What is a Microcontroller (Paralax, v2.2, student guide, 2004)

Chapter #6: Digital Display · Page 183

√Re-run the program after each change to the value variable and note which value from the list gets placed in the index variable.

Unless you tell it to make a different kind of comparison, the LOOKDOWN command checks to see if a value is equal to an entry in the list. You can also check to see if a value is greater than, less than or equal to, etc. For example, to search for an entry that is less than or equal to the value variable, use the <= operator just before the first bracket that starts the list.

√Modify SimpleLookdown.bs2 by substituting this value and LOOKDOWN statement:

value = 35

LOOKDOWN value, <= [ 7, 19, 28, 85, 167 ], index

√Experiment with different values and see if the index variable displays what you would expect.

Trick question: What happens if your value is greater than 167? This little twist in the LOOKDOWN command can cause problems because the LOOKDOWN command doesn’t make any changes to the index.

'What's a Microcontroller - SimpleLookdown.bs2

'Debug an index using a value and a lookup table.

'{$STAMP BS2}

'{$PBASIC 2.5}

LOOKDOWN value, [7, 85, 19, 167, 28], index

DEBUG ? index, CR

DEBUG "Change the value variable to a ", CR, "different number in this list:", CR, "7, 85, 19, 167, or 28.", CR, CR,

"Run the modified program and ", CR, "check to see what number the ", CR,

Page 184 · What’s a Microcontroller?

"LOOKDOWN command places in the ", CR, "index variable."

END

Example Program: DialDisplay.bs2

This example program mirrors the position of the potentiometer’s knob by lighting segments around the outside of the 7-segment LED display as shown in Figure 6-16.

Figure 6-16

Displaying the Potentiometer’s Position with the 7-Segment LED Display

√Enter and run DialDisplay.bs2.

√Twist the potentiometer’s input shaft and make sure it works.

√When you run the example program, it may not be as precise as shown in Figure 6-16. Adjust the values in the lookdown table so that that the digital display more accurately depicts the position of the potentiometer.

'What's a Microcontroller - DialDisplay.bs2

'Display POT position using 7-segment LED display.

'{$STAMP BS2} '{$PBASIC 2.5}

DEBUG "Program Running!"

OUTH = %00000000

DIRH = %11111111

DO

HIGH 5

PAUSE 100

RCTIME 5, 1, time

LOOKDOWN time, <= [40, 150, 275, 400, 550, 800], index

Chapter #6: Digital Display · Page 185

LOOKUP index, [ %11100101, %11100001, %01100001, %00100001, %00000001, %00000000 ], OUTH

LOOP

How DialDisplay.bs2 Works

This example program takes an RCTIME measurement of the potentiometer and stores it in a variable named time.

HIGH 5

PAUSE 100

RCTIME 5, 1, time

The time variable is then used in a LOOKDOWN table. The LOOKDOWN table decides which number in the list time is smaller than, then loads the entry number (0 to 5 in this case) into the index variable.

LOOKDOWN time, <= [40, 150, 275, 400, 550, 800], index

Then, the index variable is used in a LOOKUP table to choose the binary value to load into the OUTH variable.

LOOKUP index, [ %11100101, %11100001, %01100001, %00100001, %00000001, %00000000 ], OUTH

Your Turn – Adding a Segment

DialDisplay.bs2 only makes five of the six segments turn on when you turn the dial. The sequence for turning the LEDs on in DialDisplay.bs2 is E, F, A, B, C, but not D.

√Save DialDisplay.bs2 under the name DialDisplayYourTurn.bs2.

√Modify DialDisplayYourTurn.bs2 so that it causes all six outer LEDs to turn on in sequence as the potentiometer is turned. The sequence should be: E, F, A, B, C, and D.

Page 186 · What’s a Microcontroller?

SUMMARY

This chapter introduced the 7-segment LED display, and how to read a pin map. This chapter also introduced some techniques for devices and circuits that have parallel inputs. The DIRH and OUTH variables were introduced as a means of controlling the values of BASIC Stamp I/O pins P8 through P15. The LOOKUP and LOOKDOWN commands were introduced as a means for referencing the lists of values used to display letters and numbers.

Questions

1.In a 7-segment LED display, what is the active ingredient that makes the display readable when a microcontroller sends a high or low signal?

2.What does common cathode mean? What do you think common anode means?

3.What is the group of wires that conduct signals to and from a parallel device called?

4.What are the names of the commands in this chapter that are used to handle lists of values?

Exercises

1.Write an OUTH command to set P8, P10, P12 high and P9, P11, P13 low. Assuming all your I/O pins started as inputs, write the DIRH command that will cause the I/O pins to send high/low signals while leaving P14, P15 configured as inputs.

2.Write the values of OUTH required to make the letters: a, C, d, F, H, I, n, P, S.

Project

1.Spell “FISH CHIPS And dIP” over and over again with your 7-segment LED display. Make each letter last for 400 ms.

Solutions

Q1. The active ingredient is an LED.

Q2. Common cathode means that all the cathodes are connected together, i.e., they share a common connection point. Common anode would mean that all the anodes are connected together.

Q3. A parallel bus.

Q4. LOOKUP and LOOKDOWN handle list of values.

Page 188 · What’s a Microcontroller?

' Patterns of 7-Segment Display to create letters

Further Investigation

As with all texts cited in this section, this one is available for free download from www.parallax.com.

“StampWorks”, Workbook, Version 1.2, Parallax Inc., 2001

StampWorks is written by Nuts & Volts author Jon Williams, and features a wide variety of experiments. This text is a collection of 32 experiments including several that use 7-segment LED displays and other types of displays. This text makes use of the INEX 1000 board, which has many of the components you have used in this text built into it.

