RFID – radio frequency identification. Some of us have already used these things, and they have become part of everyday life. For example, with electronic vehicle tolling, door access control, public transport fare systems and so on. It sounds complex – but isn’t.
To explain RFID for the layperson, we can use a key and lock analogy. Instead of the key having a unique pattern, RFID keys hold a series of unique numbers which are read by the lock. It is up to our Arduino sketch to determine what happens when the number is read by the lock.
The key is the tag, card or other small device we carry around or have in our vehicles. We will be using a passive key, which is an integrated circuit and a small aerial. This uses power from a magnetic field associated with the lock. Here are some key or tag examples:
In this tutorial we’ll be using 125 kHz tags – for example. To continue with the analogy our lock is a small circuit board and a loop aerial. This has the capability to read the data on the IC of our key, and some locks can even write data to keys. Here is our reader (lock) example:
These readers are quite small and inexpensive – however the catch is that the loop aerial is somewhat fragile.
Setting up the RFID reader
This is a short exercise to check the reader works and communicates with the Arduino. You will need:
Simply insert the RFID reader main board into a solderless breadboard as shown below. Then use jumper wires to connect the second and third pins at the top-left of the RFID board to Arduino 5V and GND respectively. The RFID coil connects to the two pins on the top-right (they can go either way). Finally, connect a jumper wire from the bottom-left pin of the RFID board to Arduino digital pin 2:
Next, upload the following sketch to your Arduino and open the serial monitor window in the IDE:
#include <SoftwareSerial.h>
SoftwareSerial RFID(2, 3); // RX and TX
int i;
void setup()
{
RFID.begin(9600); // start serial to RFID reader
Serial.begin(9600); // start serial to PC
}
void loop()
{
if (RFID.available() > 0)
{
i = RFID.read();
Serial.print(i, DEC);
Serial.print(" ");
}
}
If you’re wondering why we used SoftwareSerial – if you connect the data line from the RFID board to the Arduino’s RX pin – you need to remove it when updating sketches, so this is more convenient.
Now start waving RFID cards or tags over the coil. You will find that they need to be parallel over the coil, and not too far away. You can experiment with covering the coil to simulate it being installed behind protective surfaces and so on. Watch this short video which shows the resulting RFID card or tag data being displayed in the Arduino IDE serial monitor.
As you can see from the example video, the reader returns the card’s unique ID number which starts with a 2 and ends with a 3. While you have the sketch operating, read the numbers from your RFID tags and note them down, you will need them for future sketches.
To do anything with the card data, we need to create some functions to retrieve the card number when it is read and place in an array for comparison against existing card data (e.g. a list of accepted cards) so your systems will know who to accept and who to deny. Using those functions, you can then make your own access system, time-logging device and so on.
Let’s demonstrate an example of this. It will check if a card presented to the reader is on an “accepted” list, and if so light a green LED, otherwise light a red LED. Use the hardware from the previous sketch, but add a typical green and red LED with 560 ohm resistor to digital pins 13 and 12 respectively. Then upload the following sketch:
#include <SoftwareSerial.h>
SoftwareSerial RFID(2, 3); // RX and TX
int data1 = 0;
int ok = -1;
int yes = 13;
int no = 12;
// use first sketch in http://wp.me/p3LK05-3Gk to get your tag numbers
int tag1[14] = {2,52,48,48,48,56,54,66,49,52,70,51,56,3};
int tag2[14] = {2,52,48,48,48,56,54,67,54,54,66,54,66,3};
int newtag[14] = { 0,0,0,0,0,0,0,0,0,0,0,0,0,0}; // used for read comparisons
void setup()
{
RFID.begin(9600); // start serial to RFID reader
Serial.begin(9600); // start serial to PC
pinMode(yes, OUTPUT); // for status LEDs
pinMode(no, OUTPUT);
}
boolean comparetag(int aa[14], int bb[14])
{
boolean ff = false;
int fg = 0;
for (int cc = 0 ; cc < 14 ; cc++)
{
if (aa[cc] == bb[cc])
{
fg++;
}
}
if (fg == 14)
{
ff = true;
}
return ff;
}
void checkmytags() // compares each tag against the tag just read
{
ok = 0; // this variable helps decision-making,
// if it is 1 we have a match, zero is a read but no match,
// -1 is no read attempt made
if (comparetag(newtag, tag1) == true)
{
ok++;
}
if (comparetag(newtag, tag2) == true)
{
ok++;
}
}
void readTags()
{
ok = -1;
if (RFID.available() > 0)
{
// read tag numbers
delay(100); // needed to allow time for the data to come in from the serial buffer.
for (int z = 0 ; z < 14 ; z++) // read the rest of the tag
{
data1 = RFID.read();
newtag[z] = data1;
}
RFID.flush(); // stops multiple reads
// do the tags match up?
checkmytags();
}
// now do something based on tag type
if (ok > 0) // if we had a match
{
Serial.println("Accepted");
digitalWrite(yes, HIGH);
delay(1000);
digitalWrite(yes, LOW);
ok = -1;
}
else if (ok == 0) // if we didn't have a match
{
Serial.println("Rejected");
digitalWrite(no, HIGH);
delay(1000);
digitalWrite(no, LOW);
ok = -1;
}
}
void loop()
{
readTags();
}
In the sketch we have a few functions that take care of reading and comparing RFID tags. Notice that the allowed tag numbers are listed at the top of the sketch, you can always add your own and more – as long as you add them to the list in the function checkmytags() which determines if the card being read is allowed or to be denied.
