4-Channel Pt100 Thermometer
Wichit Sirichote, wichit.sirichote@gmail.com
Build a circuit that displays four channels 0-100C using Pt100 as the temperature sensor.
The RTD or the Pt100 is one of the high accuracy temperature sensor for laboratory. Using the high resolution delta-sigma converter, enables designer to use a simple voltage divider circuit for measuring the resistance of the RTD without the need of any DC amplifications. This instrument shows how to use the LTC2420, 20-bit delta-sigma converter and the LM385 reference voltage for measuring four Pt100 sensors and displays on the text LCD.
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| Figure 1: The prototype board with Pt100 temperature sensor. |
The real instrument is shown in Figure 2. The circuit is simple voltage divider. VREF and R1 are fixed values. R2 is the Pt100 sensor. Its resistance is changed with temperature. We can measure the sensor's resistance easily by detecting the voltage dropped across R2. The DC signal is equal to R2*(VREF/(R1+R2)). This signal can tie to the delta-sigma ADC directly.
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| Figure 2: The real instrument is simple voltage divider with reference voltage source. |
The complete schematic is shown in Figure 3. The MCU is 40-pin DIP package Microchip PIC18F4620 running with internal oscillator. The low power 32768Hz oscillator is for 1s time base. PORTB, PB0-PB7 are for LCD 4-bit interface. The display is 16x2 text LCD. Reference voltage, +1.2V is produced by D2, LM385. This reference voltage is tied to U2 , LTC2420 and U1A, LM358. U3, CD4051 is 8-channel voltage multiplexer. This project uses only 4-channel, X0, X1, X2 and X3. The signal from the sensor for each channel is measured by the dropped voltage across each Pt100 sensor.
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| Figure 3: Hardware schematic (click to enlarge). |
Figure 4 shows the board built by PIC project board with add on the buffer circuit, LM358.
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| Figure 4: The board is modified from PIC project board. |
The sample sensor is a cheap two wires Pt100 sensor, WZP.
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| Figure 5: Pt100 sensor for four channels. |
The boot message is shown in Figure 6.
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| Figure 6: Boot message. |
The firmware provides the mode for adjusting value for each channel, +/-5C with 0.1C step. When power up the instrument, keep pressing left key will enter the Calibrate Mode. We can select channel to be adjusted by pressing the left key. The center key is for increasing +0.1C and the right key is for decreasing -0.1C. The adjusting value will be saved to internal EEPROM. Recycle the power will exit this mode and will return to normal operation. Note that the firmware for converting the Pt100's resistance to temperature is prepared for precision sensor. Thus for high accuracy Pt100, we can set all adjusting values to 0.0C.
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| Figure 7: Calibrate mode display. |
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| Figure 8: Four sensors are immersed in the water bath. |
The listings of source code is shown in Figure 9.
/* Pt100 Project 4-channel Pt100 reading instrument Copyright (C) 2011 Wichit Sirichote 4 April 2010 Fuse settings C800 161F 8F00 0081 15 July 2011 Pt100 project equation for 1.2398V reference voltage and 1k (985Ohms) + Pt100
