Arduino and MAX7219 - 50 Volt - Digital Volt Meter

Abstract

The Arduino inbuilt 10 bit ADC, can be used for constructing / measuring the 0Volt to 50Volt Digital Volt Meter. Popular MAX7219 display driver, connected with Arduino Uno will be used for displaying the measured voltage. This voltmeter can read only DC voltage.

Parts and components

Arduino Uno board
Max7219 4 Digit 7Segment Common cathode displays = 1 Nos
Or Max7219 assembled board
Resistor 27 K = 1 No
Resistor 1.2M = 1 No
IN5913B 3.3 Volt Zener diode = 1 No

Step 1: Schematic

The MAX7219 display driver chip provides a 3-wire serial (SPI) interface to drive 7-segment LED displays (common-cathode type) up to 8 digits.
The on-chip includes a BCD decoder, multiplex scan circuitry, segment and digit drivers, and an 8×8 static RAM to store the digit values.
The DIN, LOAD and CLOCK pins of MAX7219 is connected with 11,10 and 9 digital IO pins of Arduino.
A four digit, common cathode seven segment display is connected to the drive pins of MAX7219.
Otherwise, one can use the pre-assembled MAX7219 board with 4 digits, and connect the drive pins accordingly.
Voltage divider circuit is used to divide voltage into two parts (measurement and drop).
Internal Vref (1100 mV) of Arduino is selected for measurement.
The resistor R3 (1200K) and R2 (27K) forms the voltage divider, and connected to analog input pin of Arduino (A0).
IN5913B 3.3 Volt Zener diode, at A0 pin, will product the Arduino, from over voltage inputs.
Vmeasurment = (R2 / R3+R2 ) * Vin.
If Vin is 50V then Vmeasurment = (27/1227) * 50V = 1.1 V = 1100 mV.
ADC Resolution = Vref / (1024 -1).
ADC Resolution = 1.1 V / 1023 = 1100mV / 1023 = 1.075 mV.
DVM Resolution = 50 / 1023 = 48.875 mV.

Step 2: Software

The Arduino LedControl library is used for displaying digits on MAX7219.
The 1.1V internal reference is chosen for the ADC.
The voltage divider will, provide the input voltage between 0 to 1100mV.
Hence the software will measure and display the values between 00.00 Volt to 50.00 Volt.
10 bit ADC reading is mapped to 1100 mV, which is the Vref mille volt.

/*
 Measurement of 0V - 50V DC input
 Uses the MAX7219 and a 4x7 Segment display.
 
 The Arduino circuit connection for 7219:
 * MAX7219 DIN pin to digital pin 11
 * MAX7219 LOAD pin to digital pin 10
 * MAX7219 CLOCK pin to digital pin 9
 
 Name:- M.Pugazhendi
 Date:-  12thJul2016
 Version:- V0.1
 e-mail:- muthuswamy.pugazhendi@gmail.com
 * /
// include the library code:
#include <LedControl.h>
int analogPin1 = 0;     // LM35 temperature sensor connected to analog pin 2
int val = 0;           // variable to store the value read
// inputs: DIN pin, CLK pin, LOAD pin. number of chips
LedControl mydisplay = LedControl(11, 9, 10, 1);
void setup() 
{
   analogReference(INTERNAL);
  mydisplay.shutdown(0, false);  // turns on display
  mydisplay.setIntensity(0, 15); // 15 = brightest
  mydisplay.setDigit(0, 0, 9, false);
  mydisplay.setDigit(0, 1, 8, false);
  mydisplay.setDigit(0, 2, 7, false);
  mydisplay.setDigit(0, 3, 6, false);
  mydisplay.setDigit(0, 4, 5, true);
  mydisplay.setDigit(0, 5, 4, false);
  mydisplay.setDigit(0, 6, 3, false);
  mydisplay.setDigit(0, 7, 2, false);
}
void loop() 
{ 
  // read the Analog input pin
  val = analogRead(analogPin1);    
  int converted = map(val, 0, 1023, 0,5000);  
   
  converted = HexToBCD(converted);
  //Display the values
  mydisplay.setDigit(0, 0, ((converted>>12)&0x0F), false);
  mydisplay.setDigit(0, 1, ((converted>>8)&0x0F), true);
  mydisplay.setDigit(0, 2, ((converted>>4)&0x0F), false);
  mydisplay.setDigit(0, 3, (converted&0x0F), false);
                
  //One second delay between readings                  
  delay(1000);       
}
/**************************************************************************/
/*!
 * \brief Coverts hex into BCD
 *
 * This function will coverts hex into BCD
 *
 * \param[in] byte
 * \param[out] byte
 * \return Nill
 * 
 */
/**************************************************************************/
unsigned int HexToBCD(unsigned int number)
{
    unsigned char i=0;
    unsigned int k = 0;
    while(number)
    {
        k = ( k ) | ((number%10) << i*4);
        number = number / 10;
        i++;
    }
    return(k);
}