The function readTags() takes care of the actual reading of the tags/cards, by placing the currently-read tag number into an array which is them used in the comparison function checkmytags(). Then the LEDs are illuminated depending on the status of the tag at the reader. You can watch a quick demonstration of this example in this short video.
Conclusion
After working through this chapter you should now have a good foundation of knowledge on using the inexpensive RFID readers and how to call functions when a card is successfully read. For example, use some extra hardware to control a door strike, buzzer, etc.
Now it’s up to you to use them as a form of input with various access systems, tracking the movement of people or things and much more.
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Sooner or later Arduino enthusiasts and beginners alike will come across the MAX7219 IC. And for good reason, it’s a simple and somewhat inexpensive method of controlling 64 LEDs in either matrix or numeric display form. Furthermore they can be chained together to control two or more units for even more LEDs. Overall – they’re a lot of fun and can also be quite useful, so let’s get started.
Here’s an example of a MAX7219 and another IC which is a functional equivalent, the AS1107 from Austria Microsystems. You might not see the AS1107 around much, but it can be cheaper – so don’t be afraid to use that instead:
At first glance you may think that it takes a lot of real estate, but it saves some as well. As mentioned earlier, the MAX7219 can completely control 64 individual LEDs – including maintaining equal brightness, and allowing you to adjust the brightness of the LEDs either with hardware or software (or both). It can refresh the LEDs at around 800 Hz, so no more flickering, uneven LED displays.
You can even switch the display off for power saving mode, and still send it data while it is off. And another good thing – when powered up, it keeps the LEDs off, so no wacky displays for the first seconds of operation. For more technical information, here is the data sheet: MAX7219.pdf. Now to put it to work for us – we’ll demonstrate using one or more 8 x 8 LED matrix displays, as well as 8 digits of 7-segment LED numbers.
Before continuing, download and install the LedControl Arduino library as it is essential for using the MAX7219.
Controlling LED matrix displays with the MAX7219
First of all, let’s examine the hardware side of things. Here is the pinout diagram for the MAX7219:
The MAX7219 drives eight LEDs at a time, and by rapidly switching banks of eight your eyes don’t see the changes. Wiring up a matrix is very simple – if you have a common matrix with the following schematic:
connect the MAX7219 pins labelled DP, A~F to the row pins respectively, and the MAX7219 pins labelled DIG0~7 to the column pins respectively. A total example circuit with the above matrix is as follows:
The circuit is quite straight forward, except we have a resistor between 5V and MAX7219 pin 18. The MAX7219 is a constant-current LED driver, and the value of the resistor is used to set the current flow to the LEDs. Have a look at table eleven on page eleven of the data sheet:
You’ll need to know the voltage and forward current for your LED matrix or numeric display, then match the value on the table. E.g. if you have a 2V 20 mA LED, your resistor value will be 28kΩ (the values are in kΩ). Finally, the MAX7219 serial in, load and clock pins will go to Arduino digital pins which are specified in the sketch. We’ll get to that in the moment, but before that let’s return to the matrix modules.
In the last few months there has been a proliferation of inexpensive kits that contain a MAX7219 or equivalent, and an LED matrix. These are great for experimenting with and can save you a lot of work – some examples of which are shown below:
Now for the sketch. You need the following two lines at the beginning of the sketch:
The first pulls in the library, and the second line sets up an instance to control. The four parameters are as follows:
the digital pin connected to pin 1 of the MAX7219 (“data in”)
the digital pin connected to pin 13 of the MAX7219 (“CLK or clock”)
the digital pin connected to pin 12 of the MAX7219 (“LOAD”)
The number of MAX7219s connected.
If you have more than one MAX7219, connect the DOUT (“data out”) pin of the first MAX7219 to pin 1 of the second, and so on. However the CLK and LOAD pins are all connected in parallel and then back to the Arduino.
Next, two more vital functions that you’d normally put in void setup():
lc.shutdown(0,false);
lc.setIntensity(0,8);
The first line above turns the LEDs connected to the MAX7219 on. If you set TRUE, you can send data to the MAX7219 but the LEDs will stay off. The second line adjusts the brightness of the LEDs in sixteen stages. For both of those functions (and all others from the LedControl) the first parameter is the number of the MAX7219 connected. If you have one, the parameter is zero… for two MAX7219s, it’s 1 and so on.
Finally, to turn an individual LED in the matrix on or off, use:
lc.setLed(0,col,row,true);
which turns on an LED positioned at col, row connected to MAX7219 #1. Change TRUE to FALSE to turn it off. These functions are demonstrated in the following sketch:
#include "LedControl.h" // need the library
LedControl lc=LedControl(12,11,10,1); //
// pin 12 is connected to the MAX7219 pin 1
// pin 11 is connected to the CLK pin 13
// pin 10 is connected to LOAD pin 12
// 1 as we are only using 1 MAX7219
void setup()
{
// the zero refers to the MAX7219 number, it is zero for 1 chip
lc.shutdown(0,false);// turn off power saving, enables display
lc.setIntensity(0,8);// sets brightness (0~15 possible values)
lc.clearDisplay(0);// clear screen
}
void loop()
{
for (int row=0; row<8; row++)
{
for (int col=0; col<8; col++)
{
lc.setLed(0,col,row,true); // turns on LED at col, row
delay(25);
}
}
for (int row=0; row<8; row++)
{
for (int col=0; col<8; col++)
{
lc.setLed(0,col,row,false); // turns off LED at col, row
delay(25);
}
}
}
And a quick video of the results:
How about controlling two MAX7219s? Or more? The hardware modifications are easy – connect the serial data out pin from your first MAX7219 to the data in pin on the second (and so on), and the LOAD and CLOCK pins from the first MAX7219 connect to the second (and so on). You will of course still need the 5V, GND, resistor, capacitors etc. for the second and subsequent MAX7219.
