Building the display board

The display consists of 4 seven segment SA56

displays from kingbright, they are available in various

LED colours in this case we are using Red.

This enable’s the display to be viewed clearly through

the red filter on the Evatron enclosure.

                                                               

                                                                The display is a multiplexed common anode display, the                                                           seven segment displays are mounted  with the first 2 

                                                                digits the correct way up and the second two digits 

                                                                reversed so we can form the degree C character,

Building the base board

The base board is designed so it can be used either as an LED clock or thermometer board, in this project as a thermometer board.

In the schematic below the PIC16F628A multiplexes the display driving the Anodes via the PNP transistors and displays the seven segment digit code on port B.

Power is supplied via the 7805, and the temperature is read from the LM75 over I2C, SDA is multiplexed to be used alternately to drive the display and read the temperature from the temperature sensor.

Bill Of Materials

=================

QTY      PART-REFS           VALUE              

---          ---------                    -----              

Resistors

---------

8            R1-R8                    47R                

4            R9-R12                  1K2                

6            R13,R15-R19         10K                

1            R14                        1K                 

3            R20-R22             `` 820R               

Capacitors

----------

2            C1,C2                   47p                

2            C3,C4                   470u               

2            C5,C6                   100n               

Integrated Circuits

-------------------

1            U1                          PIC16F628A         

1            U2                          7805               

1            U3                          LM75              

Transistors

-----------

4            Q1-Q4               BC559AP            

1            Q5                     BC108BP            

Diodes

------

2            D1,D2               1N4004             

Miscellaneous

-------------

1            J1                      CONN-SIL4          

1            J2                      CONN-SIL8          

2            J3,J4                 TBLOCK-I2          

1            J5                      CONN-SIL2          

1            RL1                    PCJ-105D3MH        

3            SW1-SW3                                

1            X1                      CRYSTAL

Software      

The software is wirtten in Hitech C and can be downloaded here or seen below

#include <htc.h>

#include <pic16f628a.h>

#include <stdint.h>

#define SCL     TRISA4 // I2C bus

#define SDA     TRISA0 //

#define SCL_IN  PORTA,RA4    //

#define SDA_IN  PORTA,RA0    //

#define TMR0_2 (TMR0 & 1<<2)

#define LM75AD 0b10010000

__CONFIG (0x3D11);

   unsigned char mpx_cnt;

   unsigned char tempmsb;

   unsigned char templsb;

   unsigned char tempBCD1;

   unsigned char tempBCD2;

  

const int pat7seg2[10]=

{

0b10000001,//0

0b11011101,//1

0b01000011,//2

0b01010001,//3

0b00011101,//4

0b10110000,//5

0b00100001,//6

0b11010101,//7

0b00000001,//8

0b00010001,//9

};

const int pat7seg3[10]=

{

0b00000001,//0

0b00101111,//1

0b01000010,//2

0b00100010,//3

0b00101100,//4

0b00110000,//5

0b00010000,//6

0b00101011,//7

0b00000000,//8

0b00100000,//9

};

const int pat7seg4[10]=

{

0b10000001,//0

0b10101111,//1

0b11000010,//2

0b10100010,//3

0b10101100,//4

0b10110000,//5

0b10010000,//6

0b10101011,//7

0b10000000,//8

0b10100000,//9

};

void Wait()

{

   unsigned char i;

   for(i=0;i<100;i++)

      _delay(10000);

}

void i2c_dly(void)

{

_delay(20);

}

void i2c_start(void)

{

  SDA = 1;             // i2c start bit sequence

  i2c_dly();

  SCL = 1;

  i2c_dly();

  SDA = 0;

  i2c_dly();

  SCL = 0;

  i2c_dly();

}

void i2c_stop(void)

{

  SDA = 0;             // i2c stop bit sequence

  i2c_dly();

  SCL = 1;

  i2c_dly();

  SDA = 1;

  i2c_dly();

}

unsigned char i2c_rx(char ack)

{

unsigned char x, d=0;

  SDA = 1;

  for(x=0; x<8; x++) {

    d <<= 1;

    do {

      SCL = 1;

    }

    while(SCL_IN==0);    // wait for any SCL clock stretching

    i2c_dly();

    if(SDA_IN) d |= 1;

    SCL = 0;

  }

  if(ack) SDA = 0;

  else SDA = 1;

  SCL = 1;

  i2c_dly();             // send (N)ACK bit

  SCL = 0;

  SDA = 1;

  return (d) ;

}

bit i2c_tx(unsigned char d)

{

char x;

static bit b;

  for(x=8; x; x--) {

    if(d&0x80) SDA = 1;

    else SDA = 0;

    SCL = 1;

i2c_dly();

    d <<= 1;

    SCL = 0;

i2c_dly();

  }

  SDA = 1;

  i2c_dly();

  SCL = 1;

  i2c_dly();

 

  b = SDA_IN;          // possible ACK bit

  SCL = 0;

  return b;

}

void ReadI2C()

{

SDA = SCL = 1;

SCL_IN = SDA_IN = 0;

i2c_start();              // send start sequence

i2c_tx(0xE0);             // SRF08 I2C address with R/W bit clear

i2c_tx(0x00);             // SRF08 command register address

i2c_tx(0x51);             // command to start ranging in cm

i2c_stop();               // send stop sequence

}

void main()

{

   OPTION_REG = 0b11010111;

   //Initialize PORTD

   //PD0 as Output

CMCON=0B00000111;

TRISA=0B11111111;

PORTB=0b11111111;

PORTA=0b11100000;

   tempmsb=0;

   templsb=0;

 

   while(1)

   {

TRISA=0B11111111;

PORTB=0b11111111;

PORTA=0b11100000;

i2c_dly;

//Set Pointer to the Temperature Register

i2c_start();

i2c_tx(0b10010000);

i2c_tx(0b00000000);

//Eead temperature sensor

i2c_start();

i2c_tx(0b10010001);

tempmsb=i2c_rx(1);

templsb=i2c_rx(0);

i2c_stop();

tempmsb=tempmsb-6;

tempBCD1=tempmsb/10;

tempBCD2=tempmsb % 10;

TRISA=0B11111111;

   TRISB=0B11111111;

for(mpx_cnt=0;mpx_cnt < 1000000/2048/4; mpx_cnt++)

{

TRISA=0B11111111;

   TRISB=0B00000000;

while(!TMR0_2)

;

PORTB=pat7seg4[tempBCD1];

TRISA=0B11110111;

PORTA=0B11110111;

while(TMR0_2)

;

while(!TMR0_2)

;

TRISA=0B11111111;

PORTB=pat7seg3[tempBCD2];

TRISA=0B11111011;

PORTA=0B11111011;

while(TMR0_2)

;

while(!TMR0_2)

;

TRISA=0B11111111;

if((templsb & 0B10000000))

{

PORTB=pat7seg2[5];

}

else

{

PORTB=pat7seg2[0] ;

}

TRISA=0B11111101;

PORTA=0B11111101;

while(TMR0_2)

;

while(!TMR0_2)

;

TRISA=0B11111111;

PORTB=0B00100011;

TRISA=0B11111110;

PORTA=0B11111110;

while(TMR0_2)

;

}

  }

}