# 5x4 LED Display Matrix Using a Basic Stamp 2 (bs2) and Charlieplexing

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## Introduction: 5x4 LED Display Matrix Using a Basic Stamp 2 (bs2) and Charlieplexing

Have a Basic Stamp 2 and some extra LEDs sitting around? Why not play around with the concept of charlieplexing and create an output using just 5 pins.

For this instructable I will be using the BS2e but any member of the BS2 family should work.

## Step 1: Charlieplexing: What, Why, and How

Let's get the why out of the way first.

Why use charlieplexing with a Basic Stamp 2?
---Proof of concept: Learn how charlieplexing works and learn something about the BS2. This may be useful to me later using faster 8-pin chips (only 5 of them will be i/o).
---Useful reason: Basically there is none. The BS2 is far too slow to display without noticeable flickering.

What is charlieplexing?
---Charlieplexing is a method of driving a large number of LEDs with a small number of microprocessor i/o pins. I learned about charlieplexing from www.instructables.com and you can too:
Charlieplexing LEDs- The theory
How to drive a lot of LEDs from a few microcontroller pins.
Also at wikipedia: Charlieplexing

How can I drive 20 leds with 5 i/o pins?
---Please read through the three links under "What is charlieplexing?". That explains it better than I ever could. Charlieplexing is different from traditional multiplexing which needs one i/o pin for each row and each column (that would be a total of 9 i/o pins for a 5/4 display).

## Step 2: Hardware and Schematic

Materials list:

1x - Basic Stamp 2
20x - light emitting diodes (LEDs) of the same type (color and voltage drop)
5x - resistors (see below regarding resistor value)

Auxiliary/Optional:
Momentary push button as a reset switch

The Schematic:
This schematic is put together with the mechanical layout in mind. You will see the grid of LEDs set up on the left, this is the orientation for which the BS2 code has been written. Notice that each pair of LEDs have the anode connected to the cathode of the other. They are then connected to one of the five i/o pins.

Resistor Values:
You should calculate your own resistor values. Check the datasheet for your LEDs or use the LED setting on your digital multimeter to find the voltage drop of your LEDs.

Let's do some calculations:
Supply Voltage - Voltage Drop / Desired Current = Resistor Value
The BS2 supplies 5v regulated power, and can source 20ma of current. My LEDs have a 1.6v drop and operate at 20ma.
5v - 1.6v / .02amps = 155ohms

To protect your BS2 you should use the next higher resistor value from what you get with the calculation, in this case I believe it would be 180ohms. I used 220ohms because my development board has that value of resistor built into it for each i/o pin.

NOTE: I believe that since there is a resistor on each pin this effectively doubles the resistance on each led since one pin is V+ and the other is Gnd. If this is the case you should reduce the resistor values by half. The adverse effect of too high a resistor value is a dimmer LED. Can someone verify this and leave me a PM or comment so I can update this information?

Programming:
I have been using a development board that has a DB9 connector to program the chip right on the board. I do also use this chip on my solder-less breadboard and have included an In Circuit Serial Programming (ICSP) header. The header is 5 pins, pins 2 through 5 connect to pins 2-5 on a DB9 serial cable (Pin 1 is unused). Please note that to use this ICSP header pins 6 and 7 on the DB9 cable must be connected to each other.

Reset:
A momentary push reset button is optional. This just pulls pin 22 to ground when pushed.

Now it is time to build the matrix on a breadboard.

I used a terminal strip to connect one leg from each led pair together and a small jumper wire to connect the other legs. This is detailed in the closeup photo and is explained in depth here:

2. Place LED 1 with the Anode (+) toward you and the Cathode (-) away from you.
3. Place LED 2 in the same orientation with the Anode (+) in the connecting terminal strip of the LED 1 cathode.
4. Use a small jumper wire to connect the Anode of LED 1 with the Cathode of LED 2.
5. Repeat until each pair of LED's has been added to the board.

I use what would normally be the power bus strips of the bread board as bus strips for the BS2 I/O pins. Because there are only 4 bus strips I use a terminal strip for P4 (the fifth I/O connection). This can be seen on the larger picture below.

