WAM

1
WAM

• Chapter 5 Measuring Rotation

2
Adjusting Dials and Monitoring

• Dials are ideal input devices for adjustments
  such as room lighting and volume levels.
• They are also used inside devices for feedback,
  such as inside the servo to sense actual position.

3
Using a potentiometer as a variable resistor

• The device inside the dial is called a variable
  resistor or potentiometer. They are used in
  dials, joysticks, and many other devices which
  need to produce an output in reference to a
  position.
• Potentiometers can be packaged many different
  ways.

4

• The potentiometer is a resistor with two
  terminals similar to a regular resistor, but also
  has a wiper terminal to adjust where contact is
  made.

5

• The distance from the wiper to each terminal
  determines the resistance for that path. The
  minimum resistance will be 0 ohms, and the
  maximum will be the rating of the potentiometer,
  such as 10K?.

Low resistance
High resistance
High resistance
Low resistance
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Activity 1 Building/Testing Potentiometer
Circuit

• Construct the circuit and observe the LED's
  brightness at different settings of the
  potentiometer.

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• When the LED was brightest, was the potentiometer
  resistance highest or lowest in the path to the
  LED?
• When the LED was brightest, was the potentiometer
  wiper closest or furthest from terminal connected
  to Vdd?(click for answers)

8
Activity 2 Measuring Using Time

• In this activity an RC-network (Resistor-Capacitor
  ) is used to form a circuit. The capacitor is
  charged and discharged at different rates
  determined by the resistor and the capacitor
  sizes.

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Introducing the Capacitor

• The capacitor is a device which can store an
  electron charge. Its size is expressed typically
  in microfarads (?F) or millionths of Farads.
• Certain types of capacitors are polarity
  sensitive, that is, they can only be connected in
  one direction.

• Connecting a polarity sensitive capacitor
  backwards can cause the device to explode.
• Wear safety glasses.
• Ensure proper polarity when connecting.

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Build this circuitbe sure the power is off and
the capacitor is connected correctly
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Polling the RC-Time Circuit with the basic stamp

• Enter,Save, and Run PolledRcTimer.bs2 on the next
  slide (page 146 of your text).

12
See data studio scope Voltage probe
13
Record the discharge time

• Record your results next to 470 O in the chart on
  the next slide
• Replace the 470O with the other resistors below,
  and record the resulting discharge times
• Repeat this procedure using a similar chart for,
  but now using a 1000 uF capacitor in place of the
  3300 uF.

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Fill in the charts with your data
What is your conclusion about the effects of R
C on discharge time?
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Here is some data I collected
Notice how the values on the right are 1/3 of the
values on the leftthis is because RC or time on
the right is 1/3 of RC or time on the left
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• The time to discharge the capacitor is in
  proportion to the size of the resistor and
  capacitor network (RC).
• The larger the capacitance (C), the greater the
  charge it can hold, increasing time.
• The larger the resistance (R), the slower the
  capacitor will discharge, increasing time.

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Activity 3 Reading with BASIC Stamp

• The BASIC Stamp has an instruction to perform
  much of the timing operation automatically and
  with higher precision (milli or micro seconds) as
  opposed to (tenths of a second)
• RCTIME Pin, State, Variable
• Where
• Pin is the pin the RC network is connected.
• State is the initial state when timing begins.
• Variable is the memory location to store the
  results. Just like PULSOUT the time is the
  number of 2uS increments.

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Build the circuit below
The goal here will be to monitor the position of
the potentiometer dial with RCTime
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Since we can now detect much smaller timeslets
make the value of RC really small

• 10 x 104 pF
• Or 100000 pF
• Or 0.1 uF
• 10 x 103 pF
• Or 10000 pF
• Or 0.01 uF

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Enter, Save, and Run ReadPotWithRcTime.bs2 on the
next slide (page 151 of your text)
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RCTIME Program Explanation

• Declare variable time to hold results.
• DO-LOOP code block
• Set I/O P7 HIGH (5V).
• Wait 100 mS to charge capacitor and stabilize
  DEBUG screen.
• Execute RCTime instruction
• Time until capacitor discharges and P7 leaves
  defined state (1).
• Store results in variable Time.
• Display Time results.this is the number of 2
  microseconds intervals

