So V is rising up When V is as high as or just slightly higher than V the circuit toggles because of the positive feedback Output will be clamped at 4 5 V since V V Now the circuit is in state 2 Vout 4 5 V and V 4 5 R3 The R3 R4 capacitor is being discharged from Vout And V is falling down When V is as low as V or just slightly lower than V the circuit toggles again because of the positive feedback Then the output is clamped at 4 5 V again V 4 5 R3 and the R3 R4 capacitor is being charged from Vout again Therefore we see that the circuit is toggling between the two states and the output voltage is either 4 5 V or 4 5 V which forms a periodic square wave To understand the oscillation function better we draw wave forms at V V and Vout on the same scale as shown in Figure 3 b It s clear that the threshold voltages of output high and low are 4 5 R3 R3 V and 4 5 V Go back to the original circuit with reference at R3 R4 R3 R4 0 V We just need to add a DC offset 4 5 V to all the result and waveforms as shown in Figure 3 a The threshold voltages changes to 4 5 a 2 R3 R4 R4 and 4 5 R3 R4 R3 R4 b Figure 3 Waveforms of V V and Vout a with reference of 0 V b with reference of 4 5 V C Light generation and detection Light generation can be realized by a visible LED or any white light source room light flash light etc Light source intensity is determined by the detector sensitivity Phototransistor can be used as a detector Light exposure will increase the current flowing through the phototransistor Photoresistor is another choice whose resistance will change when exposed to the light Infrared IR LED and detector are also available for light generation and detection 4 EE 40 Spring 2008 Final Project D Comparator and buffer Open loop op amp can be used as a comparator Positive input connects to a fixed DC voltage used as a reference Negative input connects to the detector end When detector detects light current increases Therefore the voltage across R7 which is connected to the negative input of the op amp increases When it s lower than the reference voltage the output will be high 9 V When it s higher than the reference the output is low 0 V Thus the light on off is converted to L H of the op amp output An NMOS will be connected to the output of the comparator as a buffer since the output will be driving the LED and the logic circuit following When comparator output gate voltage of the NMOS is low the NMOS is off thus the drain is high or floating When comparator output is high the NMOS is on thus the drain is grounded This way we realize the light on off to output H L conversion E AND gate Only when both inputs are high the …
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