6. Connect the terminal strip for the LED 1 cathode to the P0 bus strip. Repeat for each odd numbered LED substituting the proper P* for each pair (see the schematic).
7. Connect the terminal strip for the LED 2 cathode to the P1 bus strip. Repeat for each odd numbered LED substituting the proper P* for each pair (see the schematic).
8. Connect each bus strip to the appropriate I/O pin on the BS2 (P0-P4).
9. Check all connections to ensure they match the schematic.
10. Celebrate.

NOTE: In the close-up you will see that it doesn't appear that I followed step 7 as the connection to the second I/O pin is on the Anode of the odd numbered LEDs. Remember that the Cathode of the even numbered LEDs is connected to the Anode of the odd numbered LEDs so the connection is the same either way. If this note confuses you, just ignore it.

## Step 4: Programming Basics

For charlieplexing to work you turn on just one led at a time. For this to work with our BS2 we need two basic steps:
1. Set the output modes for the pins by using the OUTS command.
2. Tell the BS2 which pins to use as outputs using the DIRS command

This works because the BS2 can be told which pins to drive high and low and will wait to do so until you specify which pins are outputs.

Let's see if things are hooked up correctly by just trying to blink LED 1. If you look at the schematic you can see that P0 is hooked up to the Cathode (-) of LED 1 and P1 is hooked up to the Anode of that same LED. This means we want to drive P0 low and P1 high. This can be done like so: "OUTS = %11110" which drives P4-P1 high and P0 low.

(% indicates a binary number is to follow. The lowest binary digit is always on the right. 0=LOW, 1=HIGH)

The BS2 stores that information but won't act on it until we declare which pins are outputs. This step is key as only two pins should be outputs at the same time. The rest should be inputs, which sets those pins to High Impedance mode so they will not sink any current. We need to drive P0 and P1 so we will set those to outputs and the rest to inputs like so: "DIRS = %00011".

(% indicates a binary number is to follow. The lowest binary digit is always on the right. 0=INPUT, 1=OUTPUT)

Let's put that together into some useful code:

' {\$STAMP BS2e}
' {\$PBASIC 2.5}
DO
OUTS = %11110 'Drive P0 low and P1-P4 high
DIRS = %00011 'Set P0-P1 as Outputs and P2-P4 as Inputs
PAUSE 250 'Pause for LED to remain on

DIRS = 0 'Set all pins to Input. This will turn off the LED
PAUSE 250 'Pause for LED to remain off
LOOP

## Step 5: The Development Cycle

Now that we have seen one pin work time to make sure they all work.

20led_Zig-Zag.bse
This attached code should light up each one of the 20 LEDS in a zig-zag pattern. You will notice that after each pin has bin lit I use "DIRS = 0" to turn all pins back into inputs. If you change the OUTS without turning the output pins off you may get some "ghosting" where an led that should not be lit may blink between cycles.

If you change the W1 variable at the beginning of this code to "W1 = 1" there will be only a 1 millisecond pause between each LED blink. This will cause a persistence of vision (POV) effect that makes it look like all of the LEDs are lit. This does have the effect of making the LEDs dimmer but is the essence of how we will display characters on this matrix.

20led_Interpreter_Proto.bse
I decided at this point that I had to develop some type of interpreter code to turn the crazy combinations needed to light the LEDs into a usable pattern. This file is my first attempt. You will see that at the bottom of the file the characters are stored in four lines of 5 digit binary. Each line is read in, parsed, and a subroutine is called each time an led needs to be lit.

This code works, cycling through numerals 1-0. If you do try to run it notice that it is plagued by a very slow refresh rate causing the characters to flash almost too slow to be recognized. This code is bad for many reasons.

First off, five digits of binary take up just as much room in the EEPROM as 8 digits of binary as all information is stored in groups of four bits.

Secondly, the SELECT CASE used to decide which pin needs to be lit up requires 20 cases. The BS2 is limited to 16 cases per SELECT operation. This means I had to hack around that limitation with an IF-THEN-ELSE statement.

There must be a better way. After a few hours of head scratching I discovered it.

## Step 6: A Better Interpreter

Each row of our matrix is composed of 4 LEDs, each can be on or off. The BS2 stores information in its EEPROM in groups of four bits. That correlation should make things much easier on us.

In addition to this fact, four bits correspond to the decimal numbers 0-15 for a total of 16 possibilities. This makes or SELECT CASE much easier.

