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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

Microcontroller based ISFET pH measurement system with wireless communication Gaytri Gupta#1,Rahul Kumar Verma#2

#

Department of Electronics and Communication, AMITY University, Sector-125, Noida, India ggupta@amity.edu rkverma@amity.edu

Abstract—This paper describes the experimental setup of pH measurement system which can be used for our everyday life. pH represents potential of hydrogen which is used as the unit of measure to express the degree of acidity of a substance is defined as the negative logarithm of the hydrogen ion concentration. There is a wide variety pH measurement system. The paper presents an attempt to develop an in-house low cost product for pH measurement.

The heart of this device is the ADuC814 microcontroller chip. The sensor used in this device is an ISFET sensor. This type of ion-sensitive sensor is derived from the MOSFET (metal oxide semiconductor field effect transistor). In this setup pH value can be measured and continuously monitored on the LCD and by using the RF modem the pH values can be continuously monitored on the PC also. All setups was verified and tested on all standard solutions and proved to be accurate.

I. INTRODUCTION It is truly said, “A new product must not only be created, but experimentally developed, to be successful.”

The pH meter has innumerable applications in industry and R&D laboratories. Although several attempts have been made to design analog and digital pH meters, its suffer from limitations like compactness, complexity in design, lack of storage and serial communication facilities etc., which are very important for research applications.

The microcontroller ADuC814 [1] based pH meter overcomes the above difficulties. For the software, the overall software programming job is broken down into several modules, which can be individually tested and debugged. Similarly, the hardware is also developed in small modules, which can be individually tested and debugged. In this system we have used a low power microchip ADuC814 microcontroller.

This microcontroller combine a microprocessor unit like the CPU in a desktop PC with some additional circuits called “peripherals”, plus some additional circuits on the same chip to make a small control module requiring other external devices. This device can be embedded into other electronic and mechanical devices for low-cost digital control. An ADuC814 runs in an infinite loop fetching the signals from the pH sensor and the necessary manipulations done on the data to calculate the pH. The accurate and precise determination of the values of variables such as pH and temperature is of prime importance in research and process control. Several steps are involved in getting the electrical signals.

II. PROCEDURE FOR EXPERIMENTAL SETUP The first step, sensing physical variables pH by ISFET sensor which converts the pH values into current. This current is passes through a resistance to convert it into the voltage form. The received voltage fed to the analog to digital converter. The main steps of the development of this device are 5V supply voltage to the ADuC814 microcontroller chip.

Signal acquisition from the pH sensor.

Determining the performance characteristics of the pH sensor.

Retrieve and analyze Data.

Implementation of equation into the microcontroller chip.

Simulating the results using the final modeled target system and displaying the pH values on the LCD display.

Sending the result to the PC through the RF modem Power Supply

Sensor DAC

ISFET Signal Conditioning

Computer

LCD

ADC

ZigBee

RS-232

Fig.1.Block Diagram of the experimental setup

III. WORKING OF MICROCONTROLLER BASED PH METER Fig. 2 shows the schematic diagram of an ADuC814 microcontroller based pH meter. The key point for development of this system is to interface the ISFET with microcontroller AduC814.

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Temperature

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

A. ADC Module of the ADuC814 The ADC block incorporates a 4.05 msec, 6-channel, 12-bit resolution, single-supply ADC. This block provides the user with a multi-channel multiplexer, track-and-hold amplifier, on-chip reference, offset calibration features and ADC. All components in this block are easily configured via a 3 SFR register interface. The ADC has six external input channels. Two of the ADC channels are multiplexed with the DAC outputs, ADC4 with DAC0 and ADC5 with DAC1. In the ADC, five internal signals include a temperature monitor, DAC0, DAC1, VREF and AGND. These internal channels can be selected similarly to the external channels via CS3–CS0 bits in the ADCCON2 SFR.

where an 8 bit is framed with start and stop bit. TR1 is set to 1 to start Timer1. The character byte to be transferred serially is written into the SBUF register. Data is transferred serially to the PC by using RS-232. For wireless communication ZigBee technology has been used. This is a low-cost, low-power, technology. This technology takes full advantage of the IEEE 802.15.4 physical radio specification and operates in unlicensed bands worldwide at the following frequencies: 2.400– 2.484 GHz, 902-928 MHz and 868.0–868.6 MHz. Start Initialize DAC Initialize LCD

B. DAC Module of the ADuC814 The ADuC814 incorporates two 12-bit, voltage output DACs on-chip. Each DAC has a rail-to-rail voltage output buffer capable of driving 10 kΩ/100 pF. They have two selectable ranges, 0 V to VREF (an external or the internal band gap 2.5 V reference) and 0 V to AVDD and can operate in 12-bit or 8-bit modes. DAC operation is controlled by a single special function (SFR) register, DACCON. Each DAC has two data registers, DACxH/L. The DAC0 and DAC1 outputs share pins with ADC inputs ADC4 and ADC5 respectively. When both DACs are on the number of analog inputs is reduced to four.