Y = 2592.6 * X - 297
R^2 = 0.9998
Y = A * X + B
6 August 2011 add button for calibrating the sensor +/-10.0C with 0.1C step the conversion is for Pt100 with all correction values are 00. */ #define A 2614.6 #define B -297 /* DS1302 RAM space total 31 bytes READ WRIRE 0xC1 0xC0 0xC3 0xC2 0xC5 0xC4 0xC7 0xC6 0xC9 0xC8 0xCB 0xCA .......... 0xFD 0xFC */ void show_clock(); void read_interval(); #define key1 PORTE.F2 #define key2 PORTE.F1 #define key3 PORTE.F0 #define ADC_CS1 PORTD.F5 // output bit #define ADC_SDO PORTD.F6 // input bit #define ADC_SCK PORTD.F7 // output bit #define RED LATD.F4 #define GREEN LATD.F3 #define F0 LATB.F5 #define file_number 0 // address in EEPROM //#define interval 1 // interval setting #define name 2 // station name A-Z put in front of file name, e.g. BDATA001.TXT // Gain = VREF/2^24 // Gain = 1.235V/16777216 //#define gain 0.000000073897838592529296875 // gain for LTC2420 //#define gain 1.182556152E-6 #define gain 1.172447205E-6 #define gain_PIC 3.03157*3.22265625E-3 #define offset 100E-6 // DC offset of the LTC2400 approx. 200uV int i,j,tmp; long sample; char sec, trigger; //char data[512]; //char file[32]; int ADC_buffer[8]; char file_name[12] = "xDATAxxxCSV"; // say ADATA000.TXT char num; char buffer[128]; char buffer2[32]; //short set_time[32]; unsigned long x1,x2,x3,x4,x5; unsigned long y1,y2,y3,y4,y5; unsigned long z1,z2,z3,z4,z5; unsigned long u1,u2,u3,u4,u5; unsigned long w1,w2,w3,w4,w5; // for LM35 five point moving average unsigned long v1,v2,v3,v4,v5; // for battery monitor five point moving average unsigned long s1,s2,s3,s4,s5; unsigned long t1,t2,t3,t4,t5; unsigned long r1,r2,r3,r4,r5; int secs=0; int hour; int mins; int date,month,year; short channel=0; short channel2=0; int line; // line frequency for 50 or 60 Hz noise rejection short fire=0; unsigned short character; unsigned long size; unsigned long ADC0,ADC1,ADC2,ADC3; // 4-channel analog input unsigned long ADC4,ADC5; // for LM35 (ADC4) and battery voltage (ADC5) unsigned long ADC6,ADC7; unsigned int VIN1,VIN2,VIN3,VIN4; // PIC's 10-bit ADC unsigned int period; char set_interval; char oldname; int TTL1,TTL2,TTL3; long counter1=0; int timer6=0; int timer7=0; // timeout for start recording short calibrate=0; // calibrate flag short offset1,offset2,offset3,offset4; // +127 -128 float temp_float; // EEPROM space for saving the offset in #define save_offset1 5 #define save_offset2 6 #define save_offset3 7 #define save_offset4 8 //******************* DS1302 driver ************************************** #define RST PORTC.F2 // output bit #define IO PORTD.F4 // input/output bit #define SCLK PORTD.F3 // output bit #define DIN0 PORTB.F0 //------------------------------------------------------------------------- void fire_time ()
{
secs+=4;
if (secs >= period) // test with 10 seconds
{secs = 0;
fire=1;
}
} /* read 32-bit data from LTC2400 */ unsigned long read_ADC1(void)
{
char k;
long n;
n= 0;
ADC_CS1 = 0;
for(k=0; k<32; k++)
{
n<<= 1;
ADC_SCK = 1;
n |= ADC_SDO;
ADC_SCK = 0;
}
ADC_CS1 = 1;
n&=0x1fffffff; // maskout sign bit if(n&0x10000000) return 0; // return 0 for Vin < 0 n>>=8; // get 20-bit conversion result return n; } /* 5-point moving average */
// return data that scaled with reference voltage in uV unit
unsigned long filter_ADC(void)
{
x5 = x4;
