This PIC18F46K22 project shows how to read temperature from LM335 analog temperature sensor and print its value on a three-digit seven-segment display (7-segment display).
The compiler used in this example is mikroC PRO for PIC.
The LM335 sensor is a 3-pin analog device which can measure temperature (converts temperature to analog voltage). This sensor requires an ADC module to convert the analog voltage into digital data. The PIC18F46K22 microcontroller has one ADC module with 10-bit resolution.
ADC: Analog-to-Digital Converter.
The LM335 sensor has the following features (from datasheet):
- Directly Calibrated to the Kelvin Temperature Scale
- 1°C Initial Accuracy Available
- Operates from 400 μA to 5 mA
- Less than 1-Ω Dynamic Impedance
- Easily Calibrated
- Wide Operating Temperature Range
- 200°C Overrange
- Low Cost
The LM335 has a breakdown voltage directly proportional to absolute temperature at 10 mV/°K. For example if the LM335 output voltage is equal to 3.03 (3030 mV) this means the temperature is: 303 Kelvin = 30 °Celsius.
The PIC18F46K22 is an 8-bit microcontroller which has 10-bit ADC module. The good thing with this microcontroller is the fixed voltage references. With the fixed voltage reference we get a very good result.
Normally, negative and positive references of the ADC module are VSS and VDD, but VDD is not exactly equal to 5.00V and here we should use the fixed voltage reference as a positive reference of the ADC module.
The PIC18F46K22 has 3 fixed voltage references: 1.024V, 2.048V and 4.096V. For example if we set the fixed voltage reference to 4.096V and the ADC module is configured so that the negative and the positive references are VSS and FVR (Fixed Voltage Reference) respectively, in this case the equivalent 10-bit digital value of 4.096 is 1023 and 3.00V is 3.00 * 1023/4.096 = 749 , and so on.
In this project I used 4.096 because the LM335 output is between 2.23V (temperature = -50°C) and 3.98V (temperature = +125°C).
The value of the temperature is displayed on 3-digit common anode 7-segment display, to see how to interface the PIC microcontroller with 7-segment display (4-digit counter example) using mikroC compiler, visit the following post:
Interfacing PIC microcontroller with 7-segment display | mikroC Projects
Parts Required:
- PIC18F46K22 microcontroller —-> datasheet
- 3 or 4-digit common anode 7-segment display
- LM335 temperature sensor —-> datasheet
- 3 x PNP transistor (2SA1015, 2S9015, 2N3906 …)
- 7 x 100 ohm resistor
- 3 x 4.7k ohm resistor
- 2.2k ohm resistor
- 5V source
- Breadboard
- Jumper wires
- PIC MCU Programmer (PICkit 3, PICkit 4…)
PIC18F46K22 with LM335 sensor and 7-segment display circuit:
Project circuit schematic diagram is shown below.
All the grounded terminals are connected together.
The LM335 sensor has 3 pins (from left to right):
Pin 1 for calibration, not used in this example.
Pin 2: output.
Pin 3: GND (ground).
The output pin of the LM335 sensor is connected to pin RA0 which is analog channel 0 (AN0). I chose the 2.2k ohm because as written in the datasheet for optimum accuracy the current flows through the LM335 should be 1mA. For example if the temperature = 27°C, the output will be 3.00V and assume the supply voltage is exactly 5.00V that means the current flows through the sensor is ( 5 – 3)/2.2 = 0.90mA which is good enough. Also the value 2.2k is a standard value and well used.
The 3 transistors are of the same type (PNP).
In this project the PIC18F46K22 microcontroller runs with its internal oscillator @ 16 MHz, MCLR pin is configured as an input pin.
PIC18F46K22 with LM335 sensor and 7-segment display C code:
The following C code is for mikroC PRO for PIC compiler, it was tested with version 7.2.0.
Since the 4 digits are multiplexed, we need to refresh the display very quickly (display one digit at a time, others are off), for that I used Timer0 module (used as 8-bit timer) interrupt with 1:32 prescaler, this means Timer0 overflows every 2048 microseconds { 256/[16/(4 x 32)] = 256 x 8 = 2048 microseconds }.
