Solid State Pressure Switch Systems

SOLID STATE

PRESSURE SWITCH

SYSTEMS April 7th , 2015

Department of Electronic and Electrical Engineering

Supervisor: Dr. Martin J. Burke

Stephen Brennan Student Number: 11900539 Email Address: [email protected]

1 solid state pressure switch systems

Acknowledgments

I would like to thank the staff of the Electronic and Electrical Engineering Department of

Trinity College Dublin for both providing me with the resources I needed to undertake this

project and for being extremely helpful and supportive over the duration of the project.

In particular I would like to thank Dr. Martin Burke who has been an excellent supervisor

to the project, he offered great support and guidance and was always present when help was

needed.

I would also like to thank Shane Hunt for always being on hand with excellent advice and

suggestions about the project. There were a few problems with the circuit that could not have

been solved without his knowledge.


Abstract

This project consisted of debugging and improving an electronic switch which controls

the pump in an underground water well. The circuit uses a pressure transducer to measure

pressure in a water storage tank and based on user defined pressure limits turns on or off the

pump.

The circuit was soldered onto a stripboard by a past student and the aim was to debug

this board and redesign some elements of the circuit so that it would work more effectively.

The debugging process was carried out on a modular basis. The first module to be debugged

was the pressure detection and signal processing stage, which consists of a unity gain

instrumentation amplifier to convert the differential voltage from the pressure transducer to a

single ended voltage and a low pass filter which aims to reduce the effects of noise which arise

as a result of factors like motor vibration. The second module to be debugged was the Unit

Control and Display stage which consists of an amplifier with switchable gain to convert the

displayed pressure between Bar and PSI and an ADC chip which displays the measured pressure

or the high and low threshold limits over three 7 segment displays. The third module of the

circuit was the final one to be tested; this module included the circuitry for the user defined

pressure limits, the high/low pressure threshold detection and the pump activation control.

Some positive changes were also made to the design of the circuit, just two examples of

this are the bandgap reference diode which replaced a system of OP amps to create a reference

voltage and the low pass filter which was totally redesigned. Many other changes were made to

the circuit which solve the problems experienced by the student last year and these will be

discussed in depth in the design section of the report.

At this moment in time, the final stage of the circuit has been debugged. Due to the time

consuming nature of finding and correcting the problems in the circuit a TRIAC has not yet been

attached to the system, however the TRIAC will be replaced with a simple LED to show that the

board is working correctly.


Contents Acknowledgments ........................................................................................................................... 1

Abstract ........................................................................................................................................... 2

Table Of Figures .............................................................................................................................. 5

1. Introduction ............................................................................................................................ 7

2. Literature Review .................................................................................................................... 8

3. System Design ....................................................................................................................... 10

3.1 Pressure Detection & Conversion .................................................................................. 11

3.1.1 The Pressure Transducer ........................................................................................ 11

3.1.2 The Instrumentation Amplifier .................................................................................... 14

3.1.3The Butterworth Filter .................................................................................................. 18

3.2: User Interface and Unit Control ......................................................................................... 19

3.2.1 The A/D Converter & Seven Segment Display ............................................................. 19

3.2.2 Unit Control .................................................................................................................. 20

3.3 Pump Activation ............................................................................................................. 22

3.3.1 Pressure Limits ............................................................................................................. 22

3.3.2 Threshold Detection .................................................................................................... 25

3.3.3 Pump Activation ........................................................................................................... 28

3.3.4 LED Buffer ........................................................................................................................ 31

4. Experimentation & Debugging .............................................................................................. 33

4.1 Pressure Measurement & Signal Processing .................................................................. 34

4.1.1 Pressure Transducer .................................................................................................... 34

4.1.2 Instrumentation Amplifier ........................................................................................... 34

4.1.3 Butterworth Filter ........................................................................................................ 36

4.2 User Interface and Unit Control ..................................................................................... 38

4.2.1 MAX140 & Seven Segment Display .............................................................................. 38

4.2.2 Unit Control .................................................................................................................. 43

