Real Time Engineering Solutions

Digital Voltmeter using 8051 Microcontroller:

Voltmeter is a measuring instrument, used to measure the voltage difference between two points in electrical network. Generally there are two types of voltmeters – one is analog voltmeter and the other one is digital voltmeter. In analog voltmeter a pointer moves on the scale to represent the voltage. Digital voltmeter directly displays the voltage in digits with the help of analog to digital converter

Circuit Components:

AT89C51 micro controller
ADC0804 IC
AT89C51 programming board
Variable resistor
adaptor or DC battery

Digital Voltmeter Circuit Design using 8051 Microcontroller:
In the given circuit, analog to digital converter IC data bits are connected to the PORT1 and control bits INTR, CS, WR, RD pins are connected to the P3.4, P3.5, P3.6, P3.7 respectively. LCD data pins are connected to the POTR2 of controller and control pins RS, RW, EN are connected to the P3.0, P3.1 and P3.2 respectively.

ADC0804:

This is an 8 bit analog to digital converter. This IC uses successive approximation method to convert analog values to digital. It can take only one analog data as input. The step size of this IC is varied by varying the reference voltage at pin9. If this pin is not connected, VCC will be the reference voltage.

For every 19.53mV, rise in input voltage the output is incremented by 1 value when the step size is at 5V. The conversion time of this IC depends on clock source.

ADC Features:

0 to 5V analog input voltage.
Built in clock generator.
Differential analog inputs.
Adjustable reference voltage.
Below table shows the different step sizes for different reference voltages.

In the above circuit diagram, pin9 (Vref/2) is left open so that input voltage span can be 0 to 5V.

Step size = Vref/(2 pow(n))

Where n is resolution. For ADC0804 the resolution n=8. The digital output can be calculated using the formulae

Dout = Vin/stepsize.

Vin – analog input voltage

For example, let the analog input voltage is 4V, then the digital output is Dout=4/19.53mV=204.

Steps to Convert the Analog Input to Digital:

Make CS pin LOW and send LOW to HIGH pulse to WR pin to start conversion.
cs = 0; //Make CS low

wr = 0; //Make WR low

delay_ms(1); //delay

wr = 1; //Make WR high

Continuously check the INTR pin to weather conversion is completed or not. INTR pin is set to LOW when conversion is completed otherwise this pin is HIGH.
while ( intr ); //Wait for INTR to go low

Make CS pin LOW and send HIGH to LOW pulse to RD pin to read data from ADC.
cs = 0; //Make CS low

rd = 0; //Make RD low

adc = adc_port; //Read ADC port

rd = 1;

How Digital Voltmeter Circuit Works using 8051 Microcontroller?
Initially burn the program to the at89c51 microcontroller.
Now give the connections as per the circuit diagram.
Connect variable resistor at the input of probes to vary the input analog input voltage.
make sure that maximum analog input voltage should be less than 5V DC
Connect a digital multi meter at the Pot to measure the input voltage.
Now switch on the board supply.
Now observe both LCD and digital multi meter, both displays the same voltage.
Slowly increase the analog input voltage by varying the pot, now you can observe that both multimeter and LCD displays the same voltages so that we can say that voltmeter is working properly.
Switch of the board supply.
Digital Voltmeter Circuit Applications:
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.
Digital Voltmeter Circuit Limitations:
The input analog voltage range should be 0 to 5V.
Using this system we can measure only one analog input value at a time.

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