{"id":916,"date":"2010-11-12T16:06:26","date_gmt":"2010-11-12T21:06:26","guid":{"rendered":"http:\/\/embedded-lab.com\/blog\/?p=916"},"modified":"2020-11-24T22:49:55","modified_gmt":"2020-11-25T03:49:55","slug":"digital-temperature-meter-using-an-lm35-temperature-sensor","status":"publish","type":"post","link":"https:\/\/embedded-lab.com\/blog\/digital-temperature-meter-using-an-lm35-temperature-sensor\/","title":{"rendered":"A Digital temperature meter using an LM35 temperature sensor"},"content":{"rendered":"

Introduction<\/strong><\/p>\n

A digital thermometer is a good choice of project for beginners who just stepped in to the world of microcontrollers because it provides an opportunity to learn using sensors to measure the real world signals that are analog in nature. This article describes a similar project based on a PIC16F688 microcontroller and an LM35 temperature sensor. LM35 is an analog sensor that converts the surrounding temperature to a proportional analog voltage. The output from the sensor is connected to one of the ADC channel inputs of the PIC16F688 microcontroller to derive the equivalent temperature value in digital format. The computed temperature is displayed in a 16×2 character LCD, in both \u00b0C and \u00b0F scales.<\/p>\n

Theory<\/strong><\/p>\n

The LM35 series of temperature sensors are produced by National Semiconductor Corporation and are rated to operate over a -55\u00a0\u00b0C to 150\u00b0C temperature range. These sensors do not require any external calibration and the\u00a0 output voltage is proportional to the temperature. The scale factor for temperature to voltage conversion is 10 mV per\u00a0\u00b0C. The LM35 series sensors come in different packages. The one I used is in a hermatic TO-46 transistor package where the metal case is connected to the negative pin (Gnd).<\/p>\n

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\nThe measurement of negative temperatures (below 0\u00b0C) requires a negative voltage source. However, this project does not use any negative voltage source, and therefore will demonstrate the use of sensor for measuring temperatures above 0\u00b0C (up to 100\u00b0C).<\/p>\n

The output voltage from the sensor is converted to a 10-bit digital number using the internal ADC of the PIC16F688. Since the voltage to be measured by the ADC ranges from 0 to 1.0V (that corresponds to maximum temperature range, 100 \u00b0C), the ADC requires a lower reference voltage (instead of the supply voltage Vdd = 5V) for A\/D conversion in order to get better accuracy. The lower reference voltage can be provided using a Zener diode,\u00a0 a resistor network, or sometime just simple diodes. You can derive an approximate 1.2V reference voltage by connecting two diodes and a resistor in series across the supply voltage, as shown below. As a demonstration, I am going to use this circuit in this project. I measured the output voltage across the two diodes as 1.196 V. The resistor R I used is of 3.6K, but you can use 1K too. The important thing is to measure the voltage across the two diodes as accurate as possible.<\/p>\n

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We need do some math for A\/D conversion. Our Vref is 1.196 V, and the ADC is 10-bit. So, any input voltage from 0-1.196 will be mapped to a digital number between 0-1023. The resolution of ADC is 1.196\/1024 = 0.001168 V\/Count. Therefore, the digital output corresponding to any input voltage Vin = Vin\/0.001168. Now, lets see how to get the temperature back from this whole process of converting sensor’s output to 10-bit digital number.<\/p>\n

Assume, the surrounding temperature is 26.4 \u00b0C. The sensor output will be 264 mV (0.264 V). The output of ADC will be 0.264\/0.001168 = 226. If we reverse this process, we have 226 from ADC and we can go back and find the temperature by using the sensor scale factor (10 mV\/\u00b0C),<\/p>\n

temperature = 226 * 0.001168 (V\/Count) \/ 0.01 (V\/\u00b0C) = 26.4 \u00b0C<\/em><\/p>\n

If you want to avoid floating point math in your program, just use,<\/p>\n

temperature = 226 * 1168\u00a0 = 263968<\/em><\/p>\n

While displaying this, you need to put a decimal at the fourth place from the left. So the calculated temperature is 26.3968\u00b0C, which is pretty close to the actual one. The difference is caused by quantization and rounding errors. In this project, we will display temperature accurate to one decimal place, i.e., we will divide the above number by 1000 to get 263. So the temperature will be displayed as 26.3 \u00b0C.<\/p>\n

Once you have derived the temperature back in \u00b0C, you can convert it to \u00b0F using a simple equation,<\/p>\n

temperature in \u00b0F = 9 x temperature in \u00b0C \/5 + 32<\/em><\/p>\n

In this case, the number you got for \u00b0C is scaled by 10 (263 for 26.3), you should use<\/p>\n

temperature in \u00b0F = 9 x temperature in \u00b0C \/5 + 320<\/em><\/p>\n

Since, the number for \u00b0C may not be exactly divisible by 5 (such as 263 is not), you can further eliminate the floating point by scaling it one more time by 10. So the new equation will be,<\/p>\n

temperature in \u00b0F = 9 x temperature in \u00b0C x 10 \/5 + 3200<\/em><\/p>\n

or, temperature in \u00b0F = 18 x temperature in \u00b0C\u00a0 + 3200 = 18 x 263+3200 = 7934<\/em><\/p>\n