4.3.1 Threshold Detection & Alarms ......................................................................................... 45

5. Conclusion ............................................................................................................................. 48


Appendix ......................................................................................................................................... a

Appendix A: List of components used in the circuit ................................................................... a

Appendix B: Schematic of the project ........................................................................................ b

References ....................................................................................................................................... c


Table Of Figures Figure 1: Block Diagram of the Circuit .......................................................................................... 10 Figure 2:Keller 212R Transducer Characteristics .......................................................................... 12 Figure 3: Wheatstone Bridge Representation of the Keller 212R ................................................ 12 Figure 4: Graph showing the relationship between pressure output at the transducer and the differential voltage measured from the transducer output pins ................................................. 13 Figure 5: The Transducer Output and Instrumentation amplifier shown as a potential divider network ......................................................................................................................................... 14 Figure 6: Unity Gain Instrumentation Amplifier used in this project ........................................... 15 Figure 7: Diagram of Differential Amplifier U2 [7] ........................................................................ 16 Figure 8: Sallen-Key topology 2nd order low pass filter ............................................................... 18 Figure 9: MAX140 Operating circuit [10] ...................................................................................... 19 Figure 10: Unit Control Circuit ...................................................................................................... 20 Figure 11: High and Low Limit setting circuit ................................................................................ 22 Figure 12: Threshold Detection Circuit ......................................................................................... 25 Figure 13: Effect of threshold level oscillation on the output of the comparator [11] ................ 26 Figure 14: Threshold detector with added hysteresis [12] ........................................................... 27 Figure 15: SR Latch [15] ................................................................................................................ 28 Figure 16: NOR Latch Truth Table ................................................................................................. 29 Figure 17: Table detailing the operation of the pump ................................................................. 29 Figure 18: Pump Control Operation .............................................................................................. 30 Figure 19: LED Buffer fed from a logic gate [16] ........................................................................... 31 Figure 20: LED Buffer Circuit ......................................................................................................... 32 Figure 21: Graph showing 20.1 mV offset present in the instrumentation amplifier .................. 34 Figure 22: Improved Instrumentation Amplifier (left), Original Instrumentation Amplifier (right)....................................................................................................................................................... 35 Figure 23: Sallen Key Topology 2nd Order Low Pass Filter, design from the Schematic [14] ...... 36 Figure 24: On-board layout of the Low Pass filter ........................................................................ 37 Figure 25: Simulated Frequency response of the system (simulated using MATLAB's fdatool) .. 38 Figure 26: Schematic for a common anode display [18] .............................................................. 38 Figure 27: MAX140 with A/Z and Buffer pin wiring error (left), MAX140 wired according to datasheet (Right)........................................................................................................................... 39 Figure 28: Differential voltage measured at MAX140 input pins and displayed pressure ........... 40 Figure 29: Connecting the MAX140 to the differential output of the Instrumentation amplifier 41 Figure 30: Unit Conversion using switchable gain at the differential stage ................................. 42 Figure 31: On Board Circuit (left), Circuit in the schematic (right) ............................................... 43


Figure 32: Unit Control Switch with connections to Decimal points on the Seven Segment displays .......................................................................................................................................... 44 Figure 33: Design and Experimental Threshold Values ................................................................ 45 Figure 34: NOR Latch Functionality .............................................................................................. 47


1. Introduction

In rural Ireland where no water supply infrastructure exists, it is common for houses to

draw water from a private well. The private well pumps water into a pressurised storage tank in

which the pressure level is proportional to the quantity of water in the tank, water is then

distributed from the storage tank through the household via water pipes. The relationship

between tank pressure and tank water level means that a pressure transducer can be used to

measure the quantity of water present in the storage tank by representing the pressure as a

voltage. The motor which pumps water into the storage tank is controlled by some kind of

switching device which has the capability to turn the pump off when the water pressure in the

tank reaches a predetermined high threshold value. Consumption of water in the household

will cause the pressure level in the tank to fall and when a predetermined low threshold value is

passed, this switching device will turn the pump on again, thus retaining a water pressure level

in the tank that lies between a high and low pressure thresholds.

