This topic shows how to make a simple temperature measurement station using Arduino UNO board and LM35 analog temperature sensor where temperature value is displayed on 3-digit 7-segment display.
The LM35 temperature sensor is a three pin device (VCC, OUT and GND) with an output voltage linearly related to Centigrade temperature. Since the LM35 output varies with dependent to the temperature, we need an ADC (Analog-to-Digital Converter) module to measure this voltage.
To see how to interface Arduino with 7-segment display visit the following post:
Interfacing Arduino with 7-segment display | 4-Digit counter example
Also for more details about the LM35 sensor and how to interface it with Arduino, see this project:
Arduino and LM35 temperature sensor interfacing
Hardware Required:
- Arduino UNO board
- 3-Digit (or 4-digit) common anode 7-segment display
- LM35 temperature sensor —-> datasheet
- 3 x PNP transistor (2SA1015, 2S9015, 2N3906 …)
- 8 x 100 ohm resistor
- 3 x 4.7k ohm resistor
- Breadboard
- Jumper wires
Interfacing Arduino with LM35 sensor and 7-segment display circuit:
The image below shows project circuit schematic diagram.
The LM35 sensor has 3 pins (from left to right):
Pin 1 is power supply pin, connected to Arduino 5V pin
Pin 2: output pin, connected to Arduino analog pin 0 (A0)
Pin 3: GND (ground), connected to Arduino GND pin.
The 3 transistors are of the same type (PNP).
Interfacing Arduino with LM35 sensor and 7-segment display code:
The Arduino code below doesn’t use any library for the 7-segment display.
Reading voltage quantity using the ADC gives a number between 0 and 1023 (10-bit resolution), 0V is represented by 0 and 1.1V is represented by 1023 (ADC positive reference is 1.1V) . Converting back the ADC digital value is easy, we can use the following equation:
Voltage (in Volts) = ADC reading * 1.1 / 1023
Multiplying the previous result by 100 (LM35 scale factor is 10mV/°C = 0.01V/°C) gives the actual temperature:
Temperature( °C) = ADC reading * 0.1075 , or
Temperature( °C) = ADC reading / 9.3
where: 0.1075 = 100 * 1.1 / 1023 and 9.3 = 1 / 0.1075
To use the internal 1.1V reference I used the command: analogReference(INTERNAL);
In this example I used one number after the decimal point, multiplying the temperature by 10 will remove the decimal point, so the final result is:
1 | temp = 10 * analogRead(LM35_pin) / 9.3; |
The decimal point is displayed on the screen before printing the last digit (most right) which means it displayed with digit 2.
Full Arduino 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 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 | /* * Interfacing Arduino with LM35 temperature sensor and 7-segment display. * Common anode 7-segment display is used. * This is a free software with NO WARRANTY. * http://simple-circuit.com/ */ // define LM35 data pin connection #define LM35_pin A0 // segment pins definitions #define SegA 12 #define SegB 11 #define SegC 10 #define SegD 9 #define SegE 8 #define SegF 7 #define SegG 6 #define SegDP 5 // common pins of the three digits definitions #define Dig1 4 #define Dig2 3 #define Dig3 2 // variable declarations byte current_digit; int temp; void setup() { pinMode(SegA, OUTPUT); pinMode(SegB, OUTPUT); pinMode(SegC, OUTPUT); pinMode(SegD, OUTPUT); pinMode(SegE, OUTPUT); pinMode(SegF, OUTPUT); pinMode(SegG, OUTPUT); pinMode(SegDP, OUTPUT); pinMode(Dig1, OUTPUT); pinMode(Dig2, OUTPUT); pinMode(Dig3, OUTPUT); disp_off(); // turn off the display // Timer1 module overflow interrupt configuration TCCR1A = 0; TCCR1B = 1; // enable Timer1 with prescaler = 1 ( 16 ticks each 1 µs) TCNT1 = 0; // set Timer1 preload value to 0 (reset) TIMSK1 = 1; // enable Timer1 overflow interrupt analogReference(INTERNAL); // set positive reference voltage to 1.1V } ISR(TIMER1_OVF_vect) // Timer1 interrupt service routine (ISR) { disp_off(); // turn off the display switch (current_digit) { case 1: disp((temp / 100) % 10); digitalWrite(Dig1, LOW); // turn