I'm a new Arduino user (convert?) and wanted to do something simple for my first project. As I hadn't seen anything similar to my approach I thought I would post my results to share with others. The result is a 4 digit counter I made from a pair of 2 digit common cathode 7-segment displays. Similar projects used a 75HC595 for each digit. I went for a multiplexed approach with a single shift register driving the segments (anodes) and 4 digital i/o pins each driving a NPN transistor to switch in each cathode one at a time. I connected all the 'a' segments of each display to each other and then connected that to the QA output of the shift register. See the great tutorial elsewhere on this site. Similarly, connect each of the 'b' thought 'h' segments of the display to each other and to the corresponding pin on the shift register. Next I wired in the cathode drivers by driving each one with a 2N3904 transistor so as to not overload the Arduino pins. Specific details are in the code comments. The sketch just counts from 0 to 9999 and then starts over. Not much really but I had a lot of fun making it and learned a lot about Arduino in the process. The next step is to attach the DS1307 and turn the display into yet-another-arduino-clock. Here is the code:
/*
2011-10-26
Paul Jenkins
This code is in the public domain.
//
Example code for driving a 4 digit common cathode 7-segment display with
a 74HC595 8-bit shift register and 4 NPN transistors.
The 74HC595 shift register attaches to pins 8, 11 and 12 of Arduino.
See Arduino ShiftOut tutorial for more info:
Arduino digital pins 4 to 7 drive the cathode for digits 1 through 4.
Digit 1 is the leftmost. Wire the emitter of the NPN transistor to ground.
Connect the Arduino digital pin to the base of the transistor via a
1 k-ohm resistor so as not to provide excessive base drive and damage
the transistor. Connect the collector of the transistor to the cathode of
the display.
Be sure to use current limiting resistors from the shift register outputs
Qa to Qf to the anode segments of the LED display. I used 220 ohm resistors.
- Shift Register Arduino
- 14 SER 11 Serial Input
- 11 SRCLK 12 Shift Clock
- 12 RCLK 8 Latch Enable
*/
// Define the bit-patterns for the 7-segment displays
const byte SevenSeg[16] =
{
// Hex digits
B11111100, B01100000, B11011010, B11110010, // 0123
B01100110, B10110110, B10111110, B11100000, // 4567
B11111110, B11110110, B11101110, B00111110, // 89AB
B00011010, B01111010, B10011110, B10001110, // CDEF
};
// Pin connected to latch pin (RCLK,12) of 74HC595
const int latchPin = 8;
// Pin connected to clock pin (SRCLK,11) of 74HC595
const int clockPin = 12;
// Pin connected to Data in (SER,14) of 74HC595
const int dataPin = 11;
// Pin connected to cathodes
const int disp1 = 7; // Displays are numbered from left to right
const int disp2 = 6; // as disp1 to disp4.
const int disp3 = 5; // Scan displays quickly and use POV to display
const int disp4 = 4; // 4 digits with only 1 shift register
//
void setup()
{
pinMode(latchPin, OUTPUT);
pinMode(clockPin, OUTPUT);
pinMode(dataPin, OUTPUT);
pinMode(disp1, OUTPUT);
pinMode(disp2, OUTPUT);
pinMode(disp3, OUTPUT);
pinMode(disp4, OUTPUT);
}
//
void loop()
{
for (int i=0; i < 10000; ++i)
{
// Display i as 4 digits on the display.
// No leading zero blanking.
for (int j=0; j<20; j++) // Draw the display several times to slow it down
SevenSegDisplay(i);
}
}
void DisplayADigit(int dispno, byte digit2disp)
{
// Turn off the shift register pins
// while you're shifting bits:
digitalWrite(latchPin, LOW);
AllDispOff(); // Turn off all cathode drivers.
// shift the bits out:
shiftOut(dataPin, clockPin, LSBFIRST, digit2disp);
digitalWrite(latchPin, HIGH); // Set latch high to set segments.
digitalWrite(dispno, HIGH); // Drive one cathode low to turn on display.
delay(5); // Wait a bit for POV
}
void AllDispOff()
{
// Turn all cathode drivers off
digitalWrite(disp1, LOW);
digitalWrite(disp2, LOW);
digitalWrite(disp3, LOW);
digitalWrite(disp4, LOW);
}
void SevenSegDisplay(int number)
{
int d1,d2,d3,d4; // Temporary values for thousands, hundreds, tens and units
if (number > 9999)
number = 9999; // Do some bounds checks to avoid strangeness
if (number < 0)
number = 0;
d1 = (int) (number / 1000); // Get thousands
number = number - (d1 * 1000);
d2 = (int) (number / 100); // Get hundreds
number = number - (d2 * 100);
d3 = (int) (number / 10); // Get tens
d4 = number - (d3* 10); // Get units
DisplayADigit(disp1,byte(SevenSeg[d1])); // Show thousands
DisplayADigit(disp2,byte(SevenSeg[d2])); // Show hundreds
DisplayADigit(disp3,byte(SevenSeg[d3])); // Show tens
DisplayADigit(disp4,byte(SevenSeg[d4])); // Show units
}
The shift registers are cheap.
I only have a small 30-row breadboard right now so it is packed. When I get to my bigger bb I'm going to take your suggestion and expand it to 6 digits (that's all the displays I have). Then there will be room for the time chip and battery too and I can include seconds with the time.