Chapter #7: Measuring Light · Page 189

Chapter #7: Measuring Light

DEVICES THAT CONTAIN LIGHT SENSORS

You have already worked with two different kinds of sensors. The pushbutton can be thought of as a simple pressure sensor, and the potentiometer is a position/rotation sensor. There are many kinds of sensors built into appliances and machines that are not as obvious as a button or a knob. Other common sensors measure things like temperature, smoke, vibration, tilt, and light. Although each of these different kinds of sensors can be found in one or more devices that most people use on a daily basis, light sensors are probably the most common.

One example of an every-day device that contains a light sensor is probably your television. If it can be controlled by a handheld remote, it has a built-in detector for a type of light called infrared that cannot be seen by the human eye. The handheld remote uses infrared light to transmit information about the channel, volume, and other keys that you might press to control the TV. Another common example is a digital camera. A camera’s light sensors help it adjust for various lighting conditions so that the picture looks clear regardless of whether it’s a sunny or cloudy day.

INTRODUCING THE PHOTORESISTOR

Light sensors have many different functions, and they come in different shapes, sizes, and with different price tags. Some sensors are designed to sense a particular color of light, such as blue, green, red, or infrared. Some sensors don’t care what color the light is because they react to how bright the light is. Other sensors look for only special kinds of light given off by certain chemical reactions. Light sensors also have a variety of ways to tell a microcontroller what they see. Some sensors send a voltage, some send a sequence of binary values, and others react to different kinds of light or light levels by changing resistance.

Of the light sensors that react to light by changing their resistance, the photoresistor shown in Figure 7-1 is probably the most common, least expensive and easiest to use. Its active ingredient is a chemical compound called cadmium sulfide (CdS). This compound changes resistance depending on how bright the light is that shines on its collecting surface. Bright light causes low resistance values between the two leads while dim light causes higher resistance values.

Page 190 · What’s a Microcontroller?

Figure 7-1

Photoresistor Schematic and

Part Drawing.

The photoresistor’s cadmium sulfide coated light collecting surface is shown at the top of the part’s drawing.

As with a potentiometer, you can use a photoresistor in an RC-time circuit. The program to read the photoresistor is also about the same as the one used to read the potentiometer. Even though the programming is the same, light is very different from rotation or position. The activities in this chapter focus on applications that use light (instead of position) to give the microcontroller information. Along the way, some PBASIC programming techniques will be introduced that will help you with long term data storage, and with making your programs more manageable and readable.

ACTIVITY #1: BUILDING AND TESTING THE LIGHT METER

In this activity, you will build and test an RC-time circuit that reads the value of a photoresistor. The RC-time measurement will give you an idea of the light levels sensed by the photoresistor’s light collecting surface. As with the potentiometer test, the time values measured by the RCTIME command will be displayed in the Debug Terminal.

Light Detector Test Parts

(1) Photoresistor

(1) Resistor – 220 Ω (red-red-brown)

(1) Capacitor – 0.01 µF

(1) Capacitor – 0.1 µF

(1) Jumper wire

Although there are two capacitors in the list of parts, you will only use one capacitor in the circuit at any given time.

Building the RC Time Circuit with a Photoresistor

Figure 7-2 shows a schematic of the RC-time circuit you will use in this chapter, and Figure 7-3 shows the wiring diagram. This circuit is different from the potentiometer circuit from Chapter #5, Activity #3 in two ways. First, the I/O pin used to measure the

Chapter #7: Measuring Light · Page 191

decay time is different (P2). Second, the variable resistor is now a photoresistor instead of a potentiometer.

√Build the circuit shown in Figure 7-2 and Figure 7-3.

X2

Programming the Photoresistor Test

The first example program (TestPhotoresistor.bs2) is really just a slightly revised version of ReadPotWithRcTime.bs2 from Chapter #5, Activity #3. The potentiometer circuit from Chapter #5 was connected to I/O pin P7. The circuit in this activity is connected to P2. Because of this difference, the example program has to have two commands updated to make it work. The command that was HIGH 7, since the potentiometer circuit was connected to P7, is now HIGH 2 since the photoresistor circuit is connected to P2. For the same reason, the command that was RCTIME 7, 1, time is now RCTIME 2, 1,

time.

Page 192 · What’s a Microcontroller?

Example Program: TestPhotoresistor.bs2

Instead of twisting the potentiometer’s knob, the circuit is tested by exposing the light collecting surface of the photoresistor to different light levels. When the example program is running, the Debug Terminal should display small values for bright light conditions and large values for low light conditions.

√Enter and run TestPhotoresistor.bs2.

√While watching the value of the time variable on the Debug Terminal note the value under normal lighting conditions.

√Turn the lights in the room off or cast a shadow over the circuit with your hand and check the time variable again. It should be a significantly larger number.

√If you face the photoresistor’s light collecting surface toward direct sunlight, the time variable should be fairly small.

'What's a Microcontroller - TestPhotoresistor.bs2

'Read photoresistor in RC-time circuit using RCTIME command.

'{$STAMP BS2}

'{$PBASIC 2.5}

LOOP

Your Turn – Using a Different Capacitor for Different Light Conditions

Replacing the 0.01 F capacitor with a 0.1 F capacitor can be useful for more brightly lit rooms or outdoors. The time measurements with the 0.1 F capacitor will take ten times as long, which means the value of the time variable displayed by the Debug Terminal should be ten times as large.

√Modify the circuit by replacing the 0.01 F capacitor with a 0.1 F capacitor.

√Re-run TestPhotoresistor.bs2 and verify that the RC-time measurements are roughly ten times their former values.