You will also need to make a few changes in your sketch. The first is to tell it how many MAX7219s you’re using in the following line:
LedControl lc=LedControl(12,11,10,X);
by replacing X with the quantity. Then whenever you’re using a MAX7219 function, replace the (previously used) zero with the number of the MAX7219 you wish to address. They are numbered from zero upwards, with the MAX7219 directly connected to the Arduino as unit zero, then one etc. To demonstrate this, we replicate the previous example but with two MAX7219s:
#include "LedControl.h" // need the library
LedControl lc=LedControl(12,11,10,2); //
// pin 12 is connected to the MAX7219 pin 1
// pin 11 is connected to the CLK pin 13
// pin 10 is connected to LOAD pin 12
// 1 as we are only using 1 MAX7219
void setup()
{
lc.shutdown(0,false);// turn off power saving, enables display
lc.setIntensity(0,8);// sets brightness (0~15 possible values)
lc.clearDisplay(0);// clear screen
lc.shutdown(1,false);// turn off power saving, enables display
lc.setIntensity(1,8);// sets brightness (0~15 possible values)
lc.clearDisplay(1);// clear screen
}
void loop()
{
for (int row=0; row<8; row++)
{
for (int col=0; col<8; col++)
{
lc.setLed(0,col,row,true); // turns on LED at col, row
lc.setLed(1,col,row,false); // turns on LED at col, row
delay(25);
}
}
for (int row=0; row<8; row++)
{
for (int col=0; col<8; col++)
{
lc.setLed(0,col,row,false); // turns off LED at col, row
lc.setLed(1,col,row,true); // turns on LED at col, row
delay(25);
}
}
}
And again, a quick demonstration:
Another fun use of the MAX7219 and LED matrices is to display scrolling text. For the case of simplicity we’ll use the LedControl library and the two LED matrix modules from the previous examples.
First our example sketch – it is quite long however most of this is due to defining the characters for each letter of the alphabet and so on. We’ll explain it at the other end!
// based on an orginal sketch by Arduino forum member "danigom"
// http://forum.arduino.cc/index.php?action=profile;u=188950
#include <avr/pgmspace.h>
#include <LedControl.h>
const int numDevices = 2; // number of MAX7219s used
const long scrollDelay = 75; // adjust scrolling speed
unsigned long bufferLong [14] = {0};
LedControl lc=LedControl(12,11,10,numDevices);
prog_uchar scrollText[] PROGMEM ={
" THE QUICK BROWN FOX JUMPED OVER THE LAZY DOG 1234567890 the quick brown fox jumped over the lazy dog "};
void setup(){
for (int x=0; x<numDevices; x++){
lc.shutdown(x,false); //The MAX72XX is in power-saving mode on startup
lc.setIntensity(x,8); // Set the brightness to default value
lc.clearDisplay(x); // and clear the display
}
}
void loop(){
scrollMessage(scrollText);
scrollFont();
}
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////
prog_uchar font5x7 [] PROGMEM = { //Numeric Font Matrix (Arranged as 7x font data + 1x kerning data)
B00000000, //Space (Char 0x20)
B00000000,
B00000000,
B00000000,
B00000000,
B00000000,
B00000000,
6,
B10000000, //!
B10000000,
B10000000,
B10000000,
B00000000,
B00000000,
B10000000,
2,
B10100000, //"
B10100000,
B10100000,
B00000000,
B00000000,
B00000000,
B00000000,
4,
B01010000, //#
B01010000,
B11111000,
B01010000,
B11111000,
B01010000,
B01010000,
6,
B00100000, //$
B01111000,
B10100000,
B01110000,
B00101000,
B11110000,
B00100000,
6,
B11000000, //%
B11001000,
B00010000,
B00100000,
B01000000,
B10011000,
B00011000,
6,
B01100000, //&
B10010000,
B10100000,
B01000000,
B10101000,
B10010000,
B01101000,
6,
B11000000, //'
B01000000,
B10000000,
B00000000,
B00000000,
B00000000,
B00000000,
3,
B00100000, //(
B01000000,
B10000000,
B10000000,
B10000000,
B01000000,
B00100000,
4,
B10000000, //)
B01000000,
B00100000,
B00100000,
B00100000,
B01000000,
B10000000,
4,
B00000000, //*
B00100000,
B10101000,
B01110000,
B10101000,
B00100000,
B00000000,
6,
B00000000, //+
B00100000,
B00100000,
B11111000,
B00100000,
B00100000,
B00000000,
6,
B00000000, //,
B00000000,
B00000000,
B00000000,
B11000000,
B01000000,
B10000000,
3,
B00000000, //-
B00000000,
B11111000,
B00000000,
B00000000,
B00000000,
B00000000,
6,
B00000000, //.