The aim of the project is to debug and improve a pressure switch that will drive a motor

to pump water from an underground water supply to a household. At present the device has

three core functions:

1. Measurement of the output from a pressure transducer

2. Display of pressure in Bar or PSI on a seven segment display

3. Comparison of pressure to user set high and low thresholds

4. Activate or deactivate a TRIAC triggered motor if the measured pressure from

the transducer is below or above the user defined threshold respectively.

In addition to the core functions a desired specification has also been outlined for the circuit.

The desired specifications are as follows:

• The error in the value displayed on the seven segment screen must be no more than

±1% of the transducers full scale voltage output (100mV)

• The system must operate within the Extended Industrial temperature range (-40 to 85

degrees Celsius).


2. Literature Review

The majority of pressure control switches found in water supplies are electro-

mechanical and use a snap-action switch. These switches react to a sensor and can rapidly open

or close an electrical circuit. The issue with electro-mechanical switches is that the current

flowing through the contacts cannot be instantaneously reduced to zero when the switch is

turned off and as a result of this a transient arc will momentarily link the separate contacts.

Over time this can cause the premature failure of the switch as the contacts on the switch will

be eroded [1]. In addition to this, arcing caused by switching can cause the switch to seize in the

closed position through contact welding action in a domestic water well this means that the

motor could end up continually running and will ultimately overheat. A further problem with

electro-mechanical switches is the lack of a soft start, this means that the motor can turn on at

a voltage peak and experience a surge of up to ±312𝑉𝑉 which can damage the motor or blow

fuses. The TRIAC used in a Solid State pressure switch system stops this from happening and

instead, the motor is soft started from a low voltage or the voltage crossing point.

With this limitation in mind solid state versions of the switch are a viable option as their

operation lifetime is measured in cycles [2], one cycle is a turning on and off of the switch. Just

so long as the switch remains within the cycle limit outlined by the manufacturer, the device

should work with reliable performance. The underlying problem associated with Solid State

switches though is the cost of purchasing the necessary components required to have a Solid

State Pressure transducer system. As an example, the Keller 316L solid state transducer costs

€144 [3], which does not include the switching system which must be purchased separately.

When searching for a transducer/switch system the cheapest solid state solution was €280 [4].

There were two major resources that were invaluable over the course of the project:

The first resource was the 4C13 (Instrumentation amplifiers and transducers) module note set.

These notes covered areas of circuit design that feature prominently in this project including

Instrumentation Amplifiers, filters and hysteresis. These notes contain the necessary equations

and derivations to check the current circuit design and propose improvements. The other

resource that proved to be extremely useful was the final year project report of Sarah Rouviere;


a past student of Trinity College Dublin who designed the original circuit. The report gave a

valuable insight into her design process and the difficulties which she encountered when

undertaking the project.


3. System Design

Although the design of the system was not a component of this project, debugging the

circuit would have been impossible without knowledge as to how the switch was designed and

how it functions.

Figure 1: Block Diagram of the Circuit

The block diagram displayed in figure 1 shows how the 3 constituent modules of the

circuit work together to provide the full functionality of the pressure switch. The blue area of

the diagram encompasses the pressure measurement and processing of the transducer, the

green area covers the process which leads to the activation or deactivation of the motor and

the orange area contains the Unit control and display elements of the circuit.

Within the pressure measurement and processing section, the differential voltage from

the pressure transducer is fed into a unity gain instrumentation amplifier to obtain a single

ended output. This output is then processed in a second order low pass Butterworth filter

which has a cutoff frequency of 3Hz in order to remove as much AC noise as possible. The noise

present could be as a result of the motor vibrating or from movement in the underground

water well.


