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A Portable Node of Humidity and Temperature
Sensor for Indoor Environment Monitoring
Trio Adiono
School of Electrical Engineering and Informatic, Institut
Teknologi Bandung
Gd. Achmad Bakrie Lt. III, ITB campus, Jln. Ganesha No.
10, Bandung city, West java, Indonesia
Maulana Yusuf Fathany, Syifaul Fuada, Irfan Gani
Purwanda, Sinantya Feranti Anindya
University Center of Excellence on Microelectronics,
Institut Teknologi Bandung
IC Design Laboratory, Gd. PAU Lt. IV, ITB Campus, Jln.
Tamansari No. 126, Bandung city, West Java, Indonesia
Abstract— Humidity and temperature are parameters those
are commonly implemented for monitoring tasks, including
monitoring within an indoor environment such as a smart home.
In this study, we design, develop, and demonstrate the
performance of a portable sensor system for indoor environment,
which is connected to the smartphone-based user interface for
monitoring humidity and temperature. This sensor system
consists several components those are packed into a single small
box, namely: a Zigbee communication module, STM32L100
microcontroller, DHT11 temperature sensor, BL-5C
rechargeable battery, and a charger circuit. The sensor system
and the application are then tested to gauge the performance.
Based on the test, the system is able to communicate with the
smartphone through the host. In addition, based on 24-hour
battery testing, the system requires less than 0.53958 watts to
operate, thus achieving the low-cost target.
Keywords—DHT11; Humidity and Temperature; Indoor
environment; Portable sensor system; Smart home
I. INTRODUCTION
Humidity and temperature are two of the most common
monitoring parameters in Internet of Things research, whether
for outdoor purposes (e.g. air quality measurement and
irrigation) and indoor purposes (e.g. smart home and smart
building). This is because of the fact that not only humidity
and temperature often represent crucial aspects of the
monitored object, but also due to the abundance and ease of
use of the sensors. In usual, the monitoring is conducted
through web-based or smartphone application-based user
interface, with some of the examples can be observed in [1-5].
Despite the relative ease of use and implementation of the
sensors, many of the deployment in the aforementioned
literatures can be considered impractical especially for mass-
production and integration into IoT network. In the
aforementioned literatures, the sensors are deployed using
Arduino boards, which are good enough to use for small-scale
projects, but are impractical for larger-scale implementation
due to the speed limitation and the boards’ cost. Furthermore,
it is desirable to combine the sensors and other components
such as the microcontroller, power, and connectivity modules
into one small package for lightweight implementation and
rapid installation of the sensor system. For this modular plug-
and-play approach, there have been some examples of the
devices designed following this approach as exemplified in [6-
8]. However, the aforementioned projects focused on sensor
systems for outdoor deployment, which are then monitored
using personal computer terminal that limits mobility aspect of
the system. As such, for this project, the design and
development is focused on sensor system for indoor
deployment, which is then monitored using smartphone
application.
To answer the aforementioned requirements, in this
research a portable and low-cost humidity and temperature
sensor system is designed. The designed sensor system
consists STM32L100RCT7 microcontroller as main processor,
DHT11 humidity and temperature sensor, Zigbee module for
connectivity with Raspberry Pi-based host, and BL-5C battery
and its charger circuit. The sensor system is monitored using
Android-based application developed using MIT AppInventor
2, which is connected to the Raspberry Pi-based host using
Bluetooth protocol. By designing the system, we aim to
provide comfortable and practical way for user to monitor
their home’s condition anytime and anywhere.
II. METHODS
A. System Description
Fig. 1. System architecture of the indoor monitoring system, excluding additional features such as encryption and scheduling schemes
This work is related to the previous research as described
in [9-16]. In the previous research, we propose the architecture
of indoor monitoring system (Fig. 1) which is divided into
three parts: host, user interface (smartphone), and sensor
nodes. This work is part of the sensor node development,
namely the sensor node for humidity and temperature
monitoring.
The monitoring and control of the system is conducted
through an Android-based application named ‘mySmartHome
v1.0’. The application works by interacting with the Raspberry
Pi host using Bluetooth. The host serves to translate
information and send instructions from the smartphone to the
sensor nodes through XBee module based on the given
identification address. The complete format of the message is
described in section E.
B. System Specification
The sensor system is designed to measure indoor humidity
and temperature within home environment, as well as to work
within Internet of Things network. As such, the sensor doesn’t
need to be highly precise, but just enough to get the gist of the
room’s condition. On the other hand, because the system is to
be connected to the network, the system requires fast processor
and connectivity that is both fast and uses low power.
C. Hardware Design
The system consists an STM32L100RCT7 microcontroller, DHT11 humidity and temperature sensor Xbee Pro module for Zigbee-based connectivity with Raspberry Pi-based host, and BL-5C battery and its charger circuit. The diagram block depicting the system’s structure can be observed in Fig. 2.