on digit 1 break; case 2: disp( (temp / 10) % 10); // prepare to display digit 2 digitalWrite(SegDP, LOW); // print decimal point ( . ) digitalWrite(Dig2, LOW); // turn on digit 2 break; case 3: disp(temp % 10); // prepare to display digit 3 digitalWrite(Dig3, LOW); // turn on digit 3 } current_digit = (current_digit % 3) + 1; } // main loop void loop() { temp = 10 * analogRead(LM35_pin) / 9.3; // read analog voltage and convert it to °C ( 9.3 = 1023/(1.1*100) ) delay(1000); // wait 1 second } void disp(byte number) { switch (number) { case 0: // print 0 digitalWrite(SegA, LOW); digitalWrite(SegB, LOW); digitalWrite(SegC, LOW); digitalWrite(SegD, LOW); digitalWrite(SegE, LOW); digitalWrite(SegF, LOW); digitalWrite(SegG, HIGH); digitalWrite(SegDP, HIGH); break; case 1: // print 1 digitalWrite(SegA, HIGH); digitalWrite(SegB, LOW); digitalWrite(SegC, LOW); digitalWrite(SegD, HIGH); digitalWrite(SegE, HIGH); digitalWrite(SegF, HIGH); digitalWrite(SegG, HIGH); digitalWrite(SegDP, HIGH); break; case 2: // print 2 digitalWrite(SegA, LOW); digitalWrite(SegB, LOW); digitalWrite(SegC, HIGH); digitalWrite(SegD, LOW); digitalWrite(SegE, LOW); digitalWrite(SegF, HIGH); digitalWrite(SegG, LOW); digitalWrite(SegDP, HIGH); break; case 3: // print 3 digitalWrite(SegA, LOW); digitalWrite(SegB, LOW); digitalWrite(SegC, LOW); digitalWrite(SegD, LOW); digitalWrite(SegE, HIGH); digitalWrite(SegF, HIGH); digitalWrite(SegG, LOW); digitalWrite(SegDP, HIGH); break; case 4: // print 4 digitalWrite(SegA, HIGH); digitalWrite(SegB, LOW); digitalWrite(SegC, LOW); digitalWrite(SegD, HIGH); digitalWrite(SegE, HIGH); digitalWrite(SegF, LOW); digitalWrite(SegG, LOW); digitalWrite(SegDP, HIGH); break; case 5: // print 5 digitalWrite(SegA, LOW); digitalWrite(SegB, HIGH); digitalWrite(SegC, LOW); digitalWrite(SegD, LOW); digitalWrite(SegE, HIGH); digitalWrite(SegF, LOW); digitalWrite(SegG, LOW); digitalWrite(SegDP, HIGH); break; case 6: // print 6 digitalWrite(SegA, LOW); digitalWrite(SegB, HIGH); digitalWrite(SegC, LOW); digitalWrite(SegD, LOW); digitalWrite(SegE, LOW); digitalWrite(SegF, LOW); digitalWrite(SegG, LOW); digitalWrite(SegDP, HIGH); break; case 7: // print 7 digitalWrite(SegA, LOW); digitalWrite(SegB, LOW); digitalWrite(SegC, LOW); digitalWrite(SegD, HIGH); digitalWrite(SegE, HIGH); digitalWrite(SegF, HIGH); digitalWrite(SegG, HIGH); digitalWrite(SegDP, HIGH); break; case 8: // print 8 digitalWrite(SegA, LOW); digitalWrite(SegB, LOW); digitalWrite(SegC, LOW); digitalWrite(SegD, LOW); digitalWrite(SegE, LOW); digitalWrite(SegF, LOW); digitalWrite(SegG, LOW); digitalWrite(SegDP, HIGH); break; case 9: // print 9 digitalWrite(SegA, LOW); digitalWrite(SegB, LOW); digitalWrite(SegC, LOW); digitalWrite(SegD, LOW); digitalWrite(SegE, HIGH); digitalWrite(SegF, LOW); digitalWrite(SegG, LOW); digitalWrite(SegDP, HIGH); } } void disp_off() { digitalWrite(Dig1, HIGH); digitalWrite(Dig2, HIGH); digitalWrite(Dig3, HIGH); } // end of code. |
The following video shows Proteus simulation of this project (simulation circuit is not the same as real hardware circuit, example circuit diagram is shown above):
Proteus simulation file download:
Arduino + LM35 sensor + 7-segment display
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Please sir, I don’t understand what this line does. Could you please help with some explanations? Line 70 current_digit = (current_digit % 3) + 1;
That’s because we have 3 digits and each time only one digit is turned ON, others are OFF. The MCU starts with the first digit (when current_digit equals to 1) and then 2nd digit (current_digit = 2) and then it turns ON the 3rd digit (current_digit = 3), after it rolls over to the first digit.
The % symbol is just division operation reminder.
Why were 3 different transistors used?
Hola tengo problema con el circuito lo montado pero en el display la temperatura no marca bien , esta siempre cambiando mide cosas raras no se estabiliza , porque puede ser