x4 = x3;
x3 = x2;
x2 = x1;
x1 = read_ADC1();
return ((x1+x2+x3+x4+x5)/5);
}
unsigned long filter_ADC1(void)
{
y5 = y4;
y4 = y3;
y3 = y2;
y2 = y1;
y1 = read_ADC1();
return ((y1+y2+y3+y4+y5)/5);
}
unsigned long filter_ADC2(void)
{
z5 = z4;
z4 = z3;
z3 = z2;
z2 = z1;
z1 = read_ADC1();
return ((z1+z2+z3+z4+z5)/5);
}
unsigned long filter_ADC3(void)
{
u5 = u4;
u4 = u3;
u3 = u2;
u2 = u1;
u1 = read_ADC1();
return ((u1+u2+u3+u4+u5)/5);
}
unsigned long filter_ADC4()
{
w5 = w4;
w4 = w3;
w3 = w2;
w2= w1;
w1 = read_ADC1();
return (w1+w2+w3+w4+w5)/5;
}
unsigned long filter_ADC5()
{
v5=v4;
v4=v3;
v3=v2;
v2=v1;
v1= read_ADC1();
return (v1+v2+v3+v4+v5)/5;
}
unsigned long filter_ADC6()
{
s5=s4;
s4=s3;
s3=s2;
s2=s1;
s1= read_ADC1();
return (s1+s2+s3+s4+s5)/5;
} unsigned long filter_ADC7()
{
t5=t4;
t4=t3;
t3=t2;
t2=t1;
t1= read_ADC1();
return (t1+t2+t3+t4+t5)/5;
} void read_dio()
{
if(channel2==0) VIN1=ADC_read(0);
if(channel2==1) VIN2=ADC_read(1);
if(channel2==2) VIN3=ADC_read(2);
if(channel2==3) VIN4=ADC_read(3);
channel2++;
if(channel2>3) channel2=0;
TTL1=0; TTL2=0; TTL3=0; if(PORTB&0x01) TTL1=1; if(PORTB&0x02) TTL2=1; if(PORTB&0x04) TTL3=1; } #define low_battery 4.0 // test with 6V seal lead acid short alarm_batt=0; void low_batt_detection()
{
static float d;
d=ADC_read(3)*gain_PIC;
if(d<low_battery) alarm_batt=1;
else alarm_batt =0;
// alarm_batt=1; // test
} void alarm_low_batt()
{
if(alarm_batt)
{
RED=1;
GREEN=0;
Delay_ms(100);
RED=0;
Delay_ms(100);
RED=1;
GREEN=0;
Delay_ms(100);
RED=0;
}
} short gate=0; unsigned int counter2=0; unsigned int temp3; void read_frequency()
{
gate^=1;
if(gate) T0CON.F7=1; // feed clock to timer0
else
{
T0CON.F7=0; // reset timer0
counter2=TMR0L; // must read low byte beforehand temp3=TMR0H; temp3<<=8; counter2|=temp3; TMR0H=0; TMR0L=0; } } unsigned wind; int average_wind()
{
r5=r4;
r4=r3;
r3=r2;
r2=r1;
r1=counter2;
return (r1+r2+r3+r4+r5)/20; // divided with 5 and 4 to get cycle per second
}
void wait_EOC()
{
ADC_SCK = 0;
ADC_CS1 = 0; // pull CS low to monitor conversion complete
while(ADC_SDO)
continue; // wait EOC=1 until EOC=0
ADC_CS1=1; // enter sleep
}
short scan_flag=0; void scan_all_channel()
{
scan_flag ^=1; if(scan_flag) Lcd_Custom_Out(2, 16," "); else Lcd_Custom_Out(2, 16,"."); for(i=0; i<8; i++)
{
if(channel==0)
{LATD &= 0xF8; // write 000 to MUX
ADC7=filter_ADC(); // read battery voltage
} if(channel==1)
{LATD |= 0x01; // write 001 to MUX
ADC0=filter_ADC1();
}
if(channel==2)
{LATD &=0xF8; // write 002 to MUX
LATD |= 0x02;
ADC1=filter_ADC2();
}
if(channel==3)
{
LATD |= 0x03; // write 003 to MUX
ADC2=filter_ADC3();
}
if(channel==4)
{
LATD &=0xF8;
LATD |= 0x04; // write 004 to MUX
ADC3=filter_ADC4();
} if(channel==5)
{
LATD |= 0x05; // write 005 to MUX
ADC4= filter_ADC5();
} if(channel==6)
{
LATD &=0xF8;
LATD |= 0x06; // write 005 to MUX
ADC5= filter_ADC6();
} if(channel==7)
{
LATD |= 0x07; // write 005 to MUX
ADC6= filter_ADC7();
} wait_EOC(); // wait end of conversion channel++; if(channel>7) channel=0; } } short key=0; short press_key1=0; short press_key2=0; short press_key3=0; // overflow every 4 seconds void wait_tick()
{
//while(PIR1.TMR1IF==0)
//asm CLRWDT ; // clear WDT
//PIR1.TMR1IF=0;
//TMR1H |= 0x80; // for one second tick
//GREEN=1; // tick blink every four seconds
//time();
//RED=0; fire_time(); //read_dio(); //low_batt_detection(); //alarm_low_batt(); //read_frequency(); //wind=average_wind(); scan_all_channel(); } void powerup_fill_data()