B00000000,
B00000000,
B00000000,
B00000000,
B11000000,
B11000000,
3,
B00000000, ///
B00001000,
B00010000,
B00100000,
B01000000,
B10000000,
B00000000,
6,
B01110000, //0
B10001000,
B10011000,
B10101000,
B11001000,
B10001000,
B01110000,
6,
B01000000, //1
B11000000,
B01000000,
B01000000,
B01000000,
B01000000,
B11100000,
4,
B01110000, //2
B10001000,
B00001000,
B00010000,
B00100000,
B01000000,
B11111000,
6,
B11111000, //3
B00010000,
B00100000,
B00010000,
B00001000,
B10001000,
B01110000,
6,
B00010000, //4
B00110000,
B01010000,
B10010000,
B11111000,
B00010000,
B00010000,
6,
B11111000, //5
B10000000,
B11110000,
B00001000,
B00001000,
B10001000,
B01110000,
6,
B00110000, //6
B01000000,
B10000000,
B11110000,
B10001000,
B10001000,
B01110000,
6,
B11111000, //7
B10001000,
B00001000,
B00010000,
B00100000,
B00100000,
B00100000,
6,
B01110000, //8
B10001000,
B10001000,
B01110000,
B10001000,
B10001000,
B01110000,
6,
B01110000, //9
B10001000,
B10001000,
B01111000,
B00001000,
B00010000,
B01100000,
6,
B00000000, //:
B11000000,
B11000000,
B00000000,
B11000000,
B11000000,
B00000000,
3,
B00000000, //;
B11000000,
B11000000,
B00000000,
B11000000,
B01000000,
B10000000,
3,
B00010000, //<
B00100000,
B01000000,
B10000000,
B01000000,
B00100000,
B00010000,
5,
B00000000, //=
B00000000,
B11111000,
B00000000,
B11111000,
B00000000,
B00000000,
6,
B10000000, //>
B01000000,
B00100000,
B00010000,
B00100000,
B01000000,
B10000000,
5,
B01110000, //?
B10001000,
B00001000,
B00010000,
B00100000,
B00000000,
B00100000,
6,
B01110000, //@
B10001000,
B00001000,
B01101000,
B10101000,
B10101000,
B01110000,
6,
B01110000, //A
B10001000,
B10001000,
B10001000,
B11111000,
B10001000,
B10001000,
6,
B11110000, //B
B10001000,
B10001000,
B11110000,
B10001000,
B10001000,
B11110000,
6,
B01110000, //C
B10001000,
B10000000,
B10000000,
B10000000,
B10001000,
B01110000,
6,
B11100000, //D
B10010000,
B10001000,
B10001000,
B10001000,
B10010000,
B11100000,
6,
B11111000, //E
B10000000,
B10000000,
B11110000,
B10000000,
B10000000,
B11111000,
6,
B11111000, //F
B10000000,
B10000000,
B11110000,
B10000000,
B10000000,
B10000000,
6,
B01110000, //G
B10001000,
B10000000,
B10111000,
B10001000,
B10001000,
B01111000,
6,
B10001000, //H
B10001000,
B10001000,
B11111000,
B10001000,
B10001000,
B10001000,
6,
B11100000, //I
B01000000,
B01000000,
B01000000,
B01000000,
B01000000,
B11100000,
4,
B00111000, //J
B00010000,
B00010000,
B00010000,
B00010000,
B10010000,
B01100000,
6,
B10001000, //K
B10010000,
B10100000,
B11000000,
B10100000,
B10010000,
B10001000,
6,
B10000000, //L
B10000000,
B10000000,
B10000000,
B10000000,
B10000000,
B11111000,
6,
B10001000, //M
B11011000,
B10101000,
B10101000,
B10001000,
B10001000,
B10001000,
6,
B10001000, //N
B10001000,
B11001000,
B10101000,
B10011000,
B10001000,
B10001000,
6,
B01110000, //O
B10001000,
B10001000,
B10001000,
B10001000,
B10001000,
B01110000,
6,
B11110000, //P
B10001000,
B10001000,
B11110000,
B10000000,
B10000000,
B10000000,
6,
B01110000, //Q
B10001000,
B10001000,
B10001000,
B10101000,
B10010000,
B01101000,
6,
B11110000, //R
B10001000,
B10001000,
B11110000,
B10100000,
B10010000,
B10001000,
6,
B01111000, //S
B10000000,
B10000000,
B01110000,
B00001000,
B00001000,
B11110000,
6,
B11111000, //T
B00100000,
B00100000,
B00100000,
B00100000,
B00100000,
B00100000,
6,
B10001000, //U
B10001000,
B10001000,
B10001000,
B10001000,
B10001000,
B01110000,
6,
B10001000, //V
B10001000,
B10001000,
B10001000,
B10001000,
B01010000,
B00100000,
6,
B10001000, //W
B10001000,
B10001000,
B10101000,
B10101000,
B10101000,
B01010000,
6,
B10001000, //X
B10001000,
B01010000,
B00100000,
B01010000,
B10001000,
B10001000,
6,
B10001000, //Y
B10001000,
B10001000,
B01010000,
B00100000,
B00100000,
B00100000,
6,
B11111000, //Z
B00001000,
B00010000,
B00100000,
B01000000,
B10000000,
B11111000,
6,
B11100000, //[
B10000000,
B10000000,
B10000000,
B10000000,
B10000000,
B11100000,
4,
B00000000, //(Backward Slash)
B10000000,
B01000000,
B00100000,
B00010000,
B00001000,
B00000000,
6,
B11100000, //]
B00100000,
B00100000,
B00100000,
B00100000,
B00100000,
B11100000,
4,
B00100000, //^
B01010000,
B10001000,
B00000000,
B00000000,
B00000000,
B00000000,
6,
B00000000, //_
B00000000,
B00000000,
B00000000,
B00000000,
B00000000,
B11111000,
6,
B10000000, //`
B01000000,
B00100000,
B00000000,
B00000000,
B00000000,
B00000000,
4,
B00000000, //a
B00000000,
B01110000,
B00001000,
B01111000,
B10001000,
B01111000,
6,
B10000000, //b
B10000000,
B10110000,
B11001000,
B10001000,
B10001000,
B11110000,
6,
B00000000, //c
B00000000,
B01110000,
B10001000,
B10000000,
B10001000,
B01110000,
6,
B00001000, //d
B00001000,
B01101000,
B10011000,
B10001000,
B10001000,
B01111000,
6,
B00000000, //e
B00000000,
B01110000,
B10001000,
B11111000,
B10000000,
B01110000,
6,
B00110000, //f
B01001000,
B01000000,
B11100000,
B01000000,
B01000000,
B01000000,
6,
B00000000, //g
B01111000,
B10001000,
B10001000,
B01111000,
B00001000,
B01110000,
6,
B10000000, //h
B10000000,
