Файл: Basic Analog and Digital (Students guide, v13).pdf

Chapter 1: Analog Voltage and Binary States · Page 1

Chapter #1: Analog Voltage and Binary States

This series of experiments introduces analog and digital electronics. What does that mean? In What’s a Microcontroller? we learned that analog is a “continuously variable value”. Another way to think about it is that analog electronics is analogous to nature.

There are lots of continuously variable values in nature, such as motion, light level, and sound. The position of a door as it swings open is a good example of a continuously variable value. As a door swings from all the way closed to all the way open, it visits every value in between. At one instant during its travel, it is 1/3 of the way open. At another instant, it is 1/2 way open, and so on.

INTRODUCTION TO ANALOG AND DIGITAL

Digital simply means represented by digits. Think about how many times each day you encounter analog values that are represented with digits. The temperature is 79.8 degrees. The speed limit is 45 miles per hour, etc. Not surprisingly, digital electronics represents values with digits.

The term digital is also used when referring to binary devices such as the circuitry that makes a calculator work, the microprocessor in a computer, and the BASIC Stamp microcontroller. It's true - they are all digital devices. Binary devices are digital devices using two digits, 0 and 1.

The experiments in What’s a Microcontroller? introduced a variety of techniques for interfacing with the outside world and other devices. These interfaces were mostly binary. This series of experiments extends the capabilities of interfacing by introducing several analog component interfaces and more component interface techniques.

In this first experiment, we'll build a circuit that produces an analog voltage at its output. Remember that analog voltage is continuously variable. The circuit will be adjustable so that it can produce an output anywhere between 0 and 5 volts. We'll also build a circuit called a voltage follower that uses this analog voltage to drive an LED circuit.

Page 2 · Basic Analog and Digital

Volt/Voltage: The volt is a fundamental unit of electrical measurement named after 18th century physicist Allesandro Volta, and a measurement in volts is referred to as voltage.

Most of us encounter this unit of measurement when buying batteries such as the 9 volt (DC) battery that can be used to power the Board of Education. Inside a battery there are two chemical reactions, which are separated from each other by a barrier. One of the reactions creates a surplus or electrons and the other creates a shortage of them.

The electron surplus and shortage sides of the barrier are connected to the negative and positive terminals of the battery respectively. If given a pathway around the barrier, the electrons have the potential to do work to get from the negative to the positive terminal. The volt is a measure of this potential to do work. The volt is also referred to as a unit of electric potential.

The analog voltage will also be connected to one of the BASIC Stamp I/O pins set to input mode. This binary input can actually be used to measure small variations in the analog voltage. PBASIC will be used to program the BASIC Stamp to drive a binary LED circuit, which will indicate when these variations have been detected.

The Debug Terminal is also a useful tool for displaying data the BASIC Stamp collects and sends. It will be used to monitor the binary value that the input pin receives as the analog voltage is varied.

Parts Required

For each experiment you will need a BASIC Stamp 2 and a Board of Education connected to an IBM-compatible PC with Win95/98/NT4.0. You will need to install the BASIC Stamp Editor v 2.0 or higher, which is available for free download from www.parallax.com or on the Parallax CD dated May 2003 or later. In addition, you'll need the following parts for this experiment:

(2) 470 resistors

(2) Red LEDs

(1) 100 k potentiometer.

(6) Jumper wires.

(1) LM358 op-amp

Throughout this series of experiments, we will build circuits from circuit schematics. One of the keys to learning how to read circuit schematics is learning what each symbol on the schematic means. It's also important to learn how to connect a part from the

Chapter 1: Analog Voltage and Binary States · Page 3

Analog and Digital Parts Kit to the Board of Education based on its circuit symbol in a schematic.

Circuit Schematic: Often referred to as a schematic, a circuit schematic is a map that uses symbols to show the components in a circuit and how they are connected. The components are represented by symbols such as the one that represents the LED in Figure 1-1.

Figure 1-1 shows the circuit symbol for an LED on the left and a drawing of an LED from the parts kit on the right. It also shows how the pins on an LED correspond to the terminals on the circuit symbol.

Flat spot on plastic part of LED indicating the cathode.

LED

Figure 1-2 shows a drawing of a resistor below its circuit symbol. The circuit symbol typically has the resistance value written below or next to it. The colored stripes on the part drawn below the symbol indicate its value, which is measured in ohms. The omega symbol ( ) is used to denote the ohm. You can use Appendix B to convert the color codes on the resistor to resistance values.

Page 4 · Basic Analog and Digital

Current/Amp: Current happens when electrons travel from point A to point B. Direct current is what happens when you give the surplus electrons in the negative terminal of a battery a pathway to get to the positive terminal. The amp is the measurement of how many electrons per second are traveling through the pathway.

Resistance/Ohm: Resistance is a property of a material in the pathway the electrons travel through. The more difficult it is for the electrons to get from one end of the pathway to the other, the higher the resistance. A resistor is just such a pathway, and its resistance is measured in ohms ( ).

Ohm's Law: When a resistor is used to provide a pathway between the negative and positive terminals of a battery, you have an electric circuit with voltage, resistance, and current. Ohm's Law relates the three quantities as follows:

V = I x R

V is the voltage measured in volts, I is the current measured in amps, and R is the resistance measured in ohms.

The Other Guys: Ever wonder where the words volt, amp, and ohm come from? They are all named after some of the people who made significant discoveries about electricity. We already know who the volt is named for; what about the other guys? The amp, also called the ampere, is named after 18th century physicist André Marie Ampère. The ohm is named after 19th century physicist Georg Simon Ohm.

The Potentiometer - A Source of Variable Voltage

The potentiometer (pot) has 3 pins on its underside that get plugged into the Board of Education. On the topside, it has a knob you can twist to adjust it. In this experiment, we will use variable resistance to get a variable voltage output. Figure 1-3 shows how the pins on the underside of the pot from the parts kit correspond to the circuit symbol.

Figure 1-3

Potentiometer Circuit

Symbol and Component

Pot