BASIC Stamp Input

Overview

This information is specific to the Prop-1 controller.

The Prop-1 controller (and the Basic Stamp 1 from which it was derived) communicates with the outside world through a set of eight wires ("pins"), marked on the board as P0 through P7. Any of these wires can be set up to serve as an input or output. By convention, P0-P5 are used for output and P6-P7 are used for input. Parallax even modified P6-P7 to make it easier to do input with them, and P0-P7 to make it easy to do output - but exactly how these pins are used is your choice.

In order to use a pin for input, you must:

Tell the Prop-1 which pins are inputs.
Write your program so that it looks at one or more input pins when it wants to see the status of the real world.
Connect a suitable input signal to the input pin(s).
Tell the stamp what to do if there is no input signal.

PROP-1 Input Hardware

Pin Header

Any of the P0-P7 connections can be used for input. They are located on a big array of pins:

The letters on the top say "W R B", and they mean:

The "W" column (white wire) is the actual signal input line. Each one is a separate wire going into the controller's I/O port.
The "R" column (red wire) goes to the controller's logic power supply (+5 Volts). All of the pins in the "R" column are connected together.
The "B" column (black wire) goes to ground (0 Volts). All of the pins in the "B" column are connected together.

The "W R B" pattern is repeated for all 8 pins.

The three-pin connector (white, red, black) is used in lot of Parallax products, as well as RC servos. Parallax sells a cable with the matching connector.
14-inch LCD Extension Cable
Stock#: 805-00002,451-00303
Weight: .0068 lbs
Price: $1.29 each, $0.97 in quantities of 10 [May 2006]

Pull Up, Pull Down

As we learned in the Input/Output Overview, it is a bad idea to leave a controller input disconnected. In order to always have a valid state, pull-up or pull-down resistors can be used.

Parallax has modified P6-P7 to make it easier to do input with them. Those two lines can be jumpered with pull-up or pull-down resistors. These resistors can be used to shove a 0 ("pull-down" to ground) or a 1 ("pull up" to +5 volts) into these pins.

You can choose whether to use:

pull-up = put the rectangular plug on the left two pins
pull-down = put the rectangular plug on the right two pins
nothing special = remove the jumper plug

By using these jumpers, you shove a 0 or 1 into the input, which can be safely overpowered by the switch when it is activated. You should use the jumper that does the opposite of what your switch will do when activated:

if activating the switch pulls the input low (to ground), use the pull-up option
if activating the switch pulls the input high (to +5 Volts), use the pull-down option

Kinds Of Input

Normally Open Switch

A "normally open" switch takes two wires and leaves them dangling in space ("open") most of the time. When you activate the switch, it connects the wires together, which completes the electrical circuit.

Since we prefer to use positive logic, activating the switch should send a 1 value into the controller. So the other end of the switch should be connected to +5 Volts, and when the switch is not active, we use a pull-down resistor to send in a 0.

Here is a normally open switch hooked up to a PROP-1:
Use the pull-down option: put the rectangular plug on the right two pins.

Normally Closed Switch

A "normally closed" switch takes two wires and connects them together ("closed") most of the time, which completes the electrical circuit. When you activate the switch, it disconnects the wires, leaving them dangling in space ("open").

Since we prefer to use positive logic, activating the switch should send a 1 value into the controller. So the other end of the switch should be connected to ground, and when the switch is active, we use a pull-up resistor to send in a 1.

Here is a normally closed switch hooked up to a PROP-1:
Use the pull-up option: put the rectangular plug on the left two pins.

Active Input

So far, we have illustrated the use of switches, both normally open and normally closed. These are passive components that simply make or break a connection.

But what do you do if you want the Prop-1 to act on signals from other types of sensors - things that send out a voltage when they have something to say?

The Prop-1 can indeed accept input signals from active devices, but this is trickier and involves more risk than simply using a passive switch (sometimes called "dry contacts"). If you inject the wrong type of signal, you can permanently damage the input pin on the Prop-1.

If you have any doubts, please use a mechanical relay to take the input voltage and turn it into a contact closure that can be safely used on the controller. See High Voltage Input.

If you know that your input produces a signal that is compatible with the Prop-1, you can plug it straight into the Pin Header:

High Voltage Input

Sometimes, you need to take a high voltage signal, like the line voltage switched by an X-10 module.

You can use a mechanical relay to take the input voltage and turn it into a contact closure that can be safely used on the controller.

You must select a relay with a "coil" specification that matches the input trigger voltage. (This is not the same kind of relay as is used for high voltage output. That relay will probably use a 12 VDC coil.)

Here is a relay used to interface a high input voltage to a PROP-1:
Use the pull-down option: put the rectangular plug on the right two pins.