B10110000,
B11001000,
B10001000,
B10001000,
B10001000,
6,
B01000000, //i
B00000000,
B11000000,
B01000000,
B01000000,
B01000000,
B11100000,
4,
B00010000, //j
B00000000,
B00110000,
B00010000,
B00010000,
B10010000,
B01100000,
5,
B10000000, //k
B10000000,
B10010000,
B10100000,
B11000000,
B10100000,
B10010000,
5,
B11000000, //l
B01000000,
B01000000,
B01000000,
B01000000,
B01000000,
B11100000,
4,
B00000000, //m
B00000000,
B11010000,
B10101000,
B10101000,
B10001000,
B10001000,
6,
B00000000, //n
B00000000,
B10110000,
B11001000,
B10001000,
B10001000,
B10001000,
6,
B00000000, //o
B00000000,
B01110000,
B10001000,
B10001000,
B10001000,
B01110000,
6,
B00000000, //p
B00000000,
B11110000,
B10001000,
B11110000,
B10000000,
B10000000,
6,
B00000000, //q
B00000000,
B01101000,
B10011000,
B01111000,
B00001000,
B00001000,
6,
B00000000, //r
B00000000,
B10110000,
B11001000,
B10000000,
B10000000,
B10000000,
6,
B00000000, //s
B00000000,
B01110000,
B10000000,
B01110000,
B00001000,
B11110000,
6,
B01000000, //t
B01000000,
B11100000,
B01000000,
B01000000,
B01001000,
B00110000,
6,
B00000000, //u
B00000000,
B10001000,
B10001000,
B10001000,
B10011000,
B01101000,
6,
B00000000, //v
B00000000,
B10001000,
B10001000,
B10001000,
B01010000,
B00100000,
6,
B00000000, //w
B00000000,
B10001000,
B10101000,
B10101000,
B10101000,
B01010000,
6,
B00000000, //x
B00000000,
B10001000,
B01010000,
B00100000,
B01010000,
B10001000,
6,
B00000000, //y
B00000000,
B10001000,
B10001000,
B01111000,
B00001000,
B01110000,
6,
B00000000, //z
B00000000,
B11111000,
B00010000,
B00100000,
B01000000,
B11111000,
6,
B00100000, //{
B01000000,
B01000000,
B10000000,
B01000000,
B01000000,
B00100000,
4,
B10000000, //|
B10000000,
B10000000,
B10000000,
B10000000,
B10000000,
B10000000,
2,
B10000000, //}
B01000000,
B01000000,
B00100000,
B01000000,
B01000000,
B10000000,
4,
B00000000, //~
B00000000,
B00000000,
B01101000,
B10010000,
B00000000,
B00000000,
6,
B01100000, // (Char 0x7F)
B10010000,
B10010000,
B01100000,
B00000000,
B00000000,
B00000000,
5
};
void scrollFont() {
for (int counter=0x20;counter<0x80;counter++){
loadBufferLong(counter);
delay(500);
}
}
// Scroll Message
void scrollMessage(prog_uchar * messageString) {
int counter = 0;
int myChar=0;
do {
// read back a char
myChar = pgm_read_byte_near(messageString + counter);
if (myChar != 0){
loadBufferLong(myChar);
}
counter++;
}
while (myChar != 0);
}
// Load character into scroll buffer
void loadBufferLong(int ascii){
if (ascii >= 0x20 && ascii <=0x7f){
for (int a=0;a<7;a++){ // Loop 7 times for a 5x7 font
unsigned long c = pgm_read_byte_near(font5x7 + ((ascii - 0x20) * 8) + a); // Index into character table to get row data
unsigned long x = bufferLong [a*2]; // Load current scroll buffer
x = x | c; // OR the new character onto end of current
bufferLong [a*2] = x; // Store in buffer
}
byte count = pgm_read_byte_near(font5x7 +((ascii - 0x20) * 8) + 7); // Index into character table for kerning data
for (byte x=0; x<count;x++){
rotateBufferLong();
printBufferLong();
delay(scrollDelay);
}
}
}
// Rotate the buffer
void rotateBufferLong(){
for (int a=0;a<7;a++){ // Loop 7 times for a 5x7 font
unsigned long x = bufferLong [a*2]; // Get low buffer entry
byte b = bitRead(x,31); // Copy high order bit that gets lost in rotation
x = x<<1; // Rotate left one bit
bufferLong [a*2] = x; // Store new low buffer
x = bufferLong [a*2+1]; // Get high buffer entry
x = x<<1; // Rotate left one bit
bitWrite(x,0,b); // Store saved bit
bufferLong [a*2+1] = x; // Store new high buffer
}
}
// Display Buffer on LED matrix
void printBufferLong(){
for (int a=0;a<7;a++){ // Loop 7 times for a 5x7 font
unsigned long x = bufferLong [a*2+1]; // Get high buffer entry
byte y = x; // Mask off first character
lc.setRow(3,a,y); // Send row to relevent MAX7219 chip
x = bufferLong [a*2]; // Get low buffer entry
y = (x>>24); // Mask off second character
lc.setRow(2,a,y); // Send row to relevent MAX7219 chip
y = (x>>16); // Mask off third character
lc.setRow(1,a,y); // Send row to relevent MAX7219 chip
y = (x>>8); // Mask off forth character
lc.setRow(0,a,y); // Send row to relevent MAX7219 chip
}
}
The pertinent parts are at the top of the sketch – the following line sets the number of MAX7219s in the hardware:
const int numDevices = 2;
The following can be adjusted to change the speed of text scrolling:
const long scrollDelay = 75;
… then place the text to scroll in the following (for example):
prog_uchar scrollText[] PROGMEM ={
" THE QUICK BROWN FOX JUMPED OVER THE LAZY DOG 1234567890 the quick brown fox jumped over the lazy dog "};
Finally – to scroll the text on demand, use the following:
scrollMessage(scrollText);
You can then incorporate the code into your own sketches. And a video of the example sketch in action:
Although we used the LedControl library, there are many others out there for scrolling text. One interesting example is Parola – which is incredibly customisable.
