22-11-2012, 06:15 PM
Design and Development of Microcontroller based Digital Thermo Hygrometer
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Abstract
A microcontroller based digital thermo hygrometer was designed and developed to measure the value of temperature and humidity at any place. The system was battery operated so it was portable. A microcontroller PIC16F877A was used to control the developed system’s function. The system was measure the temperature from 0°c to 100°c and the humidity % RH from 0 to 100. The reading was displayed in a seven segment display. A “C” language program was development to control the function of the microcontroller, using PCWH compiler.
Introduction
In our country some research laboratories, industries, hospitals, stores etc require to measure the temperature and humidity for research, production, treatment and diagnosis of the patients, storing food, beverage etc. Sometime, in weather monitoring, for instance, parameters such as, the temperature and humidity needed to be measured [1], thus sensors have always been given the task for doing so. Climate plays an important role in human life. The thermal comfort of human being is known to be influenced mostly by six parameters, i.e., air temperature, radiation, air flow, humidity, activity level and clothing thermal resistance [2] and [3]. The advancement in technology has made these small and reliable electronic sensors capable of monitoring environmental parameters more favorably. Monitoring systems using sensors for indoor climate and environment based parameters are explained in [4] and [5]. Combination of these sensors with data acquisition system has proved to be a better approach for temperature and relative humidity monitoring [6] and [7]. The surface acoustic waves (SAW) devices as temperature sensor is explained in [8] and [9]. These systems, however, are quite expensive and complex in nature as some of them require the use of on-chip transmitter circuit and involve fabrication processes.
Design Consideration
The system is divided into three main parts: the sensor circuit, the micro controlling unit, and the display circuit. The sensor circuit contains the IC temperature sensor and resistive humidity sensor. The analogue outputs from these sensors are analyzed and converted into digital signal which encompasses a microcontroller. The block diagram of the overall system is depicted in Fig. 1.
The Sensor Circuit
For temperature sensing, an integrated circuit temperature sensor LM35 is used [10]. The output voltage of the sensor is linearly proportional to the temperature (in Kelvin or Celsius) with the gradient of 10mV/°C and able to operate in the range of -55°C to 150°C. As the device is to be used in the tropical climate area where the temperature never drops below 0°C, the temperature range for this system has been offset to the range of 0°C to 100°C, using an op-amp.
Relative humidity measurement is performed by the humidity sensor, HIH-4002 [11]. The HIH-4002 series are linear voltage output vs % RH. The output of this sensor is also acts as the input of the microcontroller. The CA3140 op-amp is used as buffer to minimize the loading effects [12] and [13].
Microcontroller Unit
The microcontroller [14] is the heart of the whole system. Temperature sensor and Voltage sensor are input of the Microcontroller. Room heater driver circuit, air cooler driver circuit, displays and indicator circuit are output of the microcontroller .The microcontroller PIC16F877 has been used [15]. It has 40-Pin packages. It has a 10-bit A/D converter. A C language program has been developed to control the function of microcontroller using PCWH Compiler [16].
Conclusion
In recent times, the cost of electronic equipment has fallen significantly. Due to the rapid development of micro electronics, all designed components and instruments are inexpensive. A digital thermo hygrometer from the international market cost around US$ 400, while the price of the developed digital thermo hygrometer is less than US$ 60. Moreover, when the features of the presently used system are compared with the developed system, the latter emerges as a better choice in terms of cost, portability and design.