Here is the numeral 7 as stored in the EEPROM:
'7
%1111,
%1001,
%0010,
%0100,
%0100,

Each row has a decimal equivelant to 0-15 so we read a row in from memory and feed it directly to the SELECT CASE function. This means that the human readable binary matrix used to make each character (1=led on, 0=led off) is the key for the interpreter.

In order to use the same SELECT CASE for each of the 5 rows I used another select case to set the DIRS and OUTS as variables.

I first read in each of five lines of the character to variables ROW1-ROW5. The main program then calls the subroutine to display the character. This subroutine takes the first row and assigns the four possible OUTS combinations to variable outp1-outp4 and the two possible DIRS combinations to direc1 & direc2. LEDs are flashed, the row counter is incremented, and the same process is run for each of the other four rows.

This is much faster than the first interpreter program. That being said, there is still noticeable flicker. Take a look at the video, the camera makes the flicker look much worse but you get the idea. Porting this concept to a much faster chip, like a picMicro or an AVR chip would enable display of these characters without a noticeable flickering.

## Step 7: Where to Go From Here

I don't have a cnc mill or etching supplies to make circuit boards so I will not be wiring this project. If you have a mill and are interested in collaborating to move forward from here, send me a message. I'd be happy to pay for materials and shipping even happier to show something of a finished product for this project.

Other Possibilities:
1. Port this to another chip. This matrix design can be used with any chip that has 5 i/o pins available that are tri-state capable (pins that can be high, low, or input (high impedance)).
2. Using a faster chip (perhaps AVR or picMicro) you can increase the scale. With a 20pin chip you could use 14 pins to charlieplex an 8x22 display and use the remaining pins to receive serial commands from a computer or another controller. Use three more 20-pin chips and you can have a scrolling display that is 8x88 for a total of 11 characters at once (depending on the width of each character of course).

Good luck, have fun!

## Recommendations

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486 Enrolled

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We have a be nice policy.

## Questions

first of all, this is amazin. but i'm having trouble. when i try to light up either led 11-12 group led 5-6 light up. looking over at the schematic i see they both share the same pins and i'm stuck. same for the 13-14 and 7-8 group. any help would be greatly appreciated.

2 replies

jaja, maybe this is a little late, but i hade the same problem, so i connected the cathode of led 11/anode of led 12 to R1....same with led 13 and 14, just connected it to R1

i also had to switch R1 and R2.......

Howdy,

I used your project to further my understanding of Charlieplexing - thanks for writing it!  That being said, I already had a breadboard built with a functioning 20 LED grid when I arrived at your project.  I wanted to understand how to efficiently display information on the grid without doing it all manually by hand.  I had a bit of trouble initially because your code did not match my hardware design - my LEDs were in a completely different order.  I liked what I saw so I decided to work your code over a bit.

Changes:

[1] Added construction notes at the top of the file to aid in breadboard design (for new builders) or re-writing of the code should someone else show up (as I did) with a different layout.

[2] Rewrote the 'SELECT rowC' routine to accomidate the layout I already had working.  It also matches the added comments I added to the top of the file.

[3] Added the rest of the English alphabet.

[4] Cleaned up whitespace/formatting a little bit.

Hope that someone finds this useful.  Good luck!

-------------------------------------
' {\$STAMP BS2}
' {\$PBASIC 2.5}

' Circuit Construction Guide: This code assume that you are using a 4-wide x 5-tall
' grid of LEDs, configured as follows:
'
' (01) (02) (03) (04)
' (05) (06) (07) (08)
' (09) (10) (11) (12)
' (13) (14) (15) (16)
' (17) (18) (19) (20)
'
' Pins P0 though P4 are used.  The +PIN -> LED -> -PIN assignment is applied in logical order:
'
' LED -> +PIN, -PIN
'  01 -> +P0 , -P1
'  02 -> +P1 , -P0
'  03 -> +P1 , -P2
'  04 -> +P2 , -P1
'  05 -> +P2 , -P3
'  06 -> +P3 , -P2
'  07 -> +P3 , -P4
'  08 -> +P4 , -P3
'  09 -> +P0 , -P2
'  10 -> +P2 , -P0
'  11 -> +P0 , -P3
'  12 -> +P3 , -P0
'  13 -> +P0 , -P4
'  14 -> +P4 , -P0
'  15 -> +P1 , -P3
'  16 -> +P3 , -P1
'  17 -> +P1 , -P4
'  18 -> +P4 , -P1
'  19 -> +P2 , -P4
'  20 -> +P4 , -P2