Controlling LED numeric displays with the MAX7219
Using the MAX7219 and the LedControl library you can also drive numeric LED displays – up to eight digits from the one MAX7219. This gives you the ability to make various numeric displays that are clear to read and easy to control. When shopping around for numeric LED displays, make sure you have the common-cathode type.
Connecting numeric displays is quite simple, consider the following schematic which should appear familiar by now:
The schematic shows the connections for modules or groups of up to eight digits. Each digit’s A~F and dp (decimal point) anodes connect together to the MAX7219, and each digit’s cathode connects in order as well. The MAX7219 will display each digit in turn by using one cathode at a time. Of course if you want more than eight digits, connect another MAX7219 just as we did with the LED matrices previously.
The required code in the sketch is identical to the LED matrix code, however to display individual digits we use:
lc.setDigit(A, B, C, D);
where A is the MAX7219 we’re using, B is the digit to use (from a possible 0 to 7), C is the digit to display (0~9… if you use 10~15 it will display A~F respectively) and D is false/true (digit on or off). You can also send basic characters such as a dash “-” with the following:
lc.setChar(A, B,'-',false);
Now let’s put together an example of eight digits:
#include "LedControl.h" // need the library
LedControl lc=LedControl(12,11,10,1); // lc is our object
// pin 12 is connected to the MAX7219 pin 1
// pin 11 is connected to the CLK pin 13
// pin 10 is connected to LOAD pin 12
// 1 as we are only using 1 MAX7219
void setup()
{
// the zero refers to the MAX7219 number, it is zero for 1 chip
lc.shutdown(0,false);// turn off power saving, enables display
lc.setIntensity(0,8);// sets brightness (0~15 possible values)
lc.clearDisplay(0);// clear screen
}
void loop()
{
for (int a=0; a<8; a++)
{
lc.setDigit(0,a,a,true);
delay(100);
}
for (int a=0; a<8; a++)
{
lc.setDigit(0,a,8,1);
delay(100);
}
for (int a=0; a<8; a++)
{
lc.setDigit(0,a,0,false);
delay(100);
}
for (int a=0; a<8; a++)
{
lc.setChar(0,a,' ',false);
delay(100);
}
for (int a=0; a<8; a++)
{
lc.setChar(0,a,'-',false);
delay(100);
}
for (int a=0; a<8; a++)
{
lc.setChar(0,a,' ',false);
delay(100);
}
}
and the sketch in action:
Conclusion
We have only scratched the surface of what is possible with the MAX7219 and compatible parts. They’re loads of fun and quite useful as well.
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Moving on from a previous article where we explained an 8-bit ADC – in this instalment we have the Texas Instruments ADS1110 – an incredibly tiny but useful 16-bit analogue-to-digital converter IC.
It can operate between 2.7 and 5.5 V so it’s also fine for Arduino Due and other lower-voltage development boards. This is a quick guide to get you going with the ADS1110 ready for further applications. Before continuing any further, please download the data sheet (pdf) as it will be useful and referred to during this tutorial.
The ADS1110 gives you the option of a more accurate ADC than offered by the Arduino’s 10-bit ADCs – and it’s relatively easy to use. However it’s only available as a bare part in SOT23-6:
The good news is that you can order the ADS1110 mounted on a very convenient breakout board:
The ADS1110 uses the I2C bus for communication, so if this is new to you – please review the I2C tutorials before continuing. And as there’s only six pins you can’t set the bus address – instead, you can select from six variants of the ADS1110 – each with their own address (see page two of the data sheet).
As you can see the in the photo above, ours is marked “EDO” which matches to the bus address 1001000 or 0x48h. And with the example circuits we’ve used 10kΩ pull-up resistors on the I2C bus. You can use the ADS1110 as either a single-ended or differential ADC – But first we need to examine the configuration register which is used to control various attributes, and the data register.
Configuration register
Turn to page eleven of the data sheet. The configuration register is one byte in size, and as the ADS1110 resets on a power-cycle – you need to reset the register if your needs are different to the defaults. The data sheet spells it out quite neatly… bits 0 and 1 determine the gain setting for the PGA (programmable gain amplifier).
If you’re just measuring voltages or experimenting, leave these as zero for a gain of 1V/V. Next, the data rate for the ADS1110 is controlled with bits 2 and 3. If you have continuous sampling turned on, this determines the number of samples per second taken by the ADC.
After some experimenting with an Arduino Uno we found the values returned from the ADC were a bit off when using the fastest rate, so leave it as 15 SPS unless required otherwise. Bit 4 sets either continuous sampling (0) or one-off sampling (1). Ignore bits 5 and 6, however they’re always set as 0. Finally bit 7 – if you’re in one-off sampling mode, setting it to 1 requests a sample – and reading it will tell you if the returned data is new (0) or old (1). You can check that the value measured is a new value – if the first bit of the configuration byte that comes after the data is 0, it’s new. If it returns 1 the ADC conversion hasn’t finished.
