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Utilizing the Paralax Basic Stamp for paintball applications
By Bill Mills

Page 1  Page 2  Page 3

In PBASIC, unlike most flavors of BASIC, we won’t have to label each of our steps, unless we’re going to send the program there by name, like when step 5 sent the program back to Step 3 – Step 3 would need to be given a name in PBASIC.

Learning PBASIC is mainly about learning the commands that the interpreter understands, then putting them in the right order to do the job that we want done.  I’ll go over a number of commands, and some sample programs and circuits – again this is just an introduction, the Basic Stamp Manual and numerous Basic Stamp resource web sites give much more complete information.

Word to the bit, it’s a byte thang

In addition to the EEPROM that holds the Stamp’s program, and some long term data storage, the PBASIC interpreter chip (the processor on the Stamp) has a whopping 16 bytes of Random Access Memory (RAM) that PBASIC programs can use to store and manipulate numbers.  To give you an idea of the immense computer horsepower we’re talking about here, that is roughly one millionth of the minimum RAM a PC needs to run Windows95.  The Basic Stamp’s RAM is broken up into various segments, some addressable as bits (which can store a value of 1 to 0) some addressable as Bytes (which can store a value of 0 to 255) and some as Words (which can store a value of 0 to 65,535.  Most of these methods overlap, so it’s important to choose RAM variables wisely.  For example Word6 is made up of Bytes 12 and 13, so you can’t use Byte 12 to store a number if you are already using Word6.  A table of RAM addresses is found on Page 11 of the Basic Stamp Manual.

The first Word of RAM is called Port, and it is reserved, and while it can be used for storing numbers.  What you put into it has a big effect on how the stamp is operating.  The first Byte is called Pins, and you can put numbers into it, or read them out of it.  It’s easier, especially if you don’t understand the binary number system, to address the bits Pin0 through Pin7 individually.  If I/O Pin0 for example were hooked up to a push-button circuit that is designed to bring the pin voltage high when the button is pressed, we would have our program watch the value of the variable Pin0.  If the button were being pressed Pin0 would equal one, if it were not being pressed Pin0 would equal 0.  Alternatively, if we had a circuit with an LED hooked up to I/O Pin0, we could make Pin0=1 in the software to make the pin go high and light the LED, or make Pin0=0 to turn it off. 

The second Byte of Port is the Dirs Byte – it gets it’s name for direction.  All of the bits that make up this Byte determine whether the I/O pins with corresponding names are to be used for input or output.  We’ll see some of this in our example programs.

Let’s get to it!

We’ll deal with a couple of very simple circuits which are in fact, very representative of the type of things we need to run a paintgun.

The first circuit is an LED circuit.  It’s dirt simple.  We’ll attach a 470 ohm resistor to a Stamp I/O pin, and attach an LED linking that to ground.  Because LEDs are current specific, we must be sure the anode of the LED is the lead that goes to ground, so the cathode can connect to the resistor and receive the +5v signal from the stamp pin. 

This circuit is easy to wire on the Stamp Carrier board, though to miniaturize for use on a paintgun, I have assembled it using a surface mount resistor soldered right onto the base of the stamp pin, with a micro LED soldered to it, and a short length of wire running from the LED to pin 2 on the Stamp (ground).  With the SMT resistor there was no need for the pin to stick out the side of the stamp, so I simply cut it off to make my finished board smaller. Alternately, you might want to have the LED on lengthy flexible wire leads so that it can be mounted peeking out the back of a grip frame while the Stamp circuitry is concealed inside the grip.  Also, on choosing LEDs, aside from very odd LEDs (that will be labled as “high current” or “bi-color” or something similar) they are pretty much interchangeable.  You will be able to choose them based on the color, shape and size that’s right for your application.  Radio Shack and a lot of other suppliers also have LED assortments that are a grab bag of different shapes and sizes.  The current the LED will draw is minimal, so ¼ watt, or even lower wattage (like tiny surface mount) resistors will be fine what is important in choosing the resistor is the resistance value – 470 Ohms, to prevent the LED getting too much current and overloading.

With our LED circuit, setting the pin it is attached to high will light the LED, and setting that pin low will turn it off.

Our next circuit is a push-button.  We will draw power from Stamp pin 5 (+5v) and run that to one leg of a single pole, single throw (SPST) normally open pushbutton.  The other leg of the pushbutton will be tied to both an I/O pin of the Stamp, as well as to a 10KOhm resistor that leads to ground.  Again this could be wired on the Stamp Carrier, or right on the Stamp itself – the resistor tying the pin to ground, and the pushbutton leads going to Pin 5 and the I/O Pin.  With this circuit, the I/O pin will be low until the button is pressed, at which point the availability of the +5v signal will bring the I/O pin high.  The resistor acts as a current drain in case there is still a + charge left on the pin after we’ve released the button.