79.34 \u00b0F is equivalent to 26.3 \u00b0C. In this project, it will be displayed as 79.3 \u00b0F.<\/p>\n

Circuit Diagram<\/strong><\/p>\n

An external reference voltage to the internal ADC of PIC16F688 can be provided through RA1 I\/O pin. The output from the LM35 sensor is read through RA2\/AN2 ADC channel. The temperature is displayed on a 16×2 character LCD that is operating in the 4-bit mode. A 5K potentiometer is used to adjust the contrast level on the display. The detail circuit diagram is given below. Note that the PIC16F688 uses its internal clock at 4.0 MHz.<\/p>\n

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Circuit setup on the breadboard<\/p>\n

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A closer look at the LM35DH sensor and the reference voltage circuit.<\/p>\n

Software<\/strong><\/p>\n

The firmware for this project is developed with MikroC Pro for PIC compiler. The link to download the compiled HEX code is provided at the bottom of this section. The configuration bits for PIC16F688 are<\/div>\n

Oscillator -> Internal RC No Clock
\nWatchdog Timer -> Off
\nPower Up Timer -> On
\nMaster Clear Enable -> Enabled
\nCode Protect -> Off
\nData EE Read Protect -> Off
\nBrown Out Detect -> BOD Enabled, SBOREN Disabled
\nInternal External Switch Over Mode -> Enabled
\nMonitor Clock Fail-Safe -> Enabled<\/em><\/p>\n

You can set these fuses in the Edit Project window (Project-> Edit Project)<\/p>\n