Data register
As the ADS1110 is a 16-bit ADC, it returns the data over two bytes – and then follows with the value of the configuration register. So if you request three bytes the whole lot comes back. The data is in “two’s complement” form, which is a method of using signed numbers with binary. Converting those two bytes is done by some simple maths. When sampling at 15 SPS, the value returned by the ADS1110 (not the voltage) falls between -32768 and 32767. The higher byte of the value is multiplied by 256, then added to the lower byte – which is then multiplied by 2.048 and finally divided by 32768. Don’t panic, as we do this in the example sketch below.
Single-ended ADC mode
In this mode you can read a voltage that falls between zero and 2.048 V (which also happens to be the inbuilt reference voltage for the ADS1110). The example circuit is simple (from the data sheet):
Don’t forget the 10kΩ pull-up resistors on the I2C bus. The following sketch uses the ADS1110 in the default mode, and simply returns the voltage measured:
// Example 53.1 - ADS1110 single-sided voltmeter (0~2.048VDC)
#include "Wire.h"
#define ads1110 0x48
float voltage, data;
byte highbyte, lowbyte, configRegister;
void setup()
{
Serial.begin(9600);
Wire.begin();
}
void loop()
{
Wire.requestFrom(ads1110, 3);
while(Wire.available()) // ensure all the data comes in
{
highbyte = Wire.read(); // high byte * B11111111
lowbyte = Wire.read(); // low byte
configRegister = Wire.read();
}
data = highbyte * 256;
data = data + lowbyte;
Serial.print("Data >> ");
Serial.println(data, DEC);
Serial.print("Voltage >> ");
voltage = data * 2.048 ;
voltage = voltage / 32768.0;
Serial.print(voltage, DEC);
Serial.println(" V");
delay(1000);
}
Once uploaded, connect the signal to measure and open the serial monitor – you’ll be presented with something similar to:
If you need to alter the gain of the internal programmable gain amplifier of the ADC – you’ll need to write a new byte into the configuration register using:
before requesting the ADC data. This would be 0x8D, 0x8E or 0x8F for gain values of 2, 4 and 8 respectively – and use 0x8C to reset the ADS1110 back to default.
Differential ADC mode
In this mode you can read the difference between two voltages that each fall between zero and 5 V. The example circuit is simple (from the data sheet):
We must note here (and in the data sheet) that the ADS1110 can’t accept negative voltages on either of the inputs. You can use the previous sketch for the same results – and the resulting voltage will be the value of Vin- subtracted from Vin+. For example, if you had 2 V on Vin+ and 1 V on Vin- the resulting voltage would be 1 V (with the gain set to 1).
Conclusion
Once again we hope you found this of interest, and possibly useful.
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Have you ever wanted more analogue input pins on your Arduino project, but not wanted to fork out for a Mega? Or would you like to generate analogue signals? Then check out the subject of our tutorial – the NXP PCF8591 IC.
It solves both these problems as it has a single DAC (digital to analogue) converter as well as four ADCs (analogue to digital converters) – all accessible via the I2C bus. If the I2C bus is new to you, please familiarise yourself with the readings here before moving forward.
Before moving on, download the data sheet. The PCF8591 can operate on both 5V and 3.3V so if you’re using an Arduino Due, Raspberry Pi or other 3.3 V development board you’re fine. Now we’ll first explain the DAC, then the ADCs.
Using the DAC (digital-to-analogue converter)
The DAC on the PCF8591 has a resolution of 8-bits – so it can generate a theoretical signal of between zero volts and the reference voltage (Vref) in 255 steps. For demonstration purposes we’ll use a Vref of 5V, and you can use a lower Vref such as 3.3V or whatever you wish the maximum value to be … however it must be less than the supply voltage.
Note that when there is a load on the analogue output (a real-world situation), the maximum output voltage will drop – the data sheet (which you downloaded) shows a 10% drop for a 10kΩ load. Now for our demonstration circuit:
Note the use of 10kΩ pull-up resistors on the I2C bus, and the 10μF capacitor between 5V and GND. The I2C bus address is set by a combination of pins A0~A2, and with them all to GND the address is 0x90. The analogue output can be taken from pin 15 (and there’s a seperate analogue GND on pin 13. Also, connect pin 13 to GND, and circuit GND to Arduino GND.
To control the DAC we need to send two bytes of data. The first is the control byte, which simply activates the DAC and is 1000000 (or 0x40) and the next byte is the value between 0 and 255 (the output level). This is demonstrated in the following sketch:
// Example 52.1 PCF8591 DAC demo
// https://tronixstuff.com/tutorials Chapter 52
// John Boxall June 2013
#include "Wire.h"
#define PCF8591 (0x90 >> 1) // I2C bus address
void setup()
{
Wire.begin();
}
void loop()
{
for (int i=0; i<256; i++)
{
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(0x40); // control byte - turn on DAC (binary 1000000)
Wire.write(i); // value to send to DAC
Wire.endTransmission(); // end tranmission
}
for (int i=255; i>=0; --i)
{
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(0x40); // control byte - turn on DAC (binary 1000000)
Wire.write(i); // value to send to DAC
Wire.endTransmission(); // end tranmission
}
}
Did you notice the bit shift of the bus address in the #define statement? Arduino sends 7-bit addresses but the PCF8591 wants an 8-bit, so we shift the byte over by one bit.
