Voltmeter is a voltage measuring instrument. We can measure the potential difference between any two points in an electrical network using voltmeter.  There are two types of voltmeter as analog voltmeter and digital voltmeter. Analog voltmeter moves pointer on a scale but it has some limitations like accuracy of few percent of full scale. Digital voltmeter can display numerical value of voltage on a display by use of analog to digital converter (ADC). All the data processing and manipulating is in digital form, so it is very essential to use ADC. We have used ADC0804 analog-to-digital converter IC. The range of input voltage is 0-15V. Here the input voltage should be DC voltage so as to get the steady output on the LCD display. If you give the AC voltage as an input, it will display continuously running numbers as the nature of AC voltage.

1. DIGITAL VOLTMETER USING
89C51 MICROCONTROLLER
Guide,
Prof. S. A. Karmude
By,
Saylee S. Joshi

2. INTRODUCTION
• Voltmeter is a voltage measuring instrument.
• We can measure the potential difference between any
two points in an electrical network using voltmeter.
• There are two types of voltmeter as analog voltmeter
and digital voltmeter.
• Analog voltmeter moves pointer on a scale but it has
some limitations like accuracy of few percent of full
scale.
• Digital voltmeter can display numerical value of voltage
on a display by use of analog to digital converter (ADC).
• All the data processing and manipulating is in digital
form, so it is very essential to use ADC.

3. Cont….
• We have used ADC0804 analog-to-digital
converter IC. The range of input voltage is 0-
15V. Here the input voltage should be DC
voltage so as to get the steady output on the
LCD display.
• If you give the AC voltage as an input, it will
display continuously running numbers as the
nature of AC voltage.

4. Block Diagram

5. Components Required For Digital
Voltmeter Using 89c51 Microcontroller
• Microcontroller, AT89C51
• Analog‐to‐Digital Converter, ADC0804
• 16×1 LCD
• Oscillator circuit for the microcontroller
– 12MHz Crystal Capacitor
– 33pF Capacitors
• Voltage divider circuit/ Input terminals
– 200k, 100k Resistors
– 100nF Capacitor
• ADC Clock Circuit 100k Potentiometer (to adjust the back‐light of
the LCD)
– 10k Resistor
– 150pF Capacitor
• 100k Potentiometer (to adjust the back‐light of the LCD)

6. Circuit Diagram Of Digital Voltmeter
Using 89c51 Microcontroller

7. Lcd Pin Diagram
Pin no. Name Description
1 Vss Ground
2 Vdd +5V
3 Vee Contrast Adjustment ‐2V to ‐5V
4 RS Register Select
5 RW 1 ‐Read , 0‐ Write
6 E Enable Strobe
7 D0 Data Line
8 D1 Data Line
9 D2 Data Line
10 D3 Data Line
11 D4 Data Line
12 D5 Data Line
13 D6 Data Line
14 D7 Data Line
15 LED+ Backlit LED +V Vdd (Optional signal)
16 LED‐ Backlit LED –V Vss (Optional signal)

8. Components
1. 8051 Based Controller: 1
2. ADC0804: 1
3. LCD 16x1: 1
4. Potentiometer 10K: 1
5. Resistor 10K: 2
6. Resistor 200K: 1
7. Resistor 100K: 1
8. Capacitor 33pF: 2
9. Capacitor 10uF: 1
10. Capacitor 150pF: 1
11. Capacitor 100nF: 1
12. Crystal 12Mhz: 1

9. C Program For Digital Voltmeter Using
89c51 Microcontroller
#include <REGX51.H>
#include "lcd.h"
#define adc_port P1 //ADC Port
#define rd P3_7 //Read signal P3.7
#define wr P3_6 //Write signal P3.6
#define cs P3_5 //Chip Select P3.5
#define intr P3_4 //INTR signal P3.4
void conv(); //Start of conversion
function
void read(); //Read ADC function
unsigned int adc_avg,adc;
void main(){
char i;
LCD_INI();
while(1){
//Forever loop
adc_avg = 0;
for(i=0;i<10;i++){
conv(); //Start conversion
read(); //Read ADC
adc_avg += adc;
}
adc_avg = adc_avg/10;
wrt_cmd(0x80);
wrt_string("V(DC): ");
adc = adc_avg * 59;
hex2lcd((unsigned char)(adc/1000));

10. Cont….
wrt_data('.');
adc = adc%1000;
hex2lcd((unsigned char)(adc/10));
wrt_data('V');
}
}
void conv(){
cs = 0; //Make CS low
wr = 0; //Make WR low
wr = 1; //Make WR high
cs = 1; //Make CS high
while(intr); //Wait for INTR to
go low
}
void read()
{
cs = 0; //Make CS low
rd = 0; //Make RD low
adc = adc_port; //Read ADC
port
rd = 1; //Make RD high
cs = 1; //Make CS high
}

11. Advantages
• This system is used to measure the voltage in
low voltage applications.
• Used to measure the toy batteries.
• We can measure the physical quantities like
temperature, humidity, gas etc. using this
system with a little modification.

12. disadvantages
• The input analog voltage range should be 0 to
5V.
• Using this system we can measure only one
analog input value at a time.