OK, with these two circuits, we can make something happen.  For experimentation purposes, lets put a pushbutton circuit on Stamp pin 7 (I/O Pin0) and an LED circuit on Stamp pin 14 (I/O Pin7).

In PBASIC, each line of text is used for a single command.  Any time we use a ’ symbol, everything to the right of that symbol is considered to be a comment, and is ignored by the PBASIC interpreter.  Comments get stripped out of the program when it is loaded into the stamp, so they are a handy way to put notes into a program to help it make sense to a person who is studying it or modifying it.  I’ll use comments to add explanations or notations to programs in this article.  Often, hash marks (#) are added in comments to make them more visible to the reader.  The commands can also be indented with Tabs, which are also ignored by the Stamp but make the program easier to read.

'#### Stamp Button Demo Program Number One
'#### This program is found on Page 29 of the Basic Stamp 
'#### Manual
 let b2=0    'Clear the variable B2 for the button command
Loop:     'Label this part of the program as Loop
 BUTTON 7,1,200,100,b2,0,skip 
          'Go to Skip, unless I/Opin7 goes high
 Toggle 0    'Invert the status of the LED on I/O Pin 0
Skip:     'Label this part of the program as Skip
 Goto Loop   'Go back up to the part of the program 
             ' called Loop

When this program runs it is going to start by making sure that the byte variable B2 is empty.  This is important because we are going to need to let the interpreter use B2 to operate the button command.  Then we define an address point in the program to be called Loop.  Then the biggie – the BUTTON command.  There are a number of ways we could check on the status of our pushbutton, but BUTTON is really powerful, it has features built into it that save us having to write a lot of code ourselves.  Following the command are it’s arguments – information we’re giving it – separated by commas.  The button command has 7 arguments:
Pin – This is the I/O pin we want BUTTON to look at – in this case, since our pushbutton is on I/O Pin 7, we’re using 7 as the value for this argument.
Downstate – This is the state of the pin that will get BUTTON’s attention.  Since our circuit will bring the pin high when the button is pressed, we put 1 here.  We would put 0 if we wanted BUTTON to react to the pin going low.
Delay – This is a value from 0 to 255 that says how long the button must be held down before auto-repeat begins.  Auto repeat is just like a computer keyboard.  If you hold down a button it makes just one letter on your computer, but if you keep holding it down it will wait, and then begin repeating the letter.  Of we put in a value of 0 here, auto-repeat is disabled and so is debouncing.  If we put in a value of 255, debouncing is still enabled, but auto-repeat is turned off.
Rate – This value, from 0 to 255 specifies the rate at which the auto-repeat occurs.  This is the equivalent to how fast the letters appear on the computer screen when the button is held down.
Byevariable – In order for BUTTON to do it’s thing we need to give it a byte of RAM do work with.  We get to assign it which byte to use, so that it doesn’t accidentally garble some important info we were storing.
Targetstate – The downstate got BUTTON’s attention by telling it when to know that our pushbutton was pressed down, Targetstate tells it how to react.  Does it branch off to a new part of the program is the button is pushed, or if the button is not being pushed?
Address – If we do branch the program, where do we go?  We’re going to send it to Skip.

Trigger Bounce

A quick note about debouncing.  When switches or pushbuttons close, they don’t just turn on or off.  They “stutter” a little bit at the beginning and end, this is called switch bounce.  It’s too fast for us to see, but a computer can spot it.  One button push to us, can look like 2 or 3 fast presses followed by a long press followed by 2 or 3 fast presses to the computer.  Debouncing “cleans” the signal, ignoring those little bounces at the front or Back.  Simply turning off debounce is not how one writes a program for Turbo firing modes.  Turbo mode requires that the software additionally has to deal with buffering the input signal from the trigger during the paintgun’s firing cycle.  For more about Turbo mode and switch bounces, Click Here.

So, what happens when our program is loaded into the Stamp?  Nothing at first, until the button is pressed.  Then the LED will turn on.  When the button is pressed again, the button will turn off.  The TOGGLE command simply changes the output value of a pin from whatever state it is in to the opposite state.  If we hold the button down, after a pause, the LED will begin flashing.  This is the auto repeat feature.  Try changing the repeat and delay variables to see how it affects the auto-repeat performance.  Remember, each time you make a change, you’ll need to run the program again, loading it into the Stamp by pressing <alt>R.