repC       VAR Byte 'Counts character flashes (how long to display each character)
charC      VAR Byte 'what character is currently being displayed
rowC       VAR Byte 'Counter to track which row is being flashed
display    VAR Byte 'Binary to display on the currently scanning row
maxC       CON 10   'Maximum character flashes before moving to next character
charsInLib CON 36   'Total characters in the library

'Variables used for reading in each of a five row character
ROW1       VAR Byte
ROW2       VAR Byte
ROW3       VAR Byte
ROW4       VAR Byte
ROW5       VAR Byte

'Variables used to set pin directions
direc1     VAR Byte
direc2     VAR Byte

'Variables used to set pin outputs
outp1      VAR Byte
outp2      VAR Byte
outp3      VAR Byte
outp4      VAR Byte

charC = 0

'Main Program
DO
GOSUB LightLED
DIRS = 0
charC = charC + 1 'increment up to the next character
IF charC > charsInLib - 1 THEN
charC = 0
ENDIF
LOOP

READ letters + (charC * 5), ROW1
READ letters + (charC * 5) + 1, ROW2
READ letters + (charC * 5) + 2, ROW3
READ letters + (charC * 5) + 3, ROW4
READ letters + (charC * 5) + 4, ROW5
RETURN

'Subroutine to display character
LightLED:

'In depth explanation:
'5x4 led matrix interpreter functions by row of 4 leds
'Each SELECT is a different column and each case (1-16) corresponds
'to one of the 16 states possible for the column of leds.
'
'Binary should then be read out of memory and handed straight to the
'interpreter for output display.
'
'case 0    %0000
'case 1    %0001
'case 2    %0010
'case 3    %0011
'case 4    %0100
'case 5    %0101
'case 6    %0110
'case 7    %0111
'case 8    %1000
'case 9    %1001
'case 10   %1010
'case 11   %1011
'case 12   %1100
'case 13   %1101
'case 14   %1110
'case 15   %1111

DO
IF repC > maxC THEN 'How long to display the character before returning to main
repC = 0
RETURN
ENDIF
FOR rowC=1 TO 5 'Cycle through each of 5 rows
SELECT rowC
CASE 1
outp1 = %00001
outp2 = %00010
outp3 = %00010
outp4 = %00100
direc1 = %00011
direc2 = %00110
display = ROW1
CASE 2
outp1 = %00100
outp2 = %01000
outp3 = %01000
outp4 = %10000
direc1 = %01100
direc2 = %11000
display = ROW2
CASE 3
outp1 = %00001
outp2 = %00100
outp3 = %00001
outp4 = %01000
direc1 = %00101
direc2 = %01001
display = ROW3
CASE 4
outp1 = %00001
outp2 = %10000
outp3 = %00010
outp4 = %01000
direc1 = %10001
direc2 = %01010
display = ROW4
CASE 5
outp1 = %00010
outp2 = %10000
outp3 = %00100
outp4 = %10000
direc1 = %10010
direc2 = %10100
display = ROW5
ENDSELECT
DIRS = 0
SELECT display
CASE 0
PAUSE 0
CASE 1
OUTS = outp4
DIRS = direc2
PAUSE 1
CASE 2
OUTS = outp3
DIRS = direc2
PAUSE 1
CASE 3
OUTS = outp3
DIRS = direc2
PAUSE 1
OUTS = outp4
PAUSE 1
CASE 4
OUTS = outp2
DIRS = direc1
PAUSE 1
CASE 5
OUTS = outp2
DIRS = direc1
PAUSE 1
DIRS = 0
OUTS = outp4
DIRS = direc2
PAUSE 1
CASE 6
OUTS = outp2
DIRS = direc1
PAUSE 1
DIRS = 0
OUTS = outp3
DIRS = direc2
PAUSE 1
CASE 7
OUTS = outp2
DIRS = direc1
PAUSE 1
DIRS = 0
OUTS = outp3
DIRS = direc2
PAUSE 1
OUTS = outp4
PAUSE 1
CASE 8
OUTS = outp1
DIRS = direc1
PAUSE 1
CASE 9
OUTS = outp1
DIRS = direc1
PAUSE 1
DIRS = 0
OUTS = outp4
DIRS = direc2
PAUSE 1
CASE 10
OUTS = outp1
DIRS = direc1
PAUSE 1
DIRS = 0
OUTS = outp3
DIRS = direc2
PAUSE 1
CASE 11
OUTS = outp1
DIRS = direc1
PAUSE 1
DIRS = 0
OUTS = outp3
DIRS = direc2
PAUSE 1
OUTS = outp4
PAUSE 1
CASE 12
OUTS = outp1
DIRS = direc1
PAUSE 1
OUTS = outp2
PAUSE 1
CASE 13
OUTS = outp1
DIRS = direc1
PAUSE 1
OUTS = outp2
PAUSE 1
DIRS = 0
OUTS = outp4
DIRS = direc2
PAUSE 1
CASE 14
OUTS = outp1
DIRS = direc1
PAUSE 1
OUTS = outp2
PAUSE 1
DIRS = 0
OUTS = outp3
DIRS = direc2
PAUSE 1
CASE 15
OUTS = outp1
DIRS = direc1
PAUSE 1
OUTS = outp2
PAUSE 1
DIRS = 0
OUTS = outp3
DIRS = direc2
PAUSE 1
OUTS = outp4
ENDSELECT
NEXT
repC = repC + 1
LOOP
RETURN

