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Potentiometer Circuit Assignment

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                    Lab Assignment 1: Potentiometer Circuit
    Due week 0-1 at Beginning of Section in the Mechatronics Lab (9/24*-9/30)
                       * 9/24 section due date is 10/1 in Lab

Reading: Electronic Devices for the Mechanical Experimenter (course pack and on course
website, www.maelabs.ucsd.edu/mae156)

Overview
A potentiometer (pot) is a variable resistor. Pots will be used in the MAE156A project as sensors
for reading a shaft position using the microprocessor’s Analog to Digital (A/D) converter. In this
assignment you will refresh MAE170 material, learn how to wire breadboards, and read a
specification (spec.) sheet.




                                     Pot used as shaft sensor.
                                   Digikey part number CT2158
                                  See www.digikey.com for specs

Standard Symbols for Electronic Components


                          Fixed Voltage   5VDC
                                                           MCU
        Pot

                               Ground
                                                     Microprocessor Unit

1) Draw Circuit of Pot Used as Position Sensor:
       a. Draw a circuit using standard electronic symbols showing how to connect a 5K pot
          so that it will provide a 0-5VDC as the potentiometer knob is rotated. You can
          assume that a constant 5VDC is provided.




                                                 1
2) Current and Power Analysis:
       Redraw the circuit from question 1 and label the current(s) on the circuit.
       a. Calculate current(s) that flows through the circuit, assuming that the A/D converter
          on the MCU operates like an ideal voltmeter.
       b. Calculate the power dissipated in the pot.
       c. Lookup the specs of the breadboard pot at www.digikey.com. Identify:
               Price per unit in quantity of 1: _____ Price per unit in quantity of 500: _____
       d. Follow on-line link to Datasheet and identify rated specifications (specs). Make sure
          to include units. For each item indicate actual value for use in circuit for question 1.

                        i.   Max Power:         Rating: _______      As used in circuit: _____
                       ii.   Max DC Voltage: Rating: _______         As used in circuit: _____
                      iii.   Angle of Effective Rotation:     Rating: _______
                      iv.    Frictional Torque:               Rating: _______
                       v.    Linearity:                       Rating: _______




                                              2
3) Analysis of why 5K pot was selected for this circuit.
       Consider part specs, power use in circuit, and noise due to stray charge
       Remember prefixes: m=>10-3, K => 103, M=> 106


       b. What would the current be if we used a 5 pot (much smaller than the 5K pot
          used)? Would this be a good pot value for this application, and why?




       c. What would the current be if we used a 5M pot (much larger than the 5K pot
          used)? Would this be a good pot value for this application, and why?




       d. What would the current be if we used a 10K pot (twice as large than the 5K pot
          used)? Would this be a good pot value for this application, and why?




4) Calculate Angular Measurement Resolution: The pots in MAE156A machine are being
used as position sensors, and thus have resolution of minimum angular motion that can be
measured.
       e. Given that we are using a single-turn pot and an A/D converter of 10 bit resolution.
           Using your 170 knowledge (www.maelabs.ucsd.edu/mae170), calculate the
           resolution of sensing (i.e. minimum amount of rotation that can be detected) in
           degrees of rotation.




                                           3
Breadboard Familiarization:

Read the description below of the breadboard.
1. Label the resistors, R1, R2, and R3 on the breadboard that correspond to the circuit shown.

2. Use the 156A standard wiring colors to label the colors of the wires in the resistor circuit on
   the breadboard. Assume the resistors are sensors.
        5VDC => red
        Ground => black
        Sensors => blue
        Control => yellow
        Miscellaneous => green




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