There you have it – a very simple application to show how to work with the Basic Stamp.

Now let’s get a little more advanced with a MOSFET circuit, and a program like that we’d use for a paintgun.

A MOSFET is a type of transistor.  The MOSFET can handle more current than the stamp, and can provide it a reasonable level of isolation from the component it is controlling.  An IFR510 MOSFET is more than ample enough to drive most small solenoid valves used in paintguns like the Rainmaker, Shocker, Bushmaster 2000, etc.  The IFR510 and similar FET transistors are widely available from electronics suppliers, including Radio Shack.  If driving a larger solenoid, like the one used to do the mechanical work of tripping the sear in an E-Mag, or Centerflag grip on a Spyder a larger transistor  would be needed to handle the additional amperage, or multiple IFR510s could be run in series.  The IFR510 sits between it’s load and ground.  If we were using it on a paintgun to control a solenoid valve, one lead from the solenoid valve would go straight to the battery + terminal (or to an inline voltage regulator, if the battery voltage was larger than the solenoid valve could handle – I usually drive MAC valves rated for 6v at 9v straight out of the battery).  The other lead from the solenoid valve would go to the Drain lead of the MOSFET, while the Source leg of the MOSFET would go to ground.  It’s important to remember that a MOSFET goes on the ground side of the load, not on the + side.  The gate of the MOSFET then is wired directly to one of the Basic Stamp I/O pins.  When the pin is driven high by the software, the MOSFET allows current to flow between the Drain and Source.  When the pin goes low, the MOSFET does not allow power to flow.  For more info on driving high current devices, see the Basic Stamp FAQ.

At this point it should be evident that the circuitry portion of an electropneumatic paintgun can be very simple to construct with the Basic Stamp and our three circuits.  The minimum that is needed for a single solenoid paintgun is a basic stamp with one of our pushbuton circuits for the trigger (a pushbutton and a resistor), a MOSFET to link to the solenoid valve, and a battery.  That’s all!  The rest is all software, this can even run select fire, using the auto repeat feature of the BUTTON command.  Adding an LCD display only requires another couple of modular components.  The hard part here isn’t doing the electronics, but the physical work of integrating them cleanly into the paintgun.

Now let’s get a little more complex and look at a sample circuit for a practical paintgun.  We’ll also consider more carefully our looping structure, so that our program can be more modular.  I have a Basic Stamp that I have built into a Revolution Loader with an LCD and a pair of MOSFETS.  By keeping my main program modular, I need to only tweak certain sections of it to swap the loader bewteen a single solenoid Rainmaker, single solenoid SuperNova ET, and a dual solenoid automated PMI-1 conversion.

Really we’re going to want a pair of inputs for our paintgun, not just the trigger, but also a selector button that will let us switch between safe and semi-auto mode.  Instead of a single LED, we’ll use two, one to indicate that the Basic Stamp is on and getting power (and that the program inside is running OK) and one to show if we are in safe mode, or ready to fire.  We’ll use a green LED for power, and a red one for the mode indicator.  The LED’s are wired in with our LED circuit (the resistor and LED) with the green LED circuit on I/O Pin 0 (pin 7 on the Stamp), and the red on I/O Pin 1 (pin 8 on the Stamp).  We’ll put another LED circuit – the color doesn’t matter – on Pin 9 of the Stamp (I/0 pin 2) to simulate a MOSFET and solenoid valve circuit.  This LED we’ll call the valve LED, and it will be on to show is when the valve would be open if this circuit were wired into a paintgun. 

Our two pushbutton circuits we’ll put on Stamp pins 13 and 14.  The one on I/O Pin 7 we will use as our safety.  The one on I/0 pin 6 will be the trigger.  Selecting the right switch for the trigger in an electropneumatic paintgun is a subject for a whole article in itself.  The switch needs to be an SPST normally open momentary type.  From there, there are a wide range of choices for you to make that depend on the physical constraints of the paintgun, physical linkage to the trigger, and the trigger feel you want – mushy, long, definitive click, short, etc.

I should also add, that it’s a good idea to add a power switch, otherwise you’d have to disconnect the battery between uses, like with the older model Shockers.  A power switch is simply a SPST slide, rocker, or toggle switch spliced into the + lead from the battery clip.  One terminal of the switch leads to the battery + side, while the other leads to circuitry.

OK, let’s get to the software.  This is a straightforward sem-auto program, with safety and power light.  Again as a reminder, this is a copyrighted program, and may not be used in an actual paintgun.  With a light understanding of PBASIC, you should not have a problem writing your own program to meet your exact needs.

