Adjusting Dials and Monitoring

1
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.

2
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.

3

• 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.

4

• 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
5
Activity 1 Building/Testing Potentiometer
Circuit

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

6

• 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)

7
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.

8
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.

Click Here For Movie of Exploding capacitor
9
Build this circuitbe sure the power is off and
the capacitor is connected correctly
10
Polling the RC-Time Circuit with the basic stamp

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

11
See data studio scope Voltage probe
12
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, but
  now using a 1000 uF capacitor in place of the
  3300 uF.

13
Fill in the charts with your data
What is your conclusion about the effects of R
C on discharge time?
14
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
15

• 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.

16
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.

17
Build the circuit below
The goal here will be to monitor the position of
the potentiometer dial with RCTime
18
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

19
Enter, Save, and Run ReadPotWithRcTime.bs2 on the
next slide (page 151 of your text)
20
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

21
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
22

• 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

23
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

24
Build two circuits

• Potentiometer circuit from last activity

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

25

• 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

26
Goal

• To make the servo horn maintain the same position
  as the potentiometer dial

27
Method

• Write an equation that the bs2 can use to figure
  out where the servo should be at any
  potentiometer rctime value.

28
Gather your data

• Find the Potentiometer value that you want to
  correspond with the 500,1000 750 positions on
  the servo
• Use the data chart to the right

29
Plot a simple graphUse graphical analysis or any
graphing program to plot servo values vs. rctime
values
30
Here is the graph that I plotted using the data
on the previous slide
31
Write your equation using the values displayed on
the graph

• Y m x b
• Servo value (slope)(Pot value) y intercept
• Servo value (0.687)(Pot value) 501

32
Lets See if this works

• Servo value (0.687)(Pot value) 501
• If potentiometer reads 356 Servo should be at 750
• Servo value (0.687)(356) 501 746
• Not too shabby!!

33
Problem Basic stamp cannot use decimals

• Solution
• Multiply slope by 256
• Round off result
• Change equation to following
• Servo value (176)/(Pot value) 501
• Note / means multiply by something after
  first dividing by 256

34
Enter, run, and save the following code, using
your values for slope y_intercept
So, here is how we would get our servo position
from the potentiometer
35
Now lets translate this into servo motionenter
the code below, but be sure to use your
scalefactor offset
36
Many times the slope (ex 176), and the
y_intercept (ex 501) are used many times in a
program.

• The following makes this easier
• slope con 176
• Y_intercept con 501
• Now anywhere in your program you need to change
  one of these values, you will only need to change
  it once

37
Trouble shooting Choppy servos

• Adjust pause command until servo motion is smooth

38
Photo ResistorResistor whose resistance varies
with the intensity of light

• Goal Control servo direction with light
  intensity

39
Wiring the Circuit

• Replace the manual potentiometer with the photo
  resistor

40
Recompute Scale factor

• Cover the resistor and find the max rctime value
  (this will correspond with 1000 on the servo)
• Allow light to hit the resistor and find the min
  rctime value (this will correspond with 500 on
  the servo)
• Plot your graph and write your equation
• Update your previous program to work with the
  photoresistor

41
Try it

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

42
Use an accelerometer or tilt sensor to control
your servo
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44
Wire the Memsic 2125 Chip
45
How to use it

• The MX 2125 Chip converts temperature readings
  into pulse durations
• When the gas bubble collects near the sensor on
  the x axis, it heats it more, and the pulse
  durations are big
• When the gas bubble is away from the sensor on
  the x axis, it heats it less, and the pulse
  durations are small.
• The same is true for the y axis

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Programming

• Allow pulses from the chips x and y
  sensors to be received by the basic stamp and
  store the input as a variable
• Show the results on the debug terminal

48
Making the MX 2125 control a servo or servos

• Record the max and min pulsin values.
• Create your graph
• Write your equation
• Update the code for potentiometer controlled servo

• Here is some code that I wrote, of course the
  values may be different than yours