19 Student Project: Analogue Circuit Design for Temperature Control

This project report details the design and implementation of an automatic temperature controller using electronic components. The system is designed to detect and regulate environmental temperature, such as in a room, by adjusting the speed of a cooling fan driven by a DC motor. The design includes a sensing subsystem with an LM35 temperature sensor and a comparator unit built with an op-amp. The comparator compares the sensed temperature with a set-point and outputs signals that control the switching of a bipolar transistor, thereby controlling the fan's speed. The report covers the literature review of different temperature control mechanisms, the design of the controller's components (sensing unit, comparator unit, and controlled switch), circuit evaluation and analysis, and conclusions. The project's analysis involved feeding a sawtooth voltage to the comparator and observing the output control signal using a digital oscilloscope, validating the circuit's functionality. This project provides a comprehensive understanding of analogue circuit design, particularly in temperature control applications, and is a valuable resource for students studying electrical engineering.

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Student
Instructor
Analogue Circuit Design
Date

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Table of Contents
ABSTRACT....................................................................................................................................1
1: INTRODUCTION.....................................................................................................................1
2: LITERATURE REVIEW.........................................................................................................2
1.1: Manual control mechanism...............................................................................................2
1.2: Automatic control mechanism...........................................................................................3
1.3: Review of sensor temperature controller..........................................................................4
2: DESIGNING OF CONTROLLER..........................................................................................7
2.1: Controlled Switch...............................................................................................................7
2.2: Sensing unit.........................................................................................................................7
2.3: Comparator unit.................................................................................................................7
4: CIRCUIT EVALUATION AND ANALYSIS.......................................................................12
5: CONCLUSION........................................................................................................................15
BIBILIOGRAPHY......................................................................................................................16

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ABSTRACT
Temperature is one of the climatic condition that incessantly fluctuates over time. Different
places on earth are characterized by different levels of temperature depending on the
geographical position with respect to the equator. Areas located at the equator receives different
temperature relative to areas located away from the equator. Due to nature of human beings in
terms of travelling across the continents, they are subjected to environmental conditions with
conspicuous disparities to the conditions they are used to. These conditions such as temperature
need to be controlled and maintained to a stable condition so as to cause comfort and maintain
healthier environment for metabolism. To effect stable temperature condition, temperature
controllers need to be incorporated in the residential dwellings.
This mini-project dealt with designing and implementation of automatic temperature controller
using electronic devices. The system was designed in a manner that it detects the environmental
temperature of the given surrounding, such as a specific room of the residential house, and
readjust the speed of the cooling fan when the conditions exceeds the pre-set conditions. The fan
is driven by the DC motor supplied with a separate DC power supply. Sensing subsystem entails
of temperature sensor module and a comparator that gives output relay signals. The output relay
signals control the switching system of the motorized fan circuitry resulting to variable speed of
the fan.
1: INTRODUCTION
The ecosystem has adjusted itself in the manner that different living organisms survive in the
specific geographical locations on earth experiencing specific temperatures. For instance, snow
leopard on mountain ranges of Central Asia, Arctic hare in tundra regions of Greenland among
many other animals are adapted to staying in polar regions with extremely low constant
temperatures (Chatterjee, 2014). On contrary, other species of animals such as sand fox, Greater
Bilby in badlands of Queensland in Australia have evolved to survive in areas with intense heat
(Langley, 2017). Survival for the fittest also has compelled human beings adjust to specific
environmental conditions such as temperature.
Human beings most likely cannot survive in extreme temperature conditions. Exposure to these
conditions lead to deterioration of their conditions and aftermaths could be fatal such as death.
There have been debates all over concerning issue of global warming. Global warming causes
annual increase of temperature on planet earth. According to Elaine Godfrey in his report on
“Temperatures will be too high for human survival in Persian Gulf”, states that by the end the
current century, global warming will be one day a big menace in that people will rarely thrive
outside in the Persian Gulf (Godfrey, 2015). According to WHO, subjection to excessive
temperature results to physiological impact the affected people and mostly worsens the existing
conditions. This can lead to underage premature death or even disability (WHO, 2019).
The solutions for the same have led to scientific researches and inventions of temperature control
mechanisms which stabilizes surrounding temperature conditions for human comfort and