'### Semi-Auto simulation Software for single solenoid 
'### design  Copyright 2002, William R. Mills III All 
'### Rights ReservedThis program may only be used in a 
'### Basic Stamp with LEDssimulating the physical 
'### interface to a paintgun, for educationalpurposes.

'#  Configure and set variables
 symbol dwell=b10 '# call b10 dwell which is easier to 
 Symbol drop=b11 '# call b22 drop which is easier to 
 Let B2=0  '# clear b2 so BUTTON can use it for the
 Let B3=0  '# clear b3 so BUTTON can use it for the 
            'mode button
 Let dwell=30  '# set the dwell delay to 30 ms
 Let drop=30  '# set the drop delay to 30 ms
'# The dwell delay is how long the solenoid must be 
   'held open for a full and
'# complete firing cycle.  The drop delay is the minimum 
    'amount of time the
'# breech must be held open between shots to give time 
    'for a new ball to fall
'# into the breech.

'# Flash the power LED to show that the board is working 
'# when the power is turned on, and then leave it on
 Low 0   '# Make sure that the power LED is off
 Pause 100  '# Wait for 100 milliseconds – 1/10 
             'of a second
 Toggle 0   '# Turn the power LED from off to on
 Pause 100  '# Wait again
 Toggle 0   '# Turn the power LED back off
 Pause 100  '# Wait again
 Toggle 0   '# Turn the power LED back on

'##### Safe mode – Gosafe comes before safe mode so we 
       'don’t waste processor
'# time constantly setting the safe LED when 
   'we’re running the BUTTON command
'# waiting for the paintgun to get switched 
   'into semi mode.  This isn’t so important
'# now as when we’re also taking the time to 
   'send data to an LCD each time we go
'# into safe mode.

 Gosafe:    '# Label the gosafe procedure
  Low 1  '# Turn I/O pin 1 low to turn off the safe/semi 
 Safe:   '# Label the safe procedure
  Button 7,1,255,1,b3,0,skip1 
       '# If the mode button is not pressed go to Skip1
  Goto Gosemi  '# If the button was pressed we goto Gosemi
  Skip1:  '# If the button wasn’t pressed we end up here
  Goto Safe '# and then go back to safe and keep
    '# waiting for the button to be pressed

'####### Semi-auto mode, here we have to watch the 
       'trigger for button presses
'# and act on them, as well as watch the 
'# mode button and go back to safe mode
'# if it is pressed.  Notice a new command here – 
'# gosub.  Goto will go to a new
'# place in the program and just continue on. 
'#  Gosub will go to a subroutine and
'# when that subroutine is done (subroutines 
'# are ended with a return command) 
'# the program will come back and continue on, right 
'# after where the gosub command
'# was in the list.

 Gosemi:   '# That’s the label for this procedure
  High 1  '# Turn I/O pin 1 high to light the 
          '# LED and show we
          '# are live in semi-auto mode
 Semi:   '# Now were in the semi auto procedure
  Button 6,1,255,1,b2,0,skip2 
 '# If the trigger was not pressed skip ahead to skip2
  Gosub fire 
'# If the trigger was pressed execute to the 
'#firing subroutine
  Skip2:  '# whether we did or didn’t shoot we end up here
  Button 7,1,255,1,b3,0,skip1 
'# If the mode button is not pressed go to Skip3
  Goto Gosafe '# If mode was pressed we go back to safe
  Skip3:  '# Here we are at skip3
  Goto Semi: '# If we didn’t change modes 
'#look for the next trigger press

'###### The firing sub-routine – this is where 
'# we trip our firing solenoid/LED
 Fire:   '# That’s the name of the subroutine
  Low 0  '# We’ll blink off the power LED 
    '# while firing to show that
    '# we’re trying to fire even if 
    '# there’s some malfuction
  High 2  '# Give the MOSFET power to open 
     '#the solenoid valve
  Pause dwell 
    '# Now we’ll wait for the number of milliseconds we
    '# specified when we set the dwell value earlier
  Low 2  '# Now we’ll drop I/O Pin 2 and close the valve
  Pause drop '# Now we’ll pause before the next 
             '#shot to allow time for
    '# a ball to feed
  High 1  '# And now turn the power light back on
  Return  '# We’re done shooting so we return 
          '#to the line in the 
    '# program just below the gosub command that sent
    '# us here

And that’s it.  When the Stamp is turned on the power LED will blink a couple of times and then stay on.  The mode button will select between safe and live modes, which are indicated by the safety LED.  The paintgun will only fire as a semi-auto in live mode.

Continued on Page 3

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