Fig. 2. Structure of humidity and temperature sensor system
While the DHT11 sensor is a low-cost sensor for
reading humidity and temperature. The sensor allows humidity
reading at 20-80% and 0-50 °C temperature, each with 5% and
±2 °C error respectively. The basic configuration of the sensor
with STM32 microcontroller can be observed in Fig. 3. In
addition to the sensor, this system uses Zigbee for
communication with the host. The Zigbee protocol is chosen
due to its low power usage and extensive coverage range,
which also makes it suitable for power conservation purposes.
This system is also equipped with microUSB port, which can
be used for firmware update and charging the system’s
battery.
.
Fig. 3. Basic configuration of DHT11 sensor connected to STM32L100 chip
D. Software Design
The software design consists two parts, namely the design
of the software within the microcontroller and the design of
the Android application. The software in the microcontroller
serves to process the message given by the Raspberry Pi host,
while the Android application serves to control. The
flowcharts for each software are depicted in Fig. 4 and Fig. 5.
Fig. 4. Flowchart of humidity and temperature node software
Fig. 5. Flowchart of Android application
E. Packet Data Design
The message sent between the host and the sensor node is
arranged based on the data protocol from the previous work in
[17-18]. The structure of the message (Fig. 6) consists 3 Bytes
of header, 2 Bytes of address, 1 Byte for packet initialization,
varying Bytes of data payload, and 1 Byte for verification
using check sum. For this humidity and temperature sensor,
the size of data payload is 1 Byte.
Fig. 6. Packet data structure for humidity and temperature sensor node
III. RESULTS AND DISCUSSION
A. System Implementation
Based on the established specification and design, the
printed circuit board of system hardware is designed as shown
in Fig. 7 for top view and Fig. 8 for bottom view. The PCB
consists two layers and masking, with the components are
mounted and soldered on both sides. The assembled board is
then packaged as depicted in Fig. 9.
Fig. 7. Top layer of PCB for humidity and temperature node
Fig. 8. Bottom layer of PCB for humidity and temperature node
(a) (b)
(c) (d)
Fig. 9. Packaging of the humidity and temperature sensor node
Fig. 10. Microcontroller code for humidity and temperature node
Fig. 11. List of functions within dht11.c library
(a) (b)
Fig. 12. GUI of the developed android application: (a) Bluetooth connectivity to the host; (b) retrieve humidity and temperature data
The software implementation for the microcontroller is
depicted in Fig. 10 and Fig. 11, while the implementation for
the Android application is depicted in Fig. 12 and Fig. 13. The
software implementation utilizes the dht11 open library for the
microcontroller and MIT AppInventor 2 for the Android
application, respectively.
(a)
(b)
Fig. 13. Screenshot of logic blocks for the: (a) humidity; (b) temperature reading in MIT App Inventor 2
B. Performance test
To gauge the performance of the system, three tests are conducted, namely connectivity test, power usage measurement, and battery life measurement. The connectivity to the system is tested using smartphone running Android 4.4.2 (KitKat). Based on the testing conducted, the system is able to communicate with the host and user interface, with the test result showing 34% humidity and 28 °C room temperature.
Fig. 14. Functional test of humidity and temperature sensor
The power usage measurement is conducted using power supply equipped using voltage and current display. Based on the conducted measurement, there is no difference between power usage in idle state and processing state. The result of the measurement is elaborated in Table I.
TABLE I. POWER MEASUREMENT OF SENSOR NODE
Input voltage Current
Idle condition Process condition
5 VDC 42.6 mA 42.6 mA
The battery life is measured by having the system active for 24 hours. The BL-5C battery has 3.7 VDC voltage and capacity 3500 mAH, from which it can be inferred that the battery has 12.95 watt-hours. Ideally, the testing should be conducted by completely exhausting the power, but in this research the system is only activated for 24 hours to estimate the power requirements of the system. If within 24 hours the battery runs out of energy, it means the device needs at least 0.53958 watt (145,833 mA).
However, in this testing, the system is still active after 24 hours, so it can be inferred the power requirement of the system is less than 0.53958 watt.
IV. CONCLUSION AND FUTURE WORK
In this research, a sensor system to measure humidity
and temperature is designed, implemented, and evaluated. The
system is connected with an Android-based user interface
through Raspberry Pi-based host, which utilizes Bluetooth for
the former and Zigbee for the latter. Based on the testing
conducted, the system is able to work with the designed
Android application. Furthermore, the system uses low power
(< 0.53958 watt), making it suitable for low-power Internet of
Things application.
In the future, the system will be enhanced to support
better security. Furthermore, additional sensors will be added
for better measurement of air quality. The enhancement will
hopefully make the system suitable not only for safety
purposes, but also for maintaining health of the user.
ACKNOWLEDGMENT
This work is part of the “Internet of Things Devices for Smart Home System” project, which was funded by the Ministry of Research, Technology, and
Higher Education of the Republic of Indonesia (Kemenristekdikti) for the
decentralization scheme with Grant Number 009/SP2H/LT/DRPM/IV/2017
The sensor node along with the other end-devices have been exhibited in
Centrum für Büroautomation, Informationstechnologie und
Telekommunikation (CeBIT), on March 14-28, 2016 in Hannover-Germany Site: (http://www.pme.itb.ac.id/microelectronics-center-itb-on-cebit-2016-
and-ict-roadshow/
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