{
for(i=0; i<40; i++)
{
RED ^=1; // blink RED for analog scanning
//delay_ms(400);
if(channel==0)
{LATD &= 0xF8; // write 000 to MUX
ADC7=filter_ADC(); // read battery voltage
} if(channel==1)
{LATD |= 0x01; // write 001 to MUX
ADC0=filter_ADC1();
}
if(channel==2)
{LATD &=0xF8; // write 002 to MUX
LATD |= 0x02;
ADC1=filter_ADC2();
}
if(channel==3)
{
LATD |= 0x03; // write 003 to MUX
ADC2=filter_ADC3();
}
if(channel==4)
{
LATD &=0xF8;
LATD |= 0x04; // write 004 to MUX
ADC3=filter_ADC4();
} if(channel==5)
{
LATD |= 0x05; // write 005 to MUX
ADC4= filter_ADC5();
} if(channel==6)
{
LATD &=0xF8;
LATD |= 0x06; // write 005 to MUX
ADC5= filter_ADC6();
} if(channel==7)
{
LATD |= 0x07; // write 005 to MUX
ADC6= filter_ADC7();
} wait_EOC(); // wait end of conversion channel++; if(channel>7) channel=0; } } void interrupt()
{
if(INTCON.INT0IF)
{
counter1++;
INTCON.INT0IF=0; // clear INT0 flag
// RED=1;
// Delay_ms(50);
// RED=0;
}
if(PIR1.TMR1IF)
{
PIR1.TMR1IF=0; // must be cleared by software
// time(); // update clock every 4-second
} } char *text = "4-Channel Pt100"; void init_lcd()
{
Lcd_Custom_Config(&PORTB,7,6,5,4,&PORTB,2,1,3); // Initialize LCD on PORTB
Lcd_Custom_Cmd(Lcd_CURSOR_OFF); // Turn off cursor
Lcd_Custom_Out(1, 1, text);
Lcd_Custom_Out(2, 1, " Thermometer");
}
void show_temperature()
{
static float d,d1,d2,d3,d4,d5,d6,d7,d8,d9,d10,d11,fracdate;
//if(fire)
{
fire=0;
//Mmc_fat_Append();
d=ADC0*gain; //7.331371E-8; // convert to Volt VREF=1.230V
d1=ADC1*gain; //7.331371E-8;
d2=ADC2*gain; //7.331371E-8;
d3=ADC3*gain; //7.331371E-8;
d4=ADC4*gain; //7.331371E-6; // for LM35 internal sensor convert to degree C
d5=ADC5*gain; //7.331371E-5; // monitor battery voltage divided by 1000
d6=ADC6*gain; //7.331371E-5;
d7=ADC7*gain; //7.331371E-5;
d-=offset; d1-=offset; d2-=offset; d3-=offset; d4-=offset; d5-=offset; d6-=offset; d7-=offset; d= d*A + B; d1= d1*A + B; d2= d2*A + B; d3= d3*A + B; if (d>100) d=99.9; if (d1>100) d1=99.9; if (d2>100) d2=99.9; if (d3>100) d3=99.9; temp_float=1; temp_float*=offset1; d+=temp_float/10; temp_float=1; temp_float*=offset2; d1+=temp_float/10; temp_float=1; temp_float*=offset3; d2+=temp_float/10; temp_float=1; temp_float*=offset4; d3+=temp_float/10; //sprintf(buffer,"\r\n%ld,%d/%d/%d,%02d:%02d,%0.6f,%0.6f,%0.6f,%0.6f,%0.6f,%0.6f,%0.6f,%0.6f,%0.6f",sample++,date,month,year,hour,mins,fracdate,d,d1,d2,d3,d4,d5,d6,d7); sprintf(buffer,"1=%.1fC",d); Lcd_Custom_Out(1, 1,buffer ); sprintf(buffer,"3=%.1fC",d2); Lcd_Custom_Out(1,9,buffer ); sprintf(buffer,"2=%.1fC",d1); Lcd_Custom_Out(2,1,buffer ); sprintf(buffer,"4=%.1fC",d3); Lcd_Custom_Out(2, 9,buffer ); } } //short key=0; //short press_key1=0; //short press_key2=0; //short press_key3=0; void read_key()
{
key=0;
if(key1==0 && press_key1==0)
{
press_key1=1;
key=1;
}
if(key2==0 && press_key2==0)
{
press_key2=1;
key=2;
}
if((PORTE&1)==0 && press_key3==0)
{
press_key3=1;
key=3;
}
} void key_release()
{
if(key1) press_key1=0;
if(key2) press_key2=0;
if(PORTE&1)press_key3=0;
//if(key1&&(PORTE&1))timer6=0;
}
int input_channel=0; void service_key1()
{
temp_float=1; if(++input_channel>4) input_channel=1; switch (input_channel)
{
case 1: temp_float*=Eeprom_Read(save_offset1); sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
case 2: temp_float*=Eeprom_Read(save_offset2); sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
