1. FINAL YEAR PROJECT

2. A MICROCONTROLLER BASED CAPACITIVE PROJECT
NAME OF THE PROJECT
A MICROCONTROLLER BASED CAPACITIVE PROJECT

3. GROUP MEMBERS:- 1.Binita Koley 2.Suparna Bhattacharjee 3.Hrisav Kar
4.Saurav Mitra
5.Snehanshu Sengupta
6.Amartya Das

4. OBJECTIVES OF THIS STUDY
The objectives of this study are as follows:
• To design and construct a differential capacitive sensor.
• To interface the sensor to microcontroller with out any signal conditioning circuit.

5. RATIONALE FOR THE STUDY
To reduce the cost, complexity, area, and power
consumption of sensor electronic interfaces.
Direct interface circuits do not require either an intermediate signal conditioner based on amplifiers or oscillators or an ADC, but the sensor is directly connected to a general-purpose microcontroller.

6. DETAILED METHODOLOGY TO BE USED FOR CARRYING OUT THE STUDY
The direct interface circuit for differential capacitive sensors , relies on measuring the discharging time of several RC circuits using a timer embedded into the μC.

7. THE EXPECTED CONTRIBUTION FROM THE STUDY
A differential capacitive sensor can be interfaced to a general purpose μC without any intermediate
signal conditioning circuit.
Set-up cost is less as compared to it’s effective utilization to measure a very low value capacitance.

8. LIST OF ACTIVITIES TO BE CARRIED OUT TO COMPLETE THE PROJECT
Following activities had been carried out to complete the project:
1. Relevant literature survey.
2. Design of differential sensor.
3. Testing and calibration of the sensor.
4. Interfacing the Sensor to μC .
5. Analysis of data.
6. Results and conclusions.
7. Project preparation.

9. PLACES/ LABS/ EQUIPMENT AND TOOLS REQUIRED AND PLANNING OF ARRANGEMENTS
ELECTRONICS AND COMMUNICATION DEPARTMENT
MICROPROCESSOR LAB

10. EQUIPMENTS AND TOOLS REQUIRED
1. COPPER PLATE
EQUIPMENTS
AND
TOOLS
REQUIRED
2. LCD DISPLAY UNIT
3. 8 BIT MICROCONTROLLER
4. OSCILLOPE
5. PERSONAL COMPUTER AND UNIVERSAL IC PROGRAMMER

11. STATEMENT OF THE PROBLEM
Differential capacitive sensors usually have three parallel plates (the central plate being movable and the outer plates being fixed) and can be represented by two sensing capacitances (C1 and C2 ) with a common electrode as shown in the following fig 1 .

12. Ɛ0 is the electric permittivity of vaccum,
At rest, the central plate is geometrically centered, and hence C1 and C2 are equal to the nominal value C0 . For parallel plate arrangement, C0 is equal to
C0 = Ɛ0 Ɛr Ao / do
Where ,
Ɛ0 is the electric permittivity of vaccum,
Ɛr =the relative permittivity of the dielectric between plates,
A0 =the nominal area of the plates, and
d0 =the nominal distance between plates.
In operation , however the central plate of the sensor moves. Consequently, either the area of the distance between the electrodes changes and hence both C1 and C2 change, but in opposite direction. If the measurand changes the area, then both C1 and C2 linearly changes and can be expressed as
C1 = C0 (1+x) and C2 = C0 (1-x)

13. DIAGRAM OF THE DIFFERENTIAL CAPACITIVE SENSOR FABRICATED

14. METHODOLOGY OF THE STUDY
The interface circuit shown in Fig.3 has been implemented by an AT89C2051UC (Atmel) 8-bit μC running on a MHz oscillator.

16. METHODOLOGY OF THE STUDY(Pin Configuration)
The supply voltage of the μC was VCC = 5.0V
Pin Configuration:-
First, pin 9 and pin 8 of the μC is set high
pin 7 is set low which provide a short path between pin 1 and pin 2 of IC 4066
The capacitance is charged toward VCC through R7 (RI ) for a time interval longer than 5RICEQ ; Once CB is charged to VCC pin 9 is set low pin 8 and pin 7 retain their state and the internal timer of the μC starts.