The results of the sketch are shown below, we’ve connected the Vref to 5V and the oscilloscope probe and GND to the analogue output and GND respectively:
If you like curves you can generate sine waves with the sketch below. It uses a lookup table in an array which contains the necessary pre-calculated data points:
For the following DSO image dump, we changed the Vref to 3.3V – note the change in the maxima on the sine wave:
Now you can experiment with the DAC to make sound effects, signals or control other analogue circuits.
Using the ADCs (analogue-to-digital converters)
If you’ve used the analogRead() function on your Arduino (way back in Chapter One) then you’re already familiar with an ADC. With out PCF8591 we can read a voltage between zero and the Vref and it will return a value of between zero and 255 which is directly proportional to zero and the Vref.
For example, measuring 3.3V should return 168. The resolution (8-bit) of the ADC is lower than the onboard Arduino (10-bit) however the PCF8591 can do something the Arduino’s ADC cannot. But we’ll get to that in a moment.
First, to simply read the values of each ADC pin we send a control byte to tell the PCF8591 which ADC we want to read. For ADCs zero to three the control byte is 0x00, 0x01, ox02 and 0x03 respectively. Then we ask for two bytes of data back from the ADC, and store the second byte for use.
Why two bytes? The PCF8591 returns the previously measured value first – then the current byte. (See Figure 8 in the data sheet). Finally, if you’re not using all the ADC pins, connect the unused ones to GND.
The following example sketch simply retrieves values from each ADC pin one at a time, then displays them in the serial monitor:
// Example 52.3 PCF8591 ADC demo
// https://tronixstuff.com/tutorials Chapter 52
// John Boxall June 2013
#include "Wire.h"
#define PCF8591 (0x90 >> 1) // I2C bus address
#define ADC0 0x00 // control bytes for reading individual ADCs
#define ADC1 0x01
#define ADC2 0x02
#define ADC3 0x03
byte value0, value1, value2, value3;
void setup()
{
Wire.begin();
Serial.begin(9600);
}
void loop()
{
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(ADC0); // control byte - read ADC0
Wire.endTransmission(); // end tranmission
Wire.requestFrom(PCF8591, 2);
value0=Wire.read();
value0=Wire.read();
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(ADC1); // control byte - read ADC1
Wire.endTransmission(); // end tranmission
Wire.requestFrom(PCF8591, 2);
value1=Wire.read();
value1=Wire.read();
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(ADC2); // control byte - read ADC2
Wire.endTransmission(); // end tranmission
Wire.requestFrom(PCF8591, 2);
value2=Wire.read();
value2=Wire.read();
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(ADC3); // control byte - read ADC3
Wire.endTransmission(); // end tranmission
Wire.requestFrom(PCF8591, 2);
value3=Wire.read();
value3=Wire.read();
Serial.print(value0); Serial.print(" ");
Serial.print(value1); Serial.print(" ");
Serial.print(value2); Serial.print(" ");
Serial.print(value3); Serial.print(" ");
Serial.println();
}
Upon running the sketch you’ll be presented with the values of each ADC in the serial monitor. Although it was a simple demonstration to show you how to individually read each ADC, it is a cumbersome method of getting more than one byte at a time from a particular ADC.
To do this, change the control byte to request auto-increment, which is done by setting bit 2 of the control byte to 1. So to start from ADC0 we use a new control byte of binary 00000100 or hexadecimal 0x04. Then request five bytes of data (once again we ignore the first byte) which will cause the PCF8591 to return all values in one chain of bytes. This process is demonstrated in the following sketch:
// Example 52.4 PCF8591 ADC demo
// https://tronixstuff.com/tutorials Chapter 52
// John Boxall June 2013
#include "Wire.h"
#define PCF8591 (0x90 >> 1) // I2C bus address
byte value0, value1, value2, value3;
void setup()
{
Wire.begin();
Serial.begin(9600);
}
void loop()
{
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(0x04); // control byte - read ADC0 then auto-increment
Wire.endTransmission(); // end tranmission
Wire.requestFrom(PCF8591, 5);
value0=Wire.read();
value0=Wire.read();
value1=Wire.read();
value2=Wire.read();
value3=Wire.read();
Serial.print(value0); Serial.print(" ");
Serial.print(value1); Serial.print(" ");
Serial.print(value2); Serial.print(" ");
Serial.print(value3); Serial.print(" ");
Serial.println();
}
Previously we mentioned that the PCF8591 can do something that the Arduino’s ADC cannot, and this is offer a differential ADC. As opposed to the Arduino’s single-ended (i.e. it returns the difference between the positive signal voltage and GND, the differential ADC accepts two signals (that don’t necessarily have to be referenced to ground), and returns the difference between the two signals. This can be convenient for measuring small changes in voltages for load cells and so on.
Setting up the PCF8591 for differential ADC is a simple matter of changing the control byte. If you turn to page seven of the data sheet, then consider the different types of analogue input programming. Previously we used mode ’00’ for four inputs, however you can select the others which are clearly illustrated, for example:
So to set the control byte for two differential inputs, use binary 00110000 or 0x30. Then it’s a simple matter of requesting the bytes of data and working with them. As you can see there’s also combination single/differential and a complex three-differential input. However we’ll leave them for the time being.
Conclusion
Hopefully you found this of interest, whether adding a DAC to your experiments or learning a bit more about ADCs. Please consider ordering your PCF8591 from PMD Way.
This post brought to you by pmdway.com – everything for makers and electronics enthusiasts, with free delivery worldwide.
To keep up to date with new posts at tronixstuff.com, please subscribe to the mailing list in the box on the right, or follow us on twitter @tronixstuff.
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