Study On Many Kinds Of Interfaces Sensors Based On Resistive Polymer Humidity Element

Today, our survival and social activities are closely associated with humidity.With the development of social and the popularity of modernization, humidity isimportant in various fields. So the humidity sensors are also indispensable in dailylife and scientific research. The humidity sensors which could be contacted witheveryday are made of capacitive humidity elements, but it’s rare to see the humiditysensors which are made of Resistive humidity elements.It designed a sensor based on resistive polymer humidity elements with manykinds of interfaces. The resistive polymer humidity elements which were used in thispaper are designed by Microelectronics and Solid-State Electronics research groupof Jilin University electronic science and engineering college. It designed the sensoraccording to the characteristics of the resistive polymer humidity elements. As thehumidity elements have a negative temperature coefficient, so it used temperaturesensor DS18B20to increase the function of temperature compensation, so the sensorcould measure the humidity and temperature at the same time. The sensor couldmeasure the11%to95%relative humidity and the temperature of the environment,and it has the frequency interface module, the voltage interface module, the currentinterface module, the RS-232interface module, the RS-485interface module, theBluetooth interface module, the wireless sensor network module and NokiaLCD5110display module, it also has the checksum function.The sensor consists of a humidity sensor, an oscillating circuit,MCU(MicroControllerUnit), seven interface modules and a display module.Frequency interface module which is the same as oscillator circuit. Firstly, Theoscillation circuit is a original design to meet the special requirements of thehumidity element, it could satisfy the single power supply, the voltage across thehumidity element has no DC(Direct Current) component and the amplitude of thevoltage is less than1V, etc. The humidity element converts the humidity of environment into resistance value, then the oscillation circuit convert the resistancevalue into a frequency value.Secondly, drawing working curve according to the logarithmic frequency valueof the standard points (relative humidity11%,33%,54%,75%,85%,95%). Then,the logarithmic frequency of the unknown humidity through DS18B20temperaturecompensation, we could get the value of unknown humidity refer to working curve,the Nokia LCD5110could display relative humidity value and temperature values.MCU used new generation8051——STC12C5A60S2that has the advantages ofhigh speed, low power consumption and strong anti-jamming. Frequency interfacemodule convert the humidity value into frequency values through the oscillatingcircuit and MCU; Voltage interface module using three terminal adjustable shuntreference source-TL431to output the voltage value corresponding to the relativehumidity value, the range of the output voltage value is0-2.5V; Current interfacemodule used D/A converter AD421to output the current value corresponding to therelative humidity value, the range of the output voltage current is4-20mA; RS-232interface module used single power supply level converter chip MAX232which isdesigned for RS-232standard serial by Maxim, RS-485interface module used ADI’scompletely isolated RS-485/RS-422chips ADM2587E; Bluetooth interface moduleuesed BMX-03Bluetooth module; Wireless sensor network module used TI’s2.4GHzRadiofrequency on-chip system solutions chip CC2530as the hardware nodemicroprocessor, follow ZigBee protocol of the IEEE802.15.4standard. The design ofthe sensor also has the check function. It used C programming language under theKeil u Vision3environment.The experimental results show that the relative humidity that the sensor could tomeasure is between11%and95%, Accuracy of humidity sensor between33%to75%relative humidity is±5%RH, and Accuracy of humidity sensor between theother relative humidity±7%RH. The temperature measuring range is from-55℃to+125℃, the inherent temperature measurement error of temperature sensor is0.5℃.The humidity sensor is stable, reliable, fully functional and easy to use, It also hasmany kinds of interface modules to meet the different requirements of user. It’s basically meet the requirements of the experimental design. It could be made intoproducts into the market after improving.