17. METHODOLOGY OF THE STUDY(Pin Configuration)
CB is discharged toward ground through R8 ( RD ) , when the discharging exponential signal reaches the lower threshold voltage (VTH ) which is adjustable by the internal analog comparator of the μC ( pin 12 and pin 13), the timer stops.
The effect of RI in steady state can be assumed negligible whenever RI << RD.
The timer converts count T to a digital number using a high- frequency clock signal.
Time T1 = count value multiplied by clock period ( Discharge time for Capacitor CB ) is measured .

18. METHODOLOGY OF THE STUDY(Pin Configuration)
Second, Pin 9 and pin 7 of the μC is set high pin 8 is set low, CA is to charged to VCC ;
Once CA is charged to VCC pin 9 is set low pin 8 and pin 7 retain their 10 state and the timer starts and stops when discharge reach the threshold level .
Time T2 ( Discharge time for Capacitor CA ) is measured.
Third, pin 9 ,pin,8 and pin 7 all set high to charge both CA and CB . Pin 9 is set low pin 8 and pin 7 retain state to discharge through RD and T3 ( Discharge time for Capacitor CB + CA) is measured.

19. METHODOLOGY OF THE STUDY(Pin Configuration)
Assuming the linear behavior the discharging times can be expressed as
T1 = KCC0 + KCC0 x
T2 = KCC0 - KCC0 x
T3 = 2KCC0
Which shows that T1 increases with x , T2 decreases with x and T3 is in sensitive to x. Ideally T3 is equal to T1 plus T2 hence its measurement could seem unnecessary. However T1 plus T2 is more sensitive to both x and the parasitic capacitances then T3. Accordingly, the following time-based equation to estimate x:

20. EXPERIMENTAL SET-UP FIGURE

21. INPUT DATA Variation of capacitance is from
5 μf to 25 μf for a variation of distance = 0 to 35cm.

22. ANALYSIS AND FINAL RESULTS
1.The setup is working satisfactorily.
2.It is able to measure both positive and negative displacement with respect to its null position.
3.But the stray effect is not completely eliminated. The result is quite satisfactory comparing its simplicity and low cost.

23. GRAPH OF ACTUAL VS DISPLAYED RESULT

24. CONCLUSION
The differential capacitive sensors can be directly connected to a general-purpose 8-bit microcontroller without using intermediate electronics between them.
The circuit is able to measure (considering it’s simplicity and low cost ) small variation of capacitance satisfactorily.
However stray effect causing some error.

25. ADVANTAGES OF THE PROJECT
1.We are able to design a circuit that is able to measure small variation of capacitance satisfactorily.
2. The circuit is simple.
3.The circuit carries low cost.

26. SCOPE OF FUTURE STUDY
The setup is able to measure small variation of distance satisfactorily.
However micro level measurement presently not possible due to stray effect causing some fluctuation in reading.

27. REFERENCES
[ 1 ] IEEE transaction on instrumentation and measurement, Vol. 59 No. 10, October Interfacing Differential capacitive sensors to microcontrollers: A direct Approach. By ferran Reverter and Oscar casas, member, IEEE.
[ 2 ] L. K. Baxter, Capacitive sensor design and Application, IEEE Press New York, 1997.
[ 3 ] AT 89C 2051 data sheet.

28. ACKNOWLEDGEMENT THANK YOU……..
We take this opportunity to express our thanks and gratitude to all those who have accompanied and helped us with this project work.
Mr.Gautam Maity, Faculty of Electronics and Communication Department for sharing his knowledge with us,his guidance and kind support.
Mr.Mithun Maity,Faculty of Electronics and Communuication for his kind assistance.
The Library Members of our college for helping us to issue the reference books.
Each other of our group for showing dedication and commitment towards this project.
THANK YOU……..