The Limit Specification and Threshold Detection part of the circuit takes the single ended

voltage from the pressure detection and signal processing stage and compares it to a user

defined high and low pressure limit using two OP Amps in a window detector configuration. The

resulting voltage from the window detector is then sent to a NOR latch which will activate or

deactivate the motor based on whether the pressure measured by the transducer falls short of

the threshold or exceeds it.

The Display and Unit control module of the circuit uses an ADC to display the voltage

from the output of the signal processing stage or the voltage that represent the alarm limits

based on the position of the switching system. In addition to this, a switchable gain amplifier

coupled with the common anode seven segment displays allow for switching the displayed

pressure from Bars to PSI or vice versa.

3.1 Pressure Detection & Conversion

3.1.1 The Pressure Transducer

The pressure transducer used in the project was a Keller 212R. It is a solid state transducer that

uses a piezoresistive pressure capsule that can detect pressure as a result of the strain applied

to the piezo elements. The Keller 212R can be modelled as a Wheatstone bridge, some key

characteristics of the Keller 212R are:

• Accuracy: The Keller 212R is accurate to ±1% of the full scale pressure (10 Bar), this

means that at 10 Bar the output voltage will be 100𝑚𝑚𝑉𝑉 ± 1𝑚𝑚𝑉𝑉.

• Temperature: As well as guaranteed operation within the Extended Industrial range (-

40 to 85 degrees Celsius), the Keller 212R has Temperature Compensation that ensures

that the strain gauge Δ𝑅𝑅 is consistent with temperature [5].

• Water Resistance: The transducer will be used to keep measure of the pressure in a

water storage tank, water resistance is crucial. The Keller 212R has a stainless steel

body, making it ideal for operation in aqueous environments.

• Longevity: The Keller 212R has a lifetime of millions of pressure cycles meaning it will

serve as a long term solution to the problem of pressure switching.


Parameter Min. Typ. Max. Unit

Pressure Range 0(0) - 10(145) Bar(PSI)

Supply Current - - 2 mA

Supply Voltage 8 10 28 𝑉𝑉𝐷𝐷𝐷𝐷

Span Sensitivity - 0.01 - 𝑚𝑚𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉

Bridge

Resistance (𝑅𝑅𝐵𝐵)

- 3.5 - 𝑉𝑉Ω

Operating

Temperature

-40 - 85 Degrees Celsius

Linearity and

Hysteresis Error

- ±0.2 ±1 %

Figure 2:Keller 212R Transducer Characteristics

Figure 3 shows the Wheatstone bridge representation of the Keller 212R pressure

transducer. If no pressure is applied to the piezoresistive elements then the two voltage

outputs should be equal with respect to ground (𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜1 = 𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜2).

Figure 3: Wheatstone Bridge Representation of the Keller 212R

An important design consideration which is not shown in the above figure is the Voltage

Regulator that is present in the transducer, this regulator determines the DC offset seen when


the voltage output of each individual pin is measured with respect to ground. This voltage can

be altered by adjusting the supply voltage 𝑉𝑉𝐵𝐵. Initially the transducer was run off a +12𝑉𝑉 single

ended supply and this created an offset of 3.9𝑉𝑉 DC at each of the 𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜 pins however due to

common voltage considerations at the MAX140 this was changed to a bipolar source of

+6𝑉𝑉,−4𝑉𝑉 to give a 0𝑉𝑉 DC offset.

The linearity of the transducer was tested prior to carrying out any

debugging on the circuit and it was found that an internal wire had become

severed, thus rendering the transducer inoperable. Once this problem was

addressed however, the transducer showed excellent linearity:

Figure 4: Graph showing the relationship between pressure output at the transducer and the differential voltage

measured from the transducer output pins

The average error in the transistor was found to be 0.9mV out of a full scale

voltage of 100mV meaning that the transducer is accurate to 0.9% of the full

scale output. The testing was carried out on a pressure range of 0-6 bar as after 6 bars the

reference pressure monitor reached its maximum pressure value and released all contained

pressure making measurements beyond this point impossible.

