UC Berkeley EECS Department EECS 40 42 100 Lab B E Boser 1 UID PRJ1 Project 1 Light Gate Light Gate In this project you design build and test a light gate A light gate uses a light source directed to a photodetector to detect interruptions of the light beam Applications include automatic doors or intrusion alarms This project includes two main parts a Pre lab follow the direction given in this document and complete the schematic design of your Light Gate circuit Note You must have your schematic ready and checked off by a GSI before you start building the actual circuit in the lab 50 of the grade b Lab and Post lab Build and test your Light Gate circuit in the lab base on your design show your working circuit to GSI and answer the post lab questions 50 of the grade Guidlines We will be using the halogen lamp and solar cell from an earlier lab as the light source and detector and display the output on LED light emitting diode that turns on or off when the light beam is interrupted and off otherwise You are to design the electronic circuit between the solar cell and the LED Figure 1 shows a block diagram of the light gate Since the output of the solar cell is a small current it needs to be ampli ed to drive the LED A Schmitt Trigger is also needed to ensure that the light reliable turns on and off without intermediate states that are dif cult to interprete You can tie the LED directly to the output of the Schmitt Trigger through a resistor Suggested plan Pre Lab a First we determine how to best use the solar cell as a sensor We want to maximize the ratio of the output Ss on with full light beam not blocked at maximum distance to the output Ss off with the light blocked at minimum distance We use maximum and minimum distance respectively since that s the most challenging situation our circuit has to deal with if it meets these requirements it most likely will also work in the other less demanding situations We can either sense the open loop voltage Vs on Vs off from the solar cell or the short circuit current Is on Is off Fill in the table below reusing your measurements from the solar cell lab 1 pt Vs on for L 20 cm Vs off for L 10 cm rv Vs on Vs off Is on for L 20 cm Is off for L 10 cm ri Is on Is off 0 1 pt 1 1 pt 2 1 pt 3 1 pt 4 1 pt 5 Which of the two ratios rv and ri is larger Depending on the answer we are better off treating the solar cell as a voltage or current source and then design our circuit appropriately Enter your choice in the box below enter the text voltage source or current source Halogen Light Solar Cell Amplifier Schmitt Trigger RD 1k D1 Figure 1 Block diagram of the light gate 1 February 23 2010 PRJ1 v779 http ucbfeedback com 1 pt 6 b Design the ampli er circuit to gain up this signal to 5 V Von for maximum solar cell output What is the output voltage Voff for Ss off S is either V or I depending on your answer above Verify your circuit in the lab calculated Von Voff measured 1 pt 1 pt 0 1 pt 1 1 pt 2 3 c Determine thresholds Vth on and Vth off for the Schmitt Trigger to achieve reliable operation for distances between the light and the detector of 10 to 20 cm based on the range of values you determined for Von and Voff The output of the Schmitt Trigger should be a reliable indicator of the beam for any distance L between 10 cm and 20 cm calculated measured 1 pt 1 pt Vth on 5 4 1 pt 1 pt Vth off 6 7 d Draw the complete circuit diagram including the ampli er and the Schmitt Trigger Include all derivations and calculations of the component values Attach extra sheet if necessary and draw nicely for full credit 20 pts 8 e Verify your design with SPICE optional In practice you would always do this before going to the lab with a circuit of any complexity Debugging in the lab is much more time consuming than with the simulator and paper and pencil 2 February 23 2010 PRJ1 v779 http ucbfeedback com Note To simulate the complete circuit replace the sensor with a piece wise linear PWL current source in SPICE ramping the current linearly up and down between zero and an appropriate maximum value Then perform a transient simulation Please come to of ce hours if you have dif culty f Ask the GSI to check your schmatic and calculation at the start of the lab session 50 of the grade Lab Experimental Veri cation Now you are ready to build and verify the operation of your circuit It s a good idea to work in steps For example we verify rst the light sensor did that in Lab 1 already and then the ampli er and the Schmitt Trigger separately When we have convinced ourselves using e g the DVM that each part works satisfacorily we connect all parts and verify again a Build and test your ampli er circuit in the lab b Test Schmitt Trigger circuit in the lab use the power supply to apply test inputs Add the LED and verify its operation Use a smaller resistor if it is too dim c Once the ampli er circuit and Schmitt Trigger are fully functional complete the circuit d Draw your nal complete circuit diagram including all component values if different from your original design Draw nicely for full credit Use extra sheets if needed 1 pt 10 e Show your working circuit to the GSI for L 10 cm and L 20 cm Post Lab a Report dif culties you encountered if any 1 pt 12 3 February 23 2010 PRJ1 v779 http ucbfeedback com b Did you make changes to your original design If so why 1 pt 13 In a product design all involved marketing designers and product test and quality control engineers get together at this point to discuss the need for further changes and improvements Once everyone is happy you will build a few samples for further testing and show them to potential customers For these samples you design custom printed circuit boards PCBs and use an attractive package to give the entire system a professional appearance While the salesforce shops the new product around with customers the product engineers start readying the production chain Often third parties are involved e g to assemble and test the cirucit boards and to fabricate the case Password 4 February 23 2010 PRJ1 v779 http ucbfeedback com EE 42 100 Proj1 Light Gate Prelab Summary Name SID Answers 0 1 2 3 4 5 6
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