This is a project based on Arduino board which
can measure the unknown AC and DC voltages. When we connect the unknown voltage
on the breadboard circuit, the 16*2 LCD displays the voltage value. The project
uses an Arduino pro mini board whose ADC feature is used along with the
concept of Voltage Divider circuit to develop thisVoltmeter.

__Architecture of the project__
The entire project can be divided into three
basic blocks;

1) AC/DC Voltage
Sensor Unit

2) Processor Unit

3) Display Unit

The Sensor Unit takes two inputs, DC voltage and
AC voltage. The Sensor Unit scales down the input DC and AC voltages into a DC
voltage in the range of 0 to 5 V and provides the same as output.

The Processor Unit takes input voltage in the
range of 0 to 5V. This unit takes the Sensor Unit’s output as input voltage and
uses the ADC to read this voltage. An Algorithm is then applied tocalculate the
voltage. The unit then sends a 4bit data to the Display Unit which includes the
AC and DC voltage values.

The Display Unit takes the 4bit data from the
Processor Unit and produces a 16x2 display for AC and DV voltages.

**1)**

__AC/DC Voltage Sensor Unit__
A basic voltage divider circuit is used as
the AC/DC Sensing Unit to scale down the input DC and AC voltages into a DC
voltage in the range of 0 to 5 V. The Processor Unit can read this scaled down
voltage and calculate the actual AC/DC voltages.

**Design the value of R1:**

Let us first select a maximum voltage that could
be measured as 500V. When we apply 500V as ‘V’, the ‘V2’ should not be more
than 5V and hence ‘V1’ will be 500 – 5 = 495V. At very high voltages like 495,
the first thing to be taken care of is the power rating of the resistor. We are
using resistors with the power rating 0.25W, and the power consumed by the
resistor ‘R1’ should be less than this, otherwise the resistors get heated up
and catch fire.

The equation for power is, P = V1

^{2}/ R1.
Where;

P
Power rating
of the resistor

V
Voltage across the resistor

R
Resistance of the resistor

For the resistor R1 with power rating 0.25 W and
495 V across it,

0.25 = 495 * 495 / R1

Or, R1 = 980100 ohms, take 1 M ohm standard
resistor.

**Design the value of R2:**

Now the value of R2 can be calculated using the previous
equation, V = V2 * (1 + R1 / R2) as follows;

R2 = R1 / ((V / V2) – 1)

R2 = 1000000 / ((500 / 5) – 1)

R2 = 10101 ohms, take 10K ohm standard resistor.

**DC voltage as input:**

The voltage ‘V2’ is a fraction of the actual
applied voltage ‘V’. The applied voltage ‘V’ can be calculated from the
fraction of applied voltage ‘V2’ with the help of the following equation.

DC voltage, Vdc = V2 * (1 + (R1 / R2))

**AC voltage as input:**

When we are applying an AC voltage we use a
rectifier diode in series with the Voltage divider circuit to prevent the
negative cycles from entering the circuitry. No need for step down transformers
because we are already getting a voltage ‘V2’ in the range of 0 to 5 V only,
across R2.

**Requirement for Range selector:**

We require multiple ranges in avoltmeter due to
the error appears in readings because of Resistance Tolerance.

a) Decrease in the
ratio of R1/R2 decreases the error

b) There is a limit
beyond which the R1/R2 cannot decrease further:

To measure different values of V with minimum
error we need different set of R1 with a common R2. The voltage V whose value
need to be measured is connected with an R1 which gives the least ratio of
R1/R2, taking care of the fact that V2 should not go above 5V range.

(R1 / R2) > (V / 5) – 1

For example to measure V = 500V, R1 / R2 > 99,
hence we can use the set R1 = 1M and R2 = 10K which gives R1 / R2 = 100.

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