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EE 40 Spring 2008—Final Project 1EE 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 ([email protected]) 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.EE 40 Spring 2008—Final Project 2Part II: Basic Circuit—Light Sensor 1. Function analysis Figure 1. Basic circuit diagramEE 40 Spring 2008—Final Project 3This 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. Initially0=outV and there is no charge on the capacitor. However to the reference 4.5 V, V 5.4−=outV and V 5.4−=−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 5.45.4433−=>+−=−+VRRRV , so the output goes high and get clamped to 4.5 V immediately after the circuit is on. Now the circuit is in state 1: V 5.4=outV , 4335.4RRRV+=+and capacitor C3 is being charged fromEE 40 Spring 2008—Final Project 4Vout. 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 +−> VV . Now the circuit is in state 2: V 5.4−=outV and 4335.4RRRV+−=+. The 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, 4335.4RRRV+=+ and the 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 outV on the same scale as shown in Figure 3(b). It’s clear that the threshold voltages of output high and low are 4335.4RRR+×V and 4335.4RRR+×−V. Go back to the original circuit with reference at 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 434325.4RRRR++and 4345.4RRR+. (a) (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.EE 40 Spring 2008—Final Project 5 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 output is high, which will light up the red LED. Based on the functions of all the building blocks, the function of the whole circuit is as follows. When light is off, the comparator buffer output is low. So the AND output is low, thus red LED is off. However, the oscillator output will make the green LED flashing.


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Berkeley ELENG 40 - EE 40 Final Project

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