The PIC18F46K22 MCU runs at 16MHz.
The ADC module is configured so that it uses its internal clock, voltage references (negative and positive) are set to VSS and FVR respectively where the FVR is set to 4.096V:
1 2 | // initialize ADC module with voltage references: VSS - FVR(4.096V) ADC_Init_Advanced(_ADC_INTERNAL_VREFL | _ADC_INTERNAL_FVRH4); |
PIC18F46K22 ADC module is used with 10-bit resolution which means the digital value of the input analog voltage varies between 0 (0V) and 1023 (4.096V). By multiplying the digital value by 0.4 we get the temperature in Kelvin (where: 0.4 = 100*4.096/1024). The temperature in degree Celsius = Kelvin – 273 (because: 1 K = 273.16 °C).
Full mikroC code:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 | /************************************************************************************** Interfacing PIC18F46K22 MCU with LM335 temperature sensor and 7-segment display. Common anode 7-segment display is used. C Code for mikroC PRO for PIC compiler. Internal oscillator used @ 16MHz Configuration words: CONFIG1H = 0x0028 CONFIG2L = 0x0018 CONFIG2H = 0x003C CONFIG3H = 0x0037 CONFIG4L = 0x0081 CONFIG5L = 0x000F CONFIG5H = 0x00C0 CONFIG6L = 0x000F CONFIG6H = 0x00E0 CONFIG7L = 0x000F CONFIG7H = 0x0040 This is a free software with NO WARRANTY. http://simple-circuit.com/ ***************************************************************************************/ int temperature; const char seg_maps[] = { 0x01, // 0 0x4F, // 1 0x12, // 2 0x06, // 3 0x4C, // 4 0x24, // 5 0x20, // 6 0x0F, // 7 0x00, // 8 0x04, // 9 0x7E // - }; void interrupt() { static unsigned short current_digit; PORTB = 0x07; // turn all digits off if( (current_digit == 1) && (temperature < 0 || temperature >= 100) ) { if(temperature < 0) // if the temperature < 0 PORTD = seg_maps[10]; // print minus sign (-) else // if the tempearture >= 100 °C PORTD = seg_maps[1]; // print 1 PORTB.F2 = 0; // turn on digit 1 (most left) } if(current_digit == 2){ PORTD = seg_maps[ (abs(temperature) / 10) % 10 ]; PORTB.F1 = 0; // turn on digit 2 } if(current_digit == 3){ PORTD = seg_maps[ abs(temperature) % 10 ]; PORTB.F0 = 0; // turn on digit 3 } current_digit = (current_digit % 3) + 1; T0IF_bit = 0; // clear Timer0 interrupt flag } // main function void main() { OSCCON = 0x70; // set internal oscillator to 16MHz ANSELB = 0; // configure all PORTB pins as digital ANSELD = 0; // configure all PORTD pins as digital TRISB = 0xF8; // configure RB0, RB1 & RB2 as outputs TRISD = 0; // configure all PORTD pins as outputs // initialize ADC module with voltage references: VSS - FVR(4.096V) ADC_Init_Advanced(_ADC_INTERNAL_VREFL | _ADC_INTERNAL_FVRH4); T0CON = 0xC4; // enable Timer0 (8-bit mode) with 1:32 prescaler T0IF_bit = 0; // clear Timer0 interrupt flag GIE_bit = 1; // enable global interrupts TMR0IE_bit = 1; // enable Timer0 interrupt delay_ms(1000); // wait a second while(1) { int Kelvin; // read analog voltage and convert it to Kelvin (0.4 = 100*4.096/1024) Kelvin = (ADC_Get_Sample(0) + 1) * 0.4; temperature = Kelvin - 273; // convert Kelvin to °Celsius delay_ms(1000); } } // end of code. |
Proteus simulation of the project should give a result near to the one shown below where Arduino UNO board is used (simulation circuit is not the same as real hardware circuit, project hardware circuit is shown above):
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