Whats A Microcontroller v3

Digital Input – Pushbuttons · Page 63

Always disconnect power from your Board of Education or BASIC Stamp HomeWork Board before making any changes to your test circuit. From here onward, the instructions will no longer say “Disconnect power…” between each circuit modification. It is up to you to remember to do this.

Always reconnect power to your Board of Education or BASIC Stamp HomeWork Board before downloading a program to the BASIC Stamp.

Testing the Pushbutton

When the pushbutton is not pressed, the LED will be off. If the wiring is correct, when the pushbutton is pressed, the LED should be on (emitting light).

Warning Signs: If the “Pwr” LED on the Board of Education flickers, goes dim, or goes out completely when you reconnect power, it may mean that there is a short circuit from Vdd to Vss or from Vin to Vss. If this happens, disconnect power immediately and find and correct the mistake in your circuit.

The LED built into the HomeWork Board is different. It may either be labeled “Power” or “Running” and it only glows while a program is running. If a program ends, either because it executes an END command or because it runs out of commands to execute, the LED will turn off.

9 Verify that the LED in your test circuit is off.

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9Press 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, as shown on the right side of the figure, you close the connection between the terminals with conductive metal. This makes a pathway for electrons to flow through the circuit and so the LED emits light as a result.

Your Turn – Turn the LED off with a Pushbutton

Figure 3-5 shows a 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. Since this pushbutton connects a conductor across terminals 1,4 and 2,3 when pressed, it means that electricity can take the path of least resistance through the pushbutton instead of through the LED. Unlike the potential short circuits discussed in the Warning Signs box, the short circuit the pressed pushbutton creates across the LED’s terminals does not damage any circuits and serves a useful purpose.

9Build the circuit shown in Figure 3-5.

9Repeat the tests you performed on the first pushbutton circuit you built with this new circuit.

Digital Input – Pushbuttons · Page 65

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 red LED will always use about 1.7 volts, and the resistor will use the remaining 3.3 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

Page 66 · What’s a Microcontroller?

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.

9 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.

Digital Input – Pushbuttons · Page 67

BS2-IC

Vss

Binary and Circuits: The base-2 number system uses only the digits 1 and 0 to make numbers, and these binary values can be transmitted from one device to another. The BASIC Stamp interprets Vdd (5 V) as binary-1 and Vss (0 V) as binary-0. Likewise, when the BASIC Stamp sets an I/O pin to Vdd using HIGH, it sends a binary-1. When it sets an I/O pin to Vss using LOW, it sends a binary-0. This is a very common way of communicating binary numbers used by many computer chips and other devices.

Programming the BASIC Stamp to Monitor the Pushbutton

The BASIC Stamp stores the one or zero it senses at I/O pin P3 in a memory location called IN3. Here is an example program that shows how this works:

Example Program: ReadPushbuttonState.bs2

This next program makes the BASIC Stamp check the pushbutton every ¼ second and send the value of IN3 to the Debug Terminal.

Digital Input – Pushbuttons · Page 69

Your Turn – A Pushbutton with a Pull-up Resistor

The circuit you just finished working with has a resistor connected to Vss. This resistor is called a pull-down resistor because it pulls the voltage at P3 down to Vss (0 volts) when the button is not pressed. Figure 3-9 shows a pushbutton circuit that uses a pull-up resistor. This resistor pulls the voltage up to Vdd (5 volts) when the button is not pressed. The rules are now reversed. When the button is not pressed, IN3 stores the number 1, and when the button is pressed, IN3 stores the number 0.

The 220 Ω resistor is used in the pushbutton example circuits to protect the BASIC Stamp I/O pin. Although it’s a good practice for prototyping, in most products this resistor is replaced with a wire (since wires cost less than resistors).

9Modify your circuit as shown in Figure 3-9.

9Re-run ReadPushbuttonState.bs2.

9Using the Debug Terminal, verify that IN3 is 1 when the button is not pressed and 0 when the button is pressed.

Active-low vs. Active-high: The pushbutton circuit in Figure 3-9 is called active-low because it sends the BASIC Stamp a low signal (Vss) when the button is active (pressed). The pushbutton circuit in Figure 3-6 is called active-high because it sends a high signal (Vdd) when the button is active (pressed).