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survival. There are different commercial temperature controller products meant to handle
temperature stabilization for residential and industrial scale. This has really helped in mitigating
against the menace resulting from excessive temperature.
2: LITERATURE REVIEW
The ideal temperature for the room should be maintained at 24oC during the night and 33oC
during the day. To attain these conditions, the building should have inbuilt temperature control
system that keeps the room temperature checked. The temperature of the room can be controlled
by running fan that improves air circulation. When the fan rotates, it speeds up flow of air across
the room through ventilations. Cross-ventilated flow of air injects a cool breeze in the room that
speeds up sweat evaporation and discharges away hot air. As a result, there is a general reduction
of room temperature (Spark Energy, 2017).
However, at very low temperature, the effect of running fan lead to extremely low undesirable
temperature. Therefore, there need be an optimization mechanism that checks and controls the
speed of the fan with respect to the room temperature. There are two possible ways of
implementing the mechanism namely; manual temperature control mechanism and automatic
temperature control mechanism. These comes in different designs and costs with respect to the
efficiency and complexity incurred. These type of controllers are as described below.
1.1: Manual control mechanism.
This temperature control mechanism, the control panel is operated by the operator. It involves
operator who keeps on confirming the current temperature and make adjustments when the
temperature exceeds the set-point. Variation of the controller slows down the fan when the
temperature is relatively low and speeds up the fan when temperature is relatively high (Garrett-
Dyke, 2014). The manual temperature control process is as shown in figure 1 below.

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Figure 1: Manual control process mechanism.
Disadvantages of manual temperature controller.
 The operator’s latency in correcting the settings could result to temperatures
exceeding the acceptable tolerance.
 The implementation is not suitable for critical applications such hospitals and
electronic manufacturing industries.
 The method is not favorable for olds and weakling people, disable and children.
1.2: Automatic control mechanism.
The automatic temperature controller is meant to overcome shortcomings of the manual
controller mechanism. The controller mechanism is embedded with temperature sensor device
which takes in temperature and gives out different voltage magnitudes corresponding to the
levels of temperature. The controller compares the output of the temperature sensor with the set-
point (Garrett-Dyke, 2014). The speed of the fan is automatically adjusted depending on the
comparator’s output. Figure 2 below shows the automatic temperature controller process.

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Figure 2:Automatic control process Mechanism. .
Advantage of the automated mechanism
 There is no need of operator’s supervision of the speed of the fan since it is
automated system thus the workflow of the user is not disrupted.
 Optimization and performance of the fan is perfectly implemented by the
system without latency in the process (Munar, 2019).
 The system is suitable for critical applications.
1.3: Review of sensor temperature controller.
The system under discussion in this report is automatic temperature controller with a sensor
mechanism, a comparator and a controlled electric fun circuit.
The comparator is implemented by the op amp in an open loop configuration. The function of the
comparator is to make decision in accordance with two input signals compared. Two analogue

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inputs are supplied to the inverting and non-inverting nodes of the op amp and the output is logic
HIGH and LOW. One of the analogue input is the reference voltage V ref upon which the other
input V ¿is compared with to give logic output.
Figure 3: Op amp in open loop configuration.
The logic HIGH of the comparator is equivalent to the +V CC given by the condition below.
V ¿>V ref
On contrary, logic LOW of the comparator is equivalent to the −V cc on condition that
V ¿<V ref
The transition between logic HIGH and logic LOW is very rapid making the op amp to operate
as a comparator. The transition can be represented by the hysteresis loop as demonstrated below.
Figure 4: Hysteresis Loop of the op amp comparator. Ref ((Tutorials, 2019).
The hysteresis of the op amp is the resultant variance between the upper trip point +V CC and the
lower trip point −V CC .
Working automatic temperature controller.

