MASSACHVSETTS INSTITVTE OF TECHNOLOGY Department of Electrical Engineering and Computer Science 6 081 Introduction to EECS I Spring Semester 2007 Thursday Lab for Week 7 Lab on designing and using a virtual oscilloscope Lab report due Tuesday April 3rd in Lecture The constraints view of circuits In Tuesday s software lab you used the constraint view of resistor networks to get the computer to analyze circuit voltages and currents In this lab you will be designing circuits with resistors and we hope this experience will help you develop intuition about how such circuits behave Specifically you will use your understanding of resistor networks to design a virtual oscilloscope and then use that oscilloscope to investigate the behavior of a LEGO motor This lab is new and to allow us to experiment with some of the material we have made some parts optional Please also be prepared that we may make announcements if we see common difficulties which we did not predict Thursday Lab Designing and using a virtual oscilloscope In this lab you will design a resistor network that will allow you to measure voltages over a wide range Then you will write a python program that will create a graph of a sequence of voltage samples taken at regular time intervals effectively creating a virtual oscilloscope Finally you will use your virtual oscilloscope to measure the relationship between voltage and current for a LEGO motor and use your knowledge of circuits to develop a circuit model for the motor For this lab you will not need the robot but you will need a laptop and National Instruments NI Interface Box USB cable and screwdriver A number of resistors A Lego motor with connector A protoboard with built in power supply We will be using the NI box in a number of labs to interface to circuits sensors and motors The NI box can be used to measure voltages and convert those voltages to numbers that can be accessed using python We have provided a python program which will read numbers generated by the NI box specifically the numbers that are related to the voltages at the NI box terminals labeled AI0 AI1 and AI2 Note that for the NI box the voltages are measured with respect to the voltage at the NI box terminal labeled GND Our program daqtry py is an infinite loop which reads from and writes to the NI box and prints the results The program is in the subdirectory NILab in the home directory on the lab laptops You will also be using a protoboard to build some small circuits A protoboard is used for making easily modifed electrical connections between wires and circuit elements If you look at your protoboard you will notice many rows of five holes These holes are electrically connected so if you plug two wires in to two holes in the same row the wires will be connected electrically Resistor leads can also be plugged directly in to the protoboard holes Also each protoboard has several long columns of holes which are used for nodes in a circuit that have a large number of connections These columns are often used for ground and power CAREFUL the long columns are only electrically connected for HALF THE COLUMN If you have never used a protoboard have one of the staff members demonstrate the board s use Try using the NI Box Connect the NI box to your laptop it has a USB connector and try running the daqtry py program to read the voltage at AIO Please note that you will need to connect wires to the AI0 and GND terminals to connect circuitry to the NI Box You should also take note of the fact that the NI box has terminals which generate two reference voltages 5 volts and 2 5 volts We have special screwdrivers wire and wire strippers just ask if you need them Try connecting AI0 to GND and to the two reference voltages and note the readings returned by the daqtry py program PLEASE NOTE Sometime the interface to the USB port gets confused and you get a pile of error messages when you run daqtry py If so exit any python interpreter you are running unplug the NI box from the USB port wait a few seconds and then replug in the NI box Then try running daqtry py again If that does not fix the problem ask for help Checkpoint 2 15 PM Demonstrate that you can use python to read voltages from the NI box Rerange the input using a resistor network The input voltage range for the NI box AI0 input is from 0 volts to 5 volts but the power supply for the protoboard can generate voltages in the range of 0 volts to 15 volts Your job will be to design a resistor divider network that will reduce voltages in the range from zero volts to 15 volts to voltages in the range from zero volts to five volts The reduced voltage range can then be measured using the AI0 input and you can use python to post process the measured voltage and undo the effect of the resistor divider In this way you can use the NI box to effectively measure a larger range of voltages making your own volt meter To see how to design such a circuit consider the diagram below of the NI box an external resistor network and the connection points for a test voltage supplied here by the protoboard 15 variable supply and the protoboard ground If R2 10 000 10k what value should R1 have so that the voltage at AI0 with respect to GND will be five volts when the protoboard adjustable supply voltage is 15 volts Once you have decided on a resistor value for R2 explain your solution to your LA Then please use the protoboard we have given you to construct your external resistor network In order to construct your network you may need to use series and parallel combinations of the resistors we have available to generate the resistance values you need Ask you LA to show your our labeled box of resistors Finally modify daqtry py so that the printed voltage matches the test voltage You can demonstrate your volt meter using the protoboard s adjustable voltage supplies USB Cable V1 To Protoboard variable 15v AI0 R1 National Instruments V2 R2 GND To Protoboard Ground Checkpoint 3 15 PM Demonstrate that you can measure voltages from 0 to 15 volts using the NI box and your resistor network Optional Challenge if you have time The NI box has a five volt reference voltage Can you add a resistor to the divider network and then adjust the values of the resistors so that you can measure test voltages from 15 volts to 15 volts Using the Motor We have supplied you with a motor and you will be using the motor in the next lab to make a moving head for the robot For this lab you will be examining the voltage
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