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The basic schematic diagram of the system is as shown in the
figure 2. The list of components used are as follows.
1.1 Water tank:
This is the tank where the level of the liquid inside is to be
controlled. Water is pumped to the tank from above and a
level sensor measures the height of the water inside the tank.
The microcontroller controls the pump so that the liquid is at
the required level. The tank used in this setup is a stainless
steel container.
1.2 DC Pump:
The pump is a small 12V dc water pump drawing about 3A
when operating at the full-scale voltage 1.3 Level sensor:
Level Sensor Selection
When determining what type of level sensor should
be used for a given application, there are a series of questions
that must be answered:
Can the level sensor be inserted into the tank or
should it be completely external?
Should the sensor detect the level continuously or will a point
sensor be adequate?
Can the sensor come in contact with the process
fluid or must it be located in the vapor space?
Is direct measurement of the level needed or is
indirect detection of hydrostatic head (which
or preferences of the particular plant or the
A rotary potentiometer type level sensor is used in
this setup. The sensor consists of a floating arm connected to
the sliding arm of a rotary potentiometer. The level of the
floating arm, and hence the resistance, changes as the liquid
level inside the tank is changed. A voltage is applied across
the potentiometer and the change of voltage is measured
across the arm of the potentiometer. The resistance changes
from 100Ω when the floating arm is at the bottom to 10Ω
when the arm is at the top.
The level sensor is shown in Figure 1.5.
Figure.5 Principle of liquid level sensor.
A potentiometer type level sensor is a manually
adjustable resistor. The way this device works is relatively
simple. One terminal of the potentiometer is connected to a
power source. Another is hooked up to ground (a point with
no voltage or resistance and which serves as a neutral
reference point), while the third terminal runs across a strip of
resistive material. This resistive strip generally has a low
resistance at one end; its resistance gradually increases to a
maximum resistance at the other end. The third terminal
serves as the connection between the power source and
ground, and is usually interfaced to the user by means of a
knob or lever. The user can adjust the position of the third
terminal along the resistive strip in order to manually increas
or decrease resistance. By controlling resistance, a
potentiometer can determine how much current flow through
a circuit. When used to regulate current, the potentiometer i
limited by the maximum resistivity of the strip.
1.5 Microcontroller:
A PIC16F877 type microcontroller is used in this
setup as the digital controller. In general, any other type of
microcontroller with a built-in A/D converter can be used.
The PIC16F877 incorporates an 8-channel, 10-bit A/D
converter.
1.6 D/A converter:
An 12-bit MCP4921 type D/A converter is used in
this setup. In general, any other type of D/A converter can be
used with similar specifications. The complete system is as
shown in the fig. 2) Model Identification
First of all for the identification of the system, we