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Try rotating the wheel of the potentiometer while
monitoring the debug screen..why are the results
logical?
As R goes up, the time to discharge goes up and
vice versa
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• Replace the 0.1 uF Capacitor with a 0.01 uF
  capacitor
• The values you see now for RC-Time should now be
  1/10 of what they were before
• Put the 0.1 uF capacitor back in the circuit
• Make note of the highest RC-Time value

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Activity 4 Servo Control with Potentiometer

• Potentiometers are often used to control servos
• Radio controlled airplanes
• Consist of radio controlled
• Servos which control
• Flaps
• Engine throttle settings
• Rudder

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Build two circuits

• Potentiometer circuit from last activity

• Servo Circuit from Ch4
• REMEMBER THE SERVO POWER JUMPER SETTINGS!!

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• In this activity the value of RCtime is used to
  control a servo.
• The RCtime reading is offset to be within the
  controllable range of the servo (500-1000).
• You will need the largest RCtime Value recorded
  in the last activity

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Here is some sample data

• Potentiometer max counterclockwise 691
• Potentiometer max clockwise 1
• Servo max counterclockwise 1000
• Servo max clockwise 500
• WE WOULD LIKE THESE DIFFERENCE TO BE THE SAME FOR
  EASE OF CONTROL WITH THE POT

Difference of 690
Difference of 500
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Create a scale

• Make a ratio
• Servo difference 500 0.72
• Pot difference 691
• Test
• 0.72 x 691 498 (just about 500)
• 0.72 x 1 0.72 (just about 1)
• PROBLEM When full counterclockwise
  potentiometer is multiplied by the scale we get
  full clockwise servo????

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Problem

• Full Servo Counterclockwise should be 1000
• Currently 500??

• Full Clockwise should be 500
• Currently 0.724???

The Fix

• Full Servo Clockwise should be 1000
• Currently 500??
• Add 500 to 500 and get 1000 before PULSOUT

• Full Servo Clockwise should be 500
• Currently 0.724??
• Add 500 to 0.724 and get 500.724 before PULSOUT

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SCALE FACTOR

• So if the you wanted to know where an RCtime
  value of 490 would put the servo?
• (0.724) x 490 355
• 355 (500) 855somewhere around 11 OClock

OFFSET
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Another Problem

• The bs2 cannot multiply or divided by decimals
  such as the 0.724 on the previous slide

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Solutionthe /

• This is a bs2 function that will divide whatever
  you are multiplying by with 256
• Somultiply 0.724 by 256
• Result 184.344
• Round off to ones place
• Result 184
• Perform the / function

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So, here is how we would get our servo position
from the potentiometer
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Enter, run, and save the following code
So, here is how we would get our servo position
from the potentiometer
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Now modify the program, so the scaling works with
your rctime circuitAdd this line between
PULSOUT and LOOPDEBUG HOME, DEC5 time Then
run the program
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Many times the scale factor (ex 0.724), and
the offset (ex 500) are used many times in a
program.

• The following makes this easier
• Scalefactor con 184
• Offset con 500
• Now anywhere in your program you need to change
  one of these values, you will only need to change
  it once

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Enter, Run, and Save the code belowNote that it
works the same as controlservowithpot
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Using constant for calibration and easy updating

• Save the program under a new name
• Change the scalefactor and offset values to your
  unique rc circuit values and verify function
• Change pin 7 to pin 8 on your circuit and in your
  code
• Rcpin con 8
• Add this comment before the doloop
• Debug ? Rcpin
• Run the program again, and note the change

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Photo ResistorResistor whose resistance varies
with the intensity of light

• Goal Control servo direction with light
  intensity

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Wiring the Circuit

• Replace the manual potentiometer with the photo
  resistor

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Recompute Scale factor

• Cover the resistor and find the max rctime value
• Allow light to hit the resistor and find the min
  rctime value
• Subtract the above and create a new ratio
• Servo difference 500 scalefactor
• Rctime difference ?
• Dont forget to offset your results with 500

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Try it

• When the photoresistor is covered, the servo
  should move counterclockwise
• When the photoresistor is uncovered, the servo
  should move clockwise