Note that in 12-bit mode, the DAC voltage output is updated as soon as the DACL data SFR has been written, therefore, the DAC data registers should be updated as DACH first, followed by DACL.

Initialize ADC (Convert output voltage of sensor to the digital form)

TMOD register is loaded with the value 20H (Use Timer-1 in Mode-2)

The TH1 Is loaded with F7 (Set the baud rate for serial data transfer)

The SCON register is loaded with the value 50H (Serial Mode-1)

TR1 is set 1(To start Timer-1)

Use RS-232 for serial communication

ZigBee technology (For wireless communication)

Fig.3. Flowchart of pH value measurement program

The ZigBee specification provides a security toolbox approach to ensuring reliable and secure networks. Access control lists, packet freshness timers and 128-bit encryption based on the NIST Certified Advanced Encryption Standard (AES) help protect transmitted data.

Fig.2. Schematic of the system

C. Software The software initializes the LCD module. After that initializes DAC module then ADC module. The TMOD register is loaded with the value 20H, indicating the use of timer 1 in Mode2 (8-bit auto-reload). The TH1 is loaded with F7 to set the baud rate for serial data transfer (according to the crystal frequency). The SCON register is loaded with the value 50H, indicating serial Mode1,

IV. CALIBRATION OF PH METER All pH electrodes require calibration time to time. A three point calibration characterizes, electrode with a specific pH meter. Once an electrode is characterized, the electrode/meter pair can be used to determine the pH of a solution. Following procedure performs a three point

232 © 2009 ACEEE

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

calibration. A 10.00 pH, 7.00 pH and a 4.00 pH buffer solution are required.

1) Rinse electrode with DI water to remove all traces of storage solution, process medium or previous test solution. Rinse the electrode after each buffer test to prevent carry over contamination of the pH buffer solutions. Gently blot the electrode on a soft tissue to remove the excess rinse water.

2) Insert the electrode in 4.00 pH buffer solution and wait for 30 seconds to reach thermal equilibrium with this buffer solution (4.00 pH) and adjust the pH meter with the standard solution 4.00 pH.

ACKNOWLEDGMENT Time has flown by very fast while doing this project work. One of the reasons is that, this project has a lot of scope to improve in many different ways. We are obliged to Dr.Chandra Shekhar, Director, Central Electronics Engineering Research Institute, Pilani, Rajasthan, India for giving us the opportunity to carry out my project in this esteemed institute. We wish to express our deep sense, extreme depth of gratitude to our guide Dr.P.C.Panchariya, for the continuous guidance and support and the invaluable help extended to us, which helped us for the successful completion of this work.

3) Repeat step 1 and 2 for 7.00 pH buffer solution.

4) Repeat step 1 and 2 for 10.00 pH buffer solution.

5) To maximize the precision of the calibration repeat steps 1 to 4.

REFERENCES [1] [2]

Analog Devices ADuC814 Data Sheet.

Muhammad Ali Mazidi, Janice Gillispie Mizidi, The 8051 Microcontroller & Embedded System, 2nd Edition, Prentice

V. RESULT

Hall of India.

The microcontroller based pH meter has been designed by the authors. The output of the pH meter is tabulated in Table-1. The pH meter is designed for the measurement of the pH difference from 0 to 14 with an accuracy of ±0.1. The system has been made simple and low cost by employing ADuC814 microcontroller.Our method uses an ion sensitive field effect transistor (ISFET), which enables the safe and reliable pH measurement in areas in which the use of glass may present a danger. The device can be reset and pH values can be read directly and continuously with the use of AduC814 microcontroller.

[3]

James.W.Stewart

&

Kai

X.

Miao.

The

8051

microcontroller Hardware, software & interfacing, 2nd edition.

[4]

Grattarola,M.,et at., Modeling H+sensitive FET’s with SPICE, IEEE Transactions on electron Devices,vol

39,

No.4, April 1992, pp.813-818 [5]

P. Bergveld, Thirty years of ISFETOLOGY, sensors and Actuators B 88 (2003)1-20.

[6]

Duroux, C. Emde, P Bauerfeind.The ion sensitive field effect transistor (ISFET) pH electrode: A new sensor for

TABLE1.

long term ambulatotory pH monitoring , Ph

COMPARISON OF MEASURED PH VALUE WITH STANDARD BUFFER SOLUTION

[7]

C.S.Rangan,

G.R.Sharma,

and

V.S.V.Mani,

Instrumentation Devices and Systems, Tata McGraw Hill, Actual pH Value of standard buffer solution

Measured pH Value of standard buffer solution by developed System

1993.

4.00

7.00

10.00

4.02

7.06

10.04

4.09

7.05

10.05

4.00

7.01

10.07

4.08

7.00

10.00

4.07

6.98

9.98

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