Whats A Microcontroller v3

Digital Display · Page 191

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 P8 through P13 to send high/low signals while leaving P14 and 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 lists of values.

Digital Display · Page 193

'What's a Microcontroller - Ch6Prj01_FishAndChips.bs2

'Use a lookup table to store and display digits with

'a 7-segment display. Spell out the message: FISH CHIPS And dIP

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

' Patterns of 7-Segment Display to create letters

Page 194 · What’s a Microcontroller?

Measuring Light · Page 195

Chapter 7: Measuring Light

DEVICES THAT CONTAIN LIGHT SENSORS

Earlier chapters introduced pushbuttons as contact/pressure sensors and potentiometers as rotation/position sensors. Both of these sensors are common in electronic products—just think of how many devices with buttons and dials you use on a daily basis. Another common sensor found in many products is the light sensor. Here are a few examples of devices that need light sensors to function properly:

•Car headlights that automatically turn on when it’s dark

•Streetlights that automatically turn on when it gets dark

•Outdoor security lights that turn on when someone walks by (but only at night)

•Laptop displays that get brighter in well lit areas and dimmer in poorly lit areas

•Cameras with automatic exposure settings

•The sensor inside TVs, DVD players and other entertainment system components that detects the infrared light from a handheld remote

The first three examples in the list are automatic lighting, and they depend on ambient light sensors to distinguish day from night. The electronics inside those devices only needs to know whether it’s light or dark, so they can treat their light sensors as binary outputs like pushbuttons. Laptop displays and camera auto exposures adjust to area lighting conditions by getting information from their light sensors about how bright or dark it is. They have to treat their light sensors as analog outputs that supply a number that indicates how bright or dark it is, much like the Chapter 5 potentiometer examples where numbers indicated the knob’s position.

The light sensors inside TVs and other entertainment system components detect infrared (IR), which is a light that is not visible to the human eye, but can be detected by many electronic devices. For example, if you look at the end of the remote that you point at a TV or other entertainment devices, you will find a clear IR LED. When you press a button on the remote, it sends coded signals to the entertainment system component by flashing the IR LED on/off. Since we can’t see infrared light, it doesn’t look like the remote’s LED does anything when you press a button. However, if you try this while looking at the LED through the lens of a digital camera, the LED will look almost white. White light contains all the colors in the spectrum. The red green and blue sensors inside a camera chip all report that they detect light in response to white light. It so happens

Page 196 · What’s a Microcontroller?

that the red/green/blue sensors all detect the infrared light from the remote’s IR LED as well. So the camera also interprets light from an infrared LED as white.

More about infrared LEDs and detectors:

Robotics with the Boe-Bot has examples where the BASIC Stamp controlled Boe-Bot robot uses the IR LED found inside TV remotes and the IR receiver found inside TV sets for detecting objects in front of it. The Boe-Bot uses the IR LED as a tiny flashlight and the IR receiver found inside TVs to detect the IR flashlights’ reflections off objects in front of it. IR Remote for the Boe-Bot explains how TV remotes code the messages they send to TVs and has examples of how to program the BASIC Stamp microcontroller to decode messages from the remote so that you can send messages to your Boe-Bot robot, and even drive it around, all with a TV remote.

The type of light a given device senses depends on what it’s designed to do. For example, light sensors for devices that adjust to ambient lighting conditions need to sense visible light. The red, green and blue pixel sensors inside digital cameras are each sensing the levels of specific colors for a digital image. The IR sensor inside a TV is looking for infrared light that’s flashing on/off in the 40 kHz neighborhood. These are just a few examples, and each application requires a different kind of light sensor.

Figure 7-1 shows a few examples of the many light sensors available for various lightsensing requirements. From left to right, it shows a phototransistor, cadmium sulfide photoresistor, linear light sensor, blue enhanced photodiode, light to frequency converter, infrared phototransistor, and an infrared remote receiver from a TV.

Figure 7-1 Examples of Light Sensors

Measuring Light · Page 197

About the Cadmium Sulfide (CdS) Cell or Photoresistor

The Cadmium Sulfide (CdS) cell or photoresistor was one of the most common ambient light sensors in automatic lighting. With the advent of the European Union’s Restriction of use of certain Hazardous Substances (RoHS) directive, cadmium sulfide photoresistors can no longer be built into most devices imported into or manufactured in Europe. This has increased the use of a number of photoresistor replacement products, including the phototransistor and linear light sensor. As a result of these changes, this edition now features a phototransistor for detecting light levels instead of a cadmium sulfide photoresistor.

Documentation for each light sensor describes the type of light it detects in terms of wavelength. Wavelength is the measure of distance between repeating shapes or cycles. For example, picture a wave traveling through the ocean, bobbing up and down. The wavelength of that wave would be the distance between each peak (or whitecap) of the wave’s cycle. The wavelength of light can be measured in a similar way, instead we’re measuring the distance between two peaks in the electromagnetic oscillations of light. Each color of light has its own wavelength and is considered to be visible light, meaning it can be detected by the human eye. Figure 7-2 shows wavelengths for visible light as well as for some types of light the human eye cannot detect, including ultraviolet and infrared. These wavelengths are measured in nanometers, abbreviated nm. One nanometer is one billionth of a meter.

Figure 7-2 Wavelengths and their Corresponding Colors

NOTE: If you are viewing this in the grayscale printed book, you may download a full-color PDF from www.parallax.com/go/WAM.

Measuring Light · Page 199

ACTIVITY #1: BUILDING AND TESTING THE LIGHT METER

Chapter 5 introduced RC decay measurements with the RCTIME command for measuring the time it took a capacitor to lose its charge through the variable resistor inside a potentiometer. With larger resistance (to the flow of electric current), the potentiometer slowed down the rate the capacitor lost its charge, and smaller resistances sped up that rate. The decay time measurement gave an indication of the potentiometer’s resistance, which in turn made it possible for the BASIC Stamp to know the position of the potentiometer’s dial.

When placed in an RC decay circuit, a phototransistor, which conducts more or less current when more or less light shines on it, behaves a lot like the potentiometer. When more light shines on the phototransistor, it conducts more current, and the capacitor loses its charge more quickly. With less light, the phototransistor conducts less current, and the capacitor loses its charge less quickly. So the same RCTIME measurement that gave us an indication of the dial’s position with a potentiometer in Chapter 5 can now be used to measure light levels with a phototransistor.

In this activity, you will build and test an RC decay circuit that measures the time it takes a capacitor’s charge to decay through a phototransistor. The RC decay measurement will give you an idea of the light levels sensed by the phototransistor’s light-collecting surface. As with the potentiometer tests, the time values measured by the RCTIME command will be displayed in the Debug Terminal.

Light Detector Test Parts

(1) Phototransistor

(1)Resistor – 220 Ω (red-red-brown)

(2)Capacitors – 0.01 µF (labeled 103)

(1)Capacitor – 0.1 µF (labeled 104)

(1)Jumper wire

Building the RC Time Circuit with a Phototransistor

Figure 7-4 shows a schematic and wiring diagram of the RC-time circuit you will use in this chapter. 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 decay time is different (P2). Second, the potentiometer has been replaced with a phototransistor.