Here are some common relays with 110 Volt AC coils [as of October 2005]:

part number description contacts coil price

Radio Shack #275-217 125VAC/10A DPDT Plug-In Relay DPDT 10A at 125VAC 125VAC, 15mA, 4.5Ω $8.39

part number	description	contacts	coil	price
Radio Shack #275-217	125VAC/10A DPDT Plug-In Relay	DPDT 10A at 125VAC	125VAC, 15mA, 4.5Ω	$8.39

Do not try to use a solid state relay for this.

Specific Input Devices

Parallax PIR Sensor

Parallax sells a Passive Infrared (PIR) sensor that serves as a motion detector.

PIR Sensor
Stock#: 555-28027
Weight: 1 lb
Price: $7.95 [May 2006]

Features:

detects motion up to 20 feet away
Simple 3-pin connection
Single bit output - active high
Small size - easy to conceal (0.96 x 1.0 x 1.27 in.)

Parallax PROP-1 Trainer

Parallax sells a "PROP-1 Trainer" that attaches to the Prop-1 to provide output LEDs, an input push-button, and an analog input knob. It is unlikely that you will leave this gadget attached to the Prop-1 during normal operation, but it can be very helpful in early testing and learning how to use the Prop-1.

Prop-1 Trainer Board
Stock#: 31199
Weight: 0.65
Price: $19.95 [October 2005]

Features:

1 input push-button - P7
1 input pot - P6
6 output LEDs - P0-P5

Programming

Initialization

"Initialization" is the beginning of the program where things are set up for later use. Normally, the program only executes the initialization part once - when you turn it on.

In order to prepare for input and output, you need to tell the Prop-1 which of the eight pins are used for input and which are for output. [This isn't always necessary, as the Prop-1 has certain "default" values that take effect until you decide to change them. But when I write a program, I dislike making assumptions, so I explicitly tell the Prop-1 what I want to do.] This is done by setting "DIRS" equal to some value.

Another thing that I like to do in the front of the program is to clear all output bits. You can set all the bits at once by assigning a value to PINS.

Example:

  '       76543210        ' bit positions
  DIRS = %00111111        ' make P7-P6 inputs, P5-P0 outputs
  PINS = %00000000        ' all outputs off

The value that is assigned starts with "%". This means that it is a binary value, with each digit being a "0" or "1", and corresponding to an I/O pin. When setting DIRS, "1" means the corresponding pin will be used for output and "0" means input.

Testing Input

The input can be tested by using it in an "IF" statement.

Here is a simple example using positive logic ("1" means the the trigger is activated):

Main:
  IF PIN6 = 0 THEN Main

These instructions will work, but they aren't very readable. Some day you might need to come back to an old program and change it, so I prefer to put in some comments and refer to the pin and value it takes with symbols.

Here is a slightly more readable example:

  SYMBOL Trigger = 6                ' input trigger PIN6
  SYMBOL IsOff = 0

TryAgain:
  IF Trigger = IsOff THEN TryAgain  ' wait until trigger is activated

What this does:

Create an easy-to-use name for the trigger input.
Create a value that we will see if the trigger is off.
Look at the value of Trigger, and if it is off, try again.

The Prop-1 will "spin" around and around, testing the trigger input again and again - until it finally sees the trigger become active. Then the Prop-1 will break out of this "loop" and execute the rest of the program.

Example

Now, let's try a very simple program. The goal of this program is to wait until it sees a switch pressed, and then start blinking a LED.

The LED is on P0.

The switch is on P6 positive logic ("1" means the the trigger is activated). You can use either a normally open or normally closed switch.

Copy the following program from this web page, and paste it into the BASIC Stamp Editor's program window.

' {$STAMP BS1}
' {$PBASIC 1.0}

SYMBOL TheLed = 0                   ' LED is on P0
SYMBOL Trigger = PIN6               ' input trigger on P6
SYMBOL IsOff = 0

Reset:
  DIRS = %00111111                  ' make P7-P6 inputs, P5-P0 outputs
  PINS = %00000000                  ' all outputs off

Main:

TryAgain:
  IF Trigger = IsOff THEN TryAgain  ' wait until trigger is activated

  HIGH TheLed
  PAUSE 1000  ' wait 1 second

  LOW TheLed
  PAUSE 1000  ' wait 1 second

  GOTO Main   ' repeat forever
END

Here's how to test it:

Before you start the program, set the input switch to the inactive state.
Download the program from the PC to the Prop-1.
The program will start running automatically.
The LED should not be lit. Watch it for several seconds.
Activate the switch.
As long as the switch is active, the LED will blink.
When the switch is deactivated, the LED will finish its current cycle, and go out.