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The model schematic circuit is as shown below.
Figure 5: Schematic of the sensor temperature control system
The control process operates on ON/OFF control action. The temperature sensor converts
temperature from degrees Celsius to equivalent voltage signals. Input voltage signal is fed to the
inverting input of the op amp-comparator. The reference voltage of the temperature controller
system is fed to the non-inverting input of the op amp. The output of the op amp is terms of logic
HIGH and logic LOW that are equivalent to saturation voltage ± V CC of the operational amplifier.
The output signal of the comparator is used to switch ON and OFF the bipolar transistor switch
in series with the electric fan circuit. When the output of the comparator is HIGH, the NPN
bipolar transistor switch completes the circuit of the electric fan, making run. On the other hand,
when the output signal of the comparator is LOW, the NPN bipolar transistor switch is OFF, thus
introduces open circuit to the electric fan. The fun which uses a small 12 DC brushless motor is
automatically turned ON when the temperature exceeds the normal and stops when temperature
is normal.
2: DESIGNING OF CONTROLLER
Modelling of the system is broken down into segments with detailed mathematical expressions.

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2.1: Controlled Switch.
The transistor operates as an electronic switch controlled by base voltage. When the base-voltage
is more than 0.7V, the transistor is biased and thus becomes a conductor. When the base-voltage
is below 0.7V, then the transistor becomes an open circuit. The transistor for this model is a
BD437 NPN (BD437 Datasheet, 2019), bipolar transistor. It has the following characteristics;
i. Bias voltage of 0.7V
ii. Normal operation ranges between -65 degree to 150 degrees.
iii. Maximum input voltage up to 45 V.
To protect the transistor from relatively excessive current from the op amp comparator a limiting
resistor of 1 k Ω is connected incorporated.
2.2: Sensing unit.
The sensing unit consists of the temperature sensor. The sensor should have the ability to linearly
translate the temperature in centigrade to the proportional voltage signal. LM35d was the best
choice owing to its characteristics as highlighted below (TI LM35 Datasheet, 2019) ;
a) High precision sensor with a scale factor of +10mV/oC.
b) The accuracy of the sensor is between -55oC to 150oC.
c) The output impedance of the sensor is as low as 0.1Ω for 1 mA of load current.
2.3: Comparator unit.
The comparator as discussed earlier is implemented using the op amp. LM741 op amp (LM741
op amp Datasheet, 2019) is connected in the open configuration as shown below.
I. Set-point of the comparator.
The reference voltage supplied to the inverting input of the op amp comparator is as simplified in
the circuit below.

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The voltage at the inverting input is given by;
V ref = { R 2
R1+ R 2 }V
Substituting the values;
V ref = { 3.3 kΩ
100 kΩ+3.3 kΩ }12 V
V ref =0.38V
This voltage is the set-point of the whole automatic temperature controller upon which system
strives to settle room temperature.
ii. Logic output of the comparator.
A positive feedback resistor is introduced as shown in the figure below.
The positive feedback resistor R3 is initialized to be 576 kΩ . As described by the hysteresis loop
of the above configuration, logic outputs of the comparator are computed.

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Logic HIGH; V OH
For the logic HIGH, the voltage input is greater than reference voltage. Resistors R1 and R3 are
in parallel and the equivalent resistance is given by;
R= R 1× R 3
R 1+ R 3
Substituting the values;
R=100 kΩ×576 kΩ
100 kΩ+576 kΩ
R=85.2 kΩ
The reduced circuit is as shown below
Logic HIGH can be determined using voltage divider
V OH =V ( R 2
R+ R 2 )
Substituting the values;
V OH =12 V ( 3.3 kΩ
85.207 kΩ+3.3 kΩ )
V OH =0.44 V
Therefore, the minimum logic HIGH of the system equivalent to V OH =0.44 V . This voltage
drives the NPN bipolar transistor into ON position.

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Logic LOW; V OL
For the logic LOW, the voltage input is less than reference voltage. Resistors R2 and R3 are in
parallel and the equivalent resistance is given by;
R= R 2 × R 3
R 2+ R 3
Substituting the values;
R=3.3 kΩ × 576 kΩ
3.3 kΩ+576 kΩ
R=3.28 kΩ
The circuit is reduced as shown in the figure below;
Logic LOW of the comparator is determined as follows;
V OL=V { R
R+ R 1 }
V OL=12V { 3.3 kΩ
100 kΩ+3.3 kΩ }
Substituting the values;
V OL=0.38 V
Therefore, the maximum logic LOW of the system equivalent to V OL=0.38 V . This voltage
drives the NPN bipolar transistor into ON position.