'1
letters DATA %0110,
%0110,
%0110,
%0110,
%0110,
'2
%0110,
%1001,
%0010,
%0100,
%1111,
'3
%0111,
%0001,
%0011,
%0001,
%0111,
'4
%1010,
%1010,
%1111,
%0010,
%0010,
'5
%0111,
%0100,
%0111,
%0001,
%0111,
'6
%0111,
%0100,
%0111,
%0101,
%0111,
'7
%1111,
%1001,
%0010,
%0100,
%0100,
'8
%0110,
%1001,
%0110,
%1001,
%0110,
'9
%0111,
%0101,
%0111,
%0001,
%0111,
'0
%0110,
%1001,
%1001,
%1001,
%0110,
'A
%0110,
%1001,
%1001,
%1111,
%1001,
'B
%1110,
%1001,
%1110,
%1001,
%1110,
'C
%0110,
%1001,
%1000,
%1001,
%0110,
'D
%1110,
%1001,
%1001,
%1001,
%1110,
'E
%1110,
%1000,
%1100,
%1000,
%1110,
'F
%1110,
%1000,
%1100,
%1000,
%1000,
'G
%1111,
%1001,
%1000,
%1011,
%1111,
'H
%1001,
%1001,
%1111,
%1001,
%1001,
'I
%1111,
%0110,
%0110,
%0110,
%1111,
'J
%0001,
%0001,
%0001,
%1001,
%1111,
'K
%1001,
%1010,
%1100,
%1010,
%1001,
'L
%1000,
%1000,
%1000,
%1000,
%1111,
'M
%1001,
%1111,
%1001,
%1001,
%1001,
'N
%1001,
%1101,
%1011,
%1001,
%1001,
'O
%1111,
%1001,
%1001,
%1001,
%1111,
'P
%1111,
%1001,
%1111,
%1000,
%1000,
'Q
%1111,
%1001,
%1001,
%1011,
%1111,
'R
%1111,
%1001,
%1111,
%1010,
%1001,
'S
%1111,
%1000,
%1111,
%0001,
%1111,
'T
%1111,
%0110,
%0110,
%0110,
%0110,
'U
%1001,
%1001,
%1001,
%1001,
%1111,
'V
%1001,
%1001,
%1001,
%1001,
%0110,
'W
%1001,
%1001,
%1001,
%1111,
%0110,
'X
%1001,
%1001,
%0110,
%1001,
%1001,
'Y
%1001,
%1001,
%0110,
%0110,
%0110,
'Z
%1111,
%0010,
%0100,
%1000,
%1111
-------------------------------------

do you have to use a breadbord?????????????????????????