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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

[email protected]

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

user
IGBSG 2018

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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