Erwan's Blog

I got myself a cheap 4 digits 7-led display and I thought I would spend a few mns playing with it and documenting it.

12 pins : 8 for the segments (including the dots), 4 for each digit
Top row : 1, a, f, 2, 3, b
Bottomw row: e, d, dp, c, g, 4

I decided to use the Sevseg arduino library.
More about this library here.

The wiring is then as is :

Arduino pins -> 4digits display pins
2->1
6->a
11->f
3->2
4->3
7->b
10->e
9->d
13->dp
8->c
12->g
5->4

Below the schema

Note that this is a lot of digital IO’s used. A future article could focus on reducing the number of IO’s needed.
Below the arduino sketch (from the example provided with the sevseg library)

/*Written by Dean Reading, 2012.  deanreading@hotmail.com
 
 This example is a centi-second counter to demonstrate the
 use of my SevSeg library.
 */

#include "SevSeg.h"

//Create an instance of the object.
SevSeg sevseg;

//Create global variables
unsigned long timer;
int CentSec=0;

void setup() {
  //I am using a common anode display, with the digit pins connected
  //from 2-5 and the segment pins connected from 6-13
  sevseg.Begin(1,2,3,4,5,6,7,8,9,10,11,12,13);
  //Set the desired brightness (0 to 100);
  sevseg.Brightness(50);

  timer=millis();
}

void loop() {
  //Produce an output on the display
  sevseg.PrintOutput();

  //Check if 10ms has elapsed
  unsigned long mils=millis();
  if (mils-timer>=10) {
    timer=mils;
    CentSec++;
    if (CentSec==10000) { // Reset to 0 after counting for 100 seconds.
      CentSec=0;
    }
    //Update the number to be displayed, with a decimal
    //place in the correct position.
    sevseg.NewNum(CentSec,(byte) 2);
  }
}

A small break away from coding and electronic…

I wanted to offer something special to my special geek girl so here it is : a jewel (earings) made of resistors 🙂

Two times 5 * 1.8Kohm : yes, I am not cheap, not counting !

In a previous article, we had used a 74HC595 to control a ULN2803.
This enabled us to deal with 8 LED’s.

Lets now cascade two 74HC595 to deal with 16 LED’s.
To do this, we will use the serial output of 74HC595 #1 to the serial input of 74HC595 #2.

Here below the schema.

Here below the arduino sketch.
Note that we use two arrays, and that we go up and down in each array.

//the pins we are using
int latchPin = 2;
int clockPin = 3;
int dataPin = 4;


byte dataArrayA[9];
byte dataArrayB[9];
 
void setup() {
  //set all the pins used to talk to the chip
  //as output pins so we can write to them
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);
  
  dataArrayA[0] = 0xFF; //11111111
  dataArrayA[1] = 0xFE; //11111110
  dataArrayA[2] = 0xFC; //11111100
  dataArrayA[3] = 0xF8; //11111000
  dataArrayA[4] = 0xF0; //11110000
  dataArrayA[5] = 0xE0; //11100000
  dataArrayA[6] = 0xC0; //11000000
  dataArrayA[7] = 0x80; //10000000
  dataArrayA[8] = 0x00; //00000000
  
  dataArrayB[8] = 0xFF; //11111111
  dataArrayB[7] = 0xFE; //11111110
  dataArrayB[6] = 0xFC; //11111100
  dataArrayB[5] = 0xF8; //11111000
  dataArrayB[4] = 0xF0; //11110000
  dataArrayB[3] = 0xE0; //11100000
  dataArrayB[2] = 0xC0; //11000000
  dataArrayB[1] = 0x80; //10000000
  dataArrayB[0] = 0x00; //00000000

}
 
void loop() {
  for (int i = 0; i < 9; i++) {
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, dataArrayA[i]);  
    shiftOut(dataPin, clockPin, MSBFIRST, dataArrayB[i]);
    digitalWrite(latchPin, HIGH);
    delay(100);
  }

  for (int i = 8; i >= 0; i--) {
  digitalWrite(latchPin, LOW);
  shiftOut(dataPin, clockPin, MSBFIRST, dataArrayA[i]);
  shiftOut(dataPin, clockPin, MSBFIRST, dataArrayB[i]);  
  digitalWrite(latchPin, HIGH);
  delay(100);
  }
  
}

A common mistake with Arduino beginners is to use the board to deliver power .
Altough it is ok for small devices such as sensors, leds, etc, it can only deliver 40ma (per pins) which wont work for more needy devices such as motors.

There comes the transistor which lets a small current control a much larger one and your Arduino can therefore manage devices which are power supplied from a separate source.
There are 2 sort of transistors : the PNP and the NPN.
The PNP will turn on with a LOW signal whereas the NPN will turn with a HIGH signal.

A transistor has 3 pins (looking at the labelled / flat side), from left to right : (C) the collector, (B) the base, (E) the emitter.
The collector is connected to the negative pin of the device. (the positive pin of the device being connected to the positive power supply)
The base is connected to the digital pin of the arduino.
The emitter is connected to the ground.

See below the wiring

The the Arduino sketch is rather simple

int Pin = 9;

void setup(){
pinMode(Pin, OUTPUT);
}

void loop(){
digitalWrite(Pin, HIGH);
delay(2000);
digitalWrite(Pin, LOW);
delay(2000);

}