\/* Digital Thermometer using PIC16F688 and LM35\u00a0<\/span><\/pre>\n
Internal Oscillator @ 4MHz, MCLR Enabled, PWRT Enabled, WDT OFF Copyright @ Rajendra Bhatt November 8, 2010 *\/<\/span>\n\n\/\/ LCD module connections<\/span>\nsbit LCD_RS at RC4_bit;<\/span>\nsbit LCD_EN at RC5_bit;<\/span>\nsbit LCD_D4 at RC0_bit;<\/span>\nsbit LCD_D5 at RC1_bit;<\/span>\nsbit LCD_D6 at RC2_bit;<\/span>\nsbit LCD_D7 at RC3_bit;<\/span>\nsbit LCD_RS_Direction at TRISC4_bit;<\/span>\nsbit LCD_EN_Direction at TRISC5_bit;<\/span>\nsbit LCD_D4_Direction at TRISC0_bit;<\/span>\nsbit LCD_D5_Direction at TRISC1_bit;<\/span>\nsbit LCD_D6_Direction at TRISC2_bit;<\/span>\nsbit LCD_D7_Direction at TRISC3_bit;<\/span>\n\/\/ End LCD module connections<\/span>\n\/\/ Define Messages<\/span>\nchar message0[] = \"LCD Initialized\";<\/span>\nchar message1[] = \"Room Temperature\";<\/span>\n\/\/ String array to store temperature value to display<\/span>\nchar *tempC = \"000.0\";<\/span>\nchar *tempF = \"000.0\";<\/span>\n\/\/ Variables to store temperature values<\/span>\nunsigned int tempinF, tempinC;<\/span>\nunsigned long temp_value;<\/span>\nvoid Display_Temperature() {<\/span>\n \/\/ convert Temp to characters<\/span>\n if (tempinC\/10000)<\/span>\n \/\/ 48 is the decimal character code value for displaying 0 on LCD<\/span>\n tempC[0] = tempinC\/10000 + 48;<\/span>\n else tempC[0] = ' ';<\/span>\n tempC[1] = (tempinC\/1000)%10 + 48; \/\/ Extract tens digit<\/span>\n tempC[2] = (tempinC\/100)%10 + 48; \/\/ Extract ones digit<\/span>\n \/\/ convert temp_fraction to characters<\/span>\n tempC[4] = (tempinC\/10)%10 + 48; \/\/ Extract tens digit<\/span>\n \/\/ print temperature on LCD<\/span>\n Lcd_Out(2, 1, tempC);<\/span>\n if (tempinF\/10000)<\/span>\n tempF[0] = tempinF\/10000 + 48;<\/span>\n else tempF[0] = ' ';<\/span>\n tempF[1] = (tempinF\/1000)%10 + 48; \/\/ Extract tens digit<\/span>\n tempF[2] = (tempinF\/100)%10 + 48;<\/span>\n tempF[4] = (tempinF\/10)%10 + 48;<\/span>\n \/\/ print temperature on LCD<\/span>\n Lcd_Out(2, 10, tempF);<\/span>\n}<\/span>\nvoid main() {<\/span>\n ANSEL = 0b00000100; \/\/ RA2\/AN2 is analog input<\/span>\n ADCON0 = 0b01001000; \/\/ Connect AN2 to S\/H, select Vref=1.19V<\/span>\n CMCON0 = 0x07 ; \/\/ Disbale comparators<\/span>\n TRISC = 0b00000000; \/\/ PORTC All Outputs<\/span>\n TRISA = 0b00001110; \/\/ PORTA All Outputs, Except RA3 and RA2<\/span>\n Lcd_Init(); \/\/ Initialize LCD<\/span>\n Lcd_Cmd(_LCD_CLEAR); \/\/ CLEAR display<\/span>\n Lcd_Cmd(_LCD_CURSOR_OFF); \/\/ Cursor off<\/span>\n Lcd_Out(1,1,message0);<\/span>\n Delay_ms(1000);<\/span>\n Lcd_Out(1,1,message1); \/\/ Write message1 in 1st row<\/span>\n \/\/ Print degree character<\/span>\n Lcd_Chr(2,6,223);<\/span>\n Lcd_Chr(2,15,223);<\/span>\n \/\/ Different LCD displays have different char code for degree symbol<\/span>\n \/\/ if you see greek alpha letter try typing 178 instead of 223<\/span>\n Lcd_Chr(2,7,'C');<\/span>\n Lcd_Chr(2,16,'F');<\/span>\n do {<\/span>\n temp_value = ADC_Read(2);<\/span>\n temp_value = temp_value*1168;<\/span>\n tempinC = temp_value\/1000;<\/span>\n tempinC = tempinC*10;<\/span>\n tempinF = 9*tempinC\/5 + 3200;<\/span>\n Display_Temperature();<\/span>\n Delay_ms(1000); \/\/ Temperature sampling at 1 sec interval<\/span>\n } while(1);<\/span>\n}<\/span><\/pre>\n
Download HEX file<\/a><\/p>\n
Output<\/strong><\/p>\n
I took some pictures of the completed project displaying temperatures in both the scales.<\/p>\n
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Temperature goes up if you touch the sensor with your fingers<\/p>\n
Accuracy of the measurement<\/strong><\/em><\/p>\n
The accuracy of the temperature measurement highly depends upon the stability of the reference voltage. If the reference voltage drifts from the value that we considered in our calculation, the measured temperature value could be significantly off from the actual value. Using a simple diode-resistor network for deriving a reference voltage may not be a very good idea, but the purpose of this project was to demonstrate the technique, not to come up with a commercial digital thermometer product. You can also try a Zener diode or a potentiometer to derive the reference voltage. Besides, the quantization error introduced by the 10-bit ADC, rounding numbers while doing the math, and the accuracy of the sensor itself within the desired range of temperature also affect the measurement output. Read the manufacturer’s datasheet<\/a> for more details on the performance of the LM35 series sensor.<\/p>\n
Update<\/span><\/strong><\/p>\n
This design has a little flaw. It uses a voltage drop across two diodes (? 1.2 V) as a reference voltage (Vref) for A\/D conversion. However, the datasheet of PIC16F688 suggests to use Vref greater than 2.2 V to ensure 1-LSB accuracy in the A\/D conversion. Therefore, I have re-written this project but this time using a MCP1525 device for creating a precise 2.5 V reference voltage. The new design is more precise and accurate in taking temperature measurements. Read “Revised version of LM35 based digital thermometer<\/span><\/a><\/span>“<\/strong>.<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"
Introduction A digital thermometer is a good choice of project for beginners who just stepped in to the world of microcontrollers because it provides an opportunity to learn using sensors to measure the real world signals that are analog in nature. This article describes a similar project based on a PIC16F688 microcontroller and an LM35 temperature sensor. LM35 is an analog sensor that converts the surrounding temperature to a proportional analog voltage. The output from the sensor is connected to one of the ADC channel inputs of the PIC16F688 microcontroller to derive the equivalent temperature value in digital format. The<\/p>\n","protected":false},"author":2,"featured_media":918,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[245,4,38],"tags":[44,5,92,91,46],"class_list":["post-916","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-embedded-lab-projects","category-pic-projects","category-pic-tutorials","tag-adc","tag-digital-temperature-meter","tag-lm35","tag-temperature-meter","tag-thermometer"],"_links":{"self":[{"href":"https:\/\/embedded-lab.com\/blog\/wp-json\/wp\/v2\/posts\/916","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/embedded-lab.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/embedded-lab.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/embedded-lab.com\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/embedded-lab.com\/blog\/wp-json\/wp\/v2\/comments?post=916"}],"version-history":[{"count":48,"href":"https:\/\/embedded-lab.com\/blog\/wp-json\/wp\/v2\/posts\/916\/revisions"}],"predecessor-version":[{"id":14750,"href":"https:\/\/embedded-lab.com\/blog\/wp-json\/wp\/v2\/posts\/916\/revisions\/14750"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/embedded-lab.com\/blog\/wp-json\/wp\/v2\/media\/918"}],"wp:attachment":[{"href":"https:\/\/embedded-lab.com\/blog\/wp-json\/wp\/v2\/media?parent=916"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/embedded-lab.com\/blog\/wp-json\/wp\/v2\/categories?post=916"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/embedded-lab.com\/blog\/wp-json\/wp\/v2\/tags?post=916"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}