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

Page 72 · What’s a Microcontroller?

have two pins is because it needs stability. If the pushbutton only had two pins, those pins would eventually bend and break from all the pressure that the pushbutton receives when people press it.

The left side of Figure 3-2 shows how a normally open pushbutton looks when it’s not pressed. When the button is not pressed, there is a gap between the 1,4 and 2,3 terminals. This gap makes it so that the 1,4 terminal can not conduct current to the 2,3 terminal. This is called an open circuit. The name “normally open” means that the pushbutton’s normal state (not pressed) forms an open circuit. When the button is pressed, the gap between the 1,4 and 2,3 terminals is bridged by a conductive metal. This is called closed, and current can flow through the pushbutton.

Test Parts for the Pushbutton

(1) LED – pick a color

(1) Resistor - 470 Ω (yellow-violet-brown)

(1) Pushbutton - normally open

(1) Jumper wire

Building the Pushbutton Test Circuit

Figure 3-3 shows a circuit you can build to manually test the pushbutton.

Page 74 · What’s a Microcontroller?

√Press and hold the pushbutton, and verify that the LED emits light while you are holding the pushbutton down.

How the Pushbutton Circuit Works

The left side of Figure 3-4 shows what happens when the pushbutton is not pressed. The LED circuit is not connected to Vdd. It is an open circuit that cannot conduct current. By pressing the pushbutton, you close the connection between the terminals with conductive metal making a pathway for electrons to flow through the circuit.

Your Turn – A Short-Circuit

Figure 3-5 shows a different circuit that will cause the LED to behave differently. When the button is not pressed, the LED stays on. When the button is pressed, the LED turns off. The reason the LED turns off when the pushbutton is pressed is because current always follows the path of least resistance. When the pushbutton is pressed, terminals 1,4 and 2,3, have almost no resistance between them, so all the current passes through the pushbutton (short circuit) instead of the LED.

√Build the circuit shown in Figure 3-5.

√Repeat the tests you performed on the first pushbutton circuit you built with this new circuit.

Can you really do that with the LED? Up until now, the LED’s cathode has always been connected to Vss. Now, the LED is in a different place in the circuit, with its anode connected to Vdd. People often ask if this breaks any circuit rules, and the answer is no. The electrical pressure supplied by Vdd and Vss is 5 volts. The diode will always use 1.6 volts, and the resistor will always use 3.4 volts, regardless of their order.

ACTIVITY #2: READING A PUSHBUTTON WITH THE BASIC STAMP

In this activity, you will connect a pushbutton circuit to the BASIC Stamp and display whether or not the pushbutton is pressed. You will do this by writing a PBASIC program that checks the state of the pushbutton and displays it in the Debug Terminal.

Parts for a Pushbutton Circuit

(1) Pushbutton - normally open

(1) Resistor – 220 Ω (red-red-brown)

(1)Resistor – 10 kΩ (brown-black-orange)

(2)Jumper wires

Building a Pushbutton Circuit for the BASIC Stamp

Figure 3-6 shows a pushbutton circuit that is connected to BASIC Stamp I/O pin P3.

√Build the circuit shown in Figure 3-6.

Figure 3-7 shows what the BASIC Stamp sees when the button is pressed, and when it’s not pressed. When the pushbutton is pressed, the BASIC Stamp senses that Vdd is connected to P3. Inside the BASIC Stamp, this causes it to place the number 1 in a part of its memory that stores information about its I/O pins. When the pushbutton is not pressed, the BASIC Stamp cannot sense Vdd, but it can sense Vss through the 10 kΩ and 220 Ω resistors. This causes it to store the number 0 in that same memory location that stored a 1 when the pushbutton was pressed.

Page 78 · What’s a Microcontroller?

the number 1, and when the pushbutton is not pressed, the Debug Terminal displays the number 0.

Figure 3-8

Debug Terminal

Displaying

Pushbutton States

The Debug Terminal displays 1 when the pushbutton is pressed and 0 when it is not pressed.

√Enter the ReadPushbuttonState.bs2 program into the BASIC Stamp Editor.

√Run the program.

√Verify that the Debug Terminal displays the value 0 when the pushbutton is not pressed.

√Verify that the Debug Terminal displays the value 1 when the pushbutton is pressed and held.

'What's a Microcontroller - ReadPushbuttonState.bs2

'Check and send pushbutton state to Debug Terminal every 1/4 second.

'{$STAMP BS2}

'{$PBASIC 2.5}

DO

DEBUG ? IN3

PAUSE 250

LOOP

How ReadPushbuttonState.bs2 Works

The DO…LOOP in the program repeats every ¼ second because of the command PAUSE 250. Each time through the DO…LOOP, the command DEBUG ? IN3 sends the value of IN3 to the Debug Terminal. The value of IN3 is the state that I/O pin P3 senses at the instant the DEBUG command is executed.

Page 80 · What’s a Microcontroller?

ACTIVITY #3: PUSHBUTTON CONTROL OF AN LED CIRCUIT

Figure 3-10 shows a zoomed in view of a pushbutton and LED used to adjust the settings on a computer monitor. This is just one of many devices that have a pushbutton that you can press to adjust the device and an LED to show you the device’s status.

Figure 3-10

Button and LED on a Computer Monitor

The BASIC Stamp can be programmed to make decisions based on what it senses. For example, it can be programmed to decide to flash the LED on/off ten times per second when the button is pressed.

Pushbutton and LED Circuit Parts

(1) Pushbutton – normally open

(1) Resistor - 10 kΩ (brown-black-orange)

(1) LED – any color

(1) Resistor – 220 Ω (red-red-brown)

(1)Resistor – 470 Ω (yellow-violet-brown)

(2)Jumper wires

Building the Pushbutton and LED Circuits

Figure 3-11 shows the pushbutton circuit used in the activity you just finished along with the LED circuit used in Chapter #2, Activity #2.