case 3: temp_float*=Eeprom_Read(save_offset3); sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
case 4: temp_float*=Eeprom_Read(save_offset4); sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
}
Lcd_Custom_Out(2, 1,buffer);
} void service_key2()
{
temp_float=1;
switch (input_channel)
{
case 1: offset1+=1;Eeprom_Write(save_offset1,offset1); temp_float*=offset1; sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
case 2: offset2+=1;Eeprom_Write(save_offset2,offset2); temp_float*=offset2; sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
case 3: offset3+=1;Eeprom_Write(save_offset3,offset3); temp_float*=offset3; sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
case 4: offset4+=1;Eeprom_Write(save_offset4,offset4); temp_float*=offset4; sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
}
Lcd_Custom_Out(2, 1,buffer);
}
void service_key3()
{
temp_float=1;
switch (input_channel)
{
case 1: offset1-=1; Eeprom_Write(save_offset1,offset1); temp_float*=offset1; sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
case 2: offset2-=1; Eeprom_Write(save_offset2,offset2); temp_float*=offset2; sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
case 3: offset3-=1; Eeprom_Write(save_offset3,offset3); temp_float*=offset3; sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
case 4: offset4-=1; Eeprom_Write(save_offset4,offset4); temp_float*=offset4; sprintf(buffer,"CH%d %1.1f ",input_channel,temp_float/10);break;
}
Lcd_Custom_Out(2, 1,buffer);
}
void scan_key()
{
delay_ms(30);
read_key();
key_release();
if(key==1) service_key1(); // - if(key==2) service_key2(); // menu if(key==3) service_key3(); // + } void enter_calibration_mode()
{
if(key1==0)
{
calibrate=1;
Lcd_Custom_Cmd(Lcd_Clear);
Lcd_Custom_Out(1, 1, "Calibrate Mode");
}
}
void read_offset_from_eeprom()
{
offset1= Eeprom_Read(save_offset1);
offset2= Eeprom_Read(save_offset2);
offset3= Eeprom_Read(save_offset3);
offset4= Eeprom_Read(save_offset4);
}
void main() {
OSCCON=0x40; // 8MHz //OSCCON|=0x80; // enter idle mode when execute sleep instruction PORTD = 0x00; TRISD = 0x40; // PORTD is output RD6 is input bit // PORTB is output ADCON1 = 0x0A; // Configure analog inputs and Vref TRISA = 0xFF; // PORTA is ADC input TRISE |= 0x07; // PORTE is digital input PORTE = 0xff; //enable internal pullup TRISB=0x00; PORTB = 0xFF; TRISC =0x93; PORTC = 0xFF; F0=1; // 50Hz rejection frequency T1CON = 0x1F; // overflow every 4 seconds TMR1H=0; TMR1L=0; T0CON = 0xAF; // frequency input pulse T0CON.F7=0; // stop timer0 TMR0H=0; TMR0L=0; INTCON = 0xC0; // enable GLOBAL interrupt pheriferal interrupt enable INTCON2 |= 0x80; // enable PORTB pull up //WPUB=0xF7; // pull up bits on PORTB PIR1.TMR1IF = 0; // clear TMR1IF init_lcd(); Spi_Init_Advanced(MASTER_OSC_DIV4, DATA_SAMPLE_MIDDLE, CLK_IDLE_LOW, LOW_2_HIGH); RED=0; GREEN=0; // begin recording // T1CON = 0x1F; // overflow every 4 seconds PIE1.TMR1IE=1; // enable timer1 interrupt
enter_calibration_mode(); // if Key1 was pressed when power up, enter calibrate mode
offset1=0; // test for offset1
read_offset_from_eeprom(); // restore offset from eeprom
while(calibrate)
{
scan_key();
}
powerup_fill_data(); // fill data to the filter array
Lcd_Custom_Cmd(Lcd_Clear);
while(1)
{
scan_all_channel();
show_temperature();
}
} |
| Figure 9: Listings of the source program. |
Download Schematic, Firmware (hex file for programming the PIC18F4620 using PICkit2 v2.40), BOM
