I'm trying to build a very simple thermometer using shift registers, 7-segment LEDs, and a LM35. The idea of the project is to learn more about shift registers and how to use them. What I've done so far is this mess:

My mess

It's very basic, 3 75HC595N powering 3 7-segment LEDs and a LM35. I'm using this code to control the LEDs and get the information from the sensor:

 *  Digital Thermometer
 *  A cool and simple digital thermometer using:
 *    - 3x 74HC595 shift registers
 *    - 3x Common anode 7-segment displays
 *    - 1x LM35 analog thermometer
 *    - 22x 1k resistors
 *  @author Nathan Campos <nathanpc@dreamintech.net>
 *  @version 1.0

 *  LED Array Bits and pins.
 *           D
 *        ABCPDEFG
 *        11101111

const int temp = A0;
const int data =  2;
const int latch = 3;
const int clock = 4;
const int clr =  -1;

 *  Arduino setup.
void setup() {
  pinMode(data, OUTPUT);
  pinMode(latch, OUTPUT);
  pinMode(clock, OUTPUT);

  if (clr != -1) {
    pinMode(clr, OUTPUT);

 *  Clear the Shift Register.
void shift_clear() {
  if (clr != -1) {
    // The clear pin is set.
    digitalWrite(latch, LOW);

    digitalWrite(clr, LOW);
    digitalWrite(clr, HIGH);

    digitalWrite(latch, HIGH);
  } else {
    // shift all the zeros.

 *  Shift some data into the Shift Register.
 *  @param shift1 Bytes to shift into the first shift register.
 *  @param shift2 Bytes to shift into the second shift register.
 *  @param shift3 Bytes to shift into the third shift register.
void shift_write(byte shift1, byte shift2, byte shift3) {
  digitalWrite(latch, LOW);  // Pull LOW to start sending data.
  shiftOut(data, clock, LSBFIRST, shift3);
  shiftOut(data, clock, LSBFIRST, shift2);
  shiftOut(data, clock, LSBFIRST, shift1);
  digitalWrite(latch, HIGH); // Pull HIGH to stop sending data.

 *  Get the bits to light a digit in the 7-segment display.
 *  @param digit A numeric character.
 *  @param light_decimal Want to light the decimal character?
 *  @return The shift register byte to light a digit.
byte light_digit(unsigned int digit, bool light_decimal) {
  byte light = B00000000;

  // Build the binary for the shift register.
  switch (digit) {
    case 0:
      light = B11101110;
    case 1:
      light = B01100000;
    case 2:
      light = B11001101;
    case 3:
      light = B11101001;
    case 4:
      light = B01100011;
    case 5:
      light = B10101011;
    case 6:
      light = B10101111;
    case 7:
      light = B11100000;
    case 8:
      light = B11101111;
    case 9:
      light = B11100011;
      // Looks like we should return a "E" of Error.
      light = B10001111;

  if (light_decimal) {
    // Looks like we need some decimal action going.
    light = light | B00010000;

  return light;

 *  Get a digit at a specified position from a integer.
 *  @param number The number to be divided.
 *  @param digit Which digit to be get.
 *  @return The digit.
unsigned int get_digit(unsigned int number, unsigned int digit) {
  static int powers[] = { 1, 10, 100 };
  if (number <= 999) {
    return (number / powers[digit]) % 10;  // The digit.
  } else {
    return 10;  // Just so we'll get a Error digit.

 *  Light a 3 digits number.
 *  @param number The number to be shown.
void light_number(unsigned int number) {
  // Get each number.
  byte first = light_digit(get_digit(number, 2), false);
  byte second = light_digit(get_digit(number, 1), true);
  byte third = light_digit(get_digit(number, 0), false);

  // Shift the registers.
  shift_write(first, second, third);

 *  Check for the temperature and return a integer for the LEDs.
 *  @return Integer for the LEDs.
unsigned int check_temperature() {
  int reading = analogRead(temp);
  float voltage = reading * (5.0 / 1023);
  float celcius = (voltage - 0.5) * 100;


  //Serial.println(" C");
  return celcius * 10;

 *  Arduino main loop.
void loop() {

The problem is that the readings from the Arduino serial monitor are "incorrect" since the temperature of the room is ~23C:

0.85 - 0.74 - 0.73 - 0.78 - 0.80 - 0.72 - 0.71 - 0.81 - 0.61 - 0.62 - 0.71 - 0.81 - 0.79 - 0.69 - 0.74 - 0.78 - 0.80 - 0.77 - 0.74 - 0.78 - 0.80 - 0.71 - 0.67 - 0.71 - 0.67 - 0.71 - 0.67

When I measure the voltage between the Ground pin and the Output pin I get the correct output for the current temperature (0.23):

Ground pin to Output pin voltage

If I measured the voltage between the Ground and +Vs pin I get 3.84V:

Voltage between the Ground and Input pin

Since this is a learning project I would love to know why this is happening and how to correct it. I think the problem might be related to the power that the LED array is using, if needed I can use a external 5V power supply for them and let the Arduino powering only with the sensor.

  • \$\begingroup\$ We don't need 10 Megapixel images here, and they take a long time to load. If you scale your images back to 500 px wide, they'll still fill the screen width, and people won't have to wait to read your question. \$\endgroup\$ – The Photon Mar 9 '13 at 15:15
  • \$\begingroup\$ @ThePhoton Sorry, I've reduced their size to 630px wide. \$\endgroup\$ – Nathan Campos Mar 9 '13 at 15:18
  • 2
    \$\begingroup\$ Haha, I wrote my comment, then noticed the big Canon in your Gravatar. \$\endgroup\$ – The Photon Mar 9 '13 at 15:20
  • \$\begingroup\$ @ThePhoton I thought about doing some hi-res macro shots, but then I realized it was faster to use my Galaxy Nexus crappy camera. :P \$\endgroup\$ – Nathan Campos Mar 9 '13 at 15:22
  • 1
    \$\begingroup\$ Also in your code, float celcius = (voltage - 0.5) * 100;. Why the 0.5 V offset? The LM35 datasheet says it gives an output of 10 mV / degree, with 0 offset for celsius scale, so you shouldn't need that. \$\endgroup\$ – The Photon Mar 9 '13 at 15:43

Looking at your photo, you seem to have +Vs of the LM35 connected to +5V on the Arduino, so if you are reading 3.84V there, that is definitely a problem to address before trying to fix anything else.

The likely problem is this: Te 7 segment displays (or some wiring fault on the breadboard) is drawing too much current from the Arduino, and ultimately from the USB connector. This means that the voltage regulation provided by the USB power supply is no longer working effectively, and so the supply voltage that you measured as 3.84V is likely wandering, and in any case low.

While many if not all the chips in this project will operate off voltages lower than the 5V that V+ is supposed to be at, there are some aspects that will be sensitive to lower V+.

One notable V+ -sensitive feature is the A-to-D convertor in the Atmega328. (As The Photon commented.) The convertor can be set to choose between a couple of different reference voltages. Your code indicates that you have it set to use the V+ (VCC) value (5V) as the reference. If that reference is low (eg: 3.8V) then your input voltage from the LM35 will measure as a number that is scaled up accordingly.

And I'm fairly certain that if your V+ is low, it will change according to how many LEDs are illuminated and drawing current, so your measurements will vary according to what digits are displaying, and vice versa, of course.

Once you've fixed the power supply issue, you might assess whether to set the A/D convertor to use the built-in 1.1V reference instead, as this will give you higher resolution and more stable converted values. See the Atmega328 docs for more details on the A/D convertor and options.


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