R² = 0.9997

0

10

20

30

40

50

60

70

0 2 4 6

Tran

sduc

er O

utpu

t (m

V)

Pressure (bars)

Graph showing the Transducer Linearity

Volts (mV)

Linear (Volts (mV))

Pressure Volts

(mV)

0.36 4.4

0.79 8.7

0.89 9.8

0.94 10.3

1.34 14.3

1.71 17.6

2.08 21.7

2.22 23.1

2.61 26.9

2.98 31.9

3.17 32.6

3.54 36.3

3.79 38.8

4.24 43.2

4.62 47.1

5.01 50.8

5.16 52.4

5.38 54.7

5.67 57.6


3.1.2 The Instrumentation Amplifier

The output from the pressure transducer is a differential voltage. In order to convert this

differential voltage into a single ended voltage an instrumentation amplifier is used, the

instrumentation amplifier has a large input impedance; typically around 45𝑀𝑀Ω and so acts as a

buffer amplifier for the pressure transducer. This can be proven by modeling the voltage

output of the Pressure transducer as a potential divider circuit where 𝑉𝑉𝑜𝑜 is the output voltage

from the transducer, 𝑅𝑅𝐵𝐵 is the bridge voltage from the transducer and 𝑅𝑅𝑖𝑖 is the input

impedance of the amplifier [7][8].

Figure 5: The Transducer Output and Instrumentation amplifier shown as a potential divider network

The voltage across the input of the instrumentation amplifier can be found by using the

equation for the voltage output of the potential divider:

𝑉𝑉 =𝑅𝑅𝑖𝑖

𝑅𝑅𝐵𝐵 + 𝑅𝑅𝑖𝑖𝑉𝑉𝑜𝑜

Dividing top and bottom by 𝑅𝑅𝑖𝑖 gives the following equation:

𝑉𝑉 =1

1 + 𝑅𝑅𝐵𝐵𝑅𝑅𝑖𝑖

𝑉𝑉𝑜𝑜

It is clear from this equation that if 𝑅𝑅𝑖𝑖 is significantly larger than 𝑅𝑅𝐵𝐵, the voltage at the

instrumentation amplifier will be equal to the output voltage of the transducer. For this reason

the instrumentation amplifier is essential in the design of the circuit.


Figure 6: Unity Gain Instrumentation Amplifier used in this project

The gain of the unity gain amplifier was designed to be unity. It is clear from the lack of

gain resistors and negative feedback resistors in U1 and U3 that the first stage is unity gain, in

order to make the second stage unity gain the differential amplifier configuration U2 will be

explored.

𝑉𝑉1

𝑉𝑉2 𝑉𝑉𝑜𝑜


Figure 7: Diagram of Differential Amplifier U2 [7]

By using the principle of Superposition the overall output voltage can be found, this is

done by finding the contribution to the output voltage of 𝑉𝑉1 and 𝑉𝑉2. From above:

𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜1 = −𝑅𝑅2𝑅𝑅1𝑉𝑉1

If the input contribution from 𝑉𝑉1 is then neglected (ie if 𝑉𝑉1 is tied to ground) it can be seen that

the voltage output from 𝑉𝑉2 is:

𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜2 =𝑅𝑅1 + 𝑅𝑅2𝑅𝑅1

𝑉𝑉+

Where 𝑉𝑉+ is the voltage measured at the output of the potential divider:

𝑉𝑉+ =𝑅𝑅4 + 𝑅𝑅5

𝑅𝑅3 + 𝑅𝑅4 + 𝑅𝑅5𝑉𝑉2

Substituting this into the original equation for 𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜2:

𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜2 =(𝑅𝑅1 + 𝑅𝑅2)

𝑅𝑅1�

𝑅𝑅4 + 𝑅𝑅5𝑅𝑅3 + 𝑅𝑅4 + 𝑅𝑅5

�𝑉𝑉2

The output is the combined contribution from 𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜1 and 𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜2:

𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜 = 𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜1 + 𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜2 = (𝑅𝑅1 + 𝑅𝑅2)

𝑅𝑅1�

𝑅𝑅4 + 𝑅𝑅5𝑅𝑅3 + 𝑅𝑅4 + 𝑅𝑅5

�𝑉𝑉2 −𝑅𝑅2𝑅𝑅1𝑉𝑉1


The values for the resistors are chosen such that 𝑅𝑅1 = 𝑅𝑅2 = 𝑅𝑅3 = 𝑅𝑅4 + 𝑅𝑅5 in order to achieve

unity gain. The final voltage output from the differential amplifier is therefore:

𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜 =𝑅𝑅2𝑅𝑅1

(𝑉𝑉2 − 𝑉𝑉1) =𝑅𝑅4 + 𝑅𝑅5𝑅𝑅3

(𝑉𝑉2 − 𝑉𝑉1)

The differential gain of the amp is therefore:

𝐴𝐴𝑑𝑑 =𝑅𝑅2𝑅𝑅1

=𝑅𝑅4 + 𝑅𝑅5𝑅𝑅3

The Common Mode Rejection ratio of the amplifier is dependent on the mismatch in the

resistors[7]:

𝐶𝐶𝑀𝑀𝑅𝑅𝑅𝑅Δ𝑅𝑅 = �1 + 𝑅𝑅2𝑅𝑅1� � 1

4Δ𝑅𝑅�

When designing the instrumentation amplifier an infinite CMRR is a desirable quality as

we want to reject all common mode voltage and just amplify the difference between the two

input voltages. In order to achieve infinite CMRR 𝑅𝑅1(𝑅𝑅4 + 𝑅𝑅5) = 𝑅𝑅3𝑅𝑅2. In addition to this the

resistor values are set such that 𝑅𝑅1 = 𝑅𝑅2 = 𝑅𝑅3 = 𝑅𝑅4 . In the previous design the value of these

resistors was chosen to be 1𝑉𝑉Ω however in order to ensure that the transducer was not getting

loaded by the instrumentation amplifier, the values of resistors 𝑅𝑅1,𝑅𝑅2,𝑅𝑅3 were increased

to 12𝑉𝑉Ω and resistor 𝑅𝑅4 was chosen to be 10𝑉𝑉Ω.

Another modification that was performed on the original circuit was the addition of the

potentiometer 𝑅𝑅5 which allows for the correction of any offsets present in the instrumentation

amplifier stage due to resistor mismatches. The value for 𝑅𝑅4 was chosen to be less than the

other resistors in case the offset was to be corrected with a value of resistance less than 12𝑉𝑉Ω

and the potentiometer in series allows for a maximum resistance of 14.7𝑉𝑉Ω. The variation in

resistance is 4.7𝑉𝑉Ω and with the 1% resistors the maximum offset that can be expected would

be much less than this meaning that the error that arises due to resistor tolerances could be

effectively dialed out.


3.1.3The Butterworth Filter

The filter that was soldered onto the board was completely removed and replaced with a 2nd

order low pass filter which was designed using the Sallen-Key topology. The main design

consideration for the filter was that it was to remove any signals that were above 3Hz.

Figure 8: Sallen-Key topology 2nd order low pass filter

The key design parameter for a low pass filter is the cut off frequency, 𝑓𝑓𝐷𝐷 which represents the

point at which the ratio of the input and output voltages has a magnitude of 0.707 or -3dB. In

this case the cutoff frequency will be set to 3Hz. First, the cutoff frequency is converted into

rad/s and the relationship between the cutoff frequency and the resistors/capacitors is shown:

𝜔𝜔𝑐𝑐 = 2𝜋𝜋𝑓𝑓𝑐𝑐 = 1�𝑅𝑅6𝑅𝑅7𝐷𝐷1𝐷𝐷2

For a second order filter, the Q factor is 1.414 and in order to have unity gain:

𝐶𝐶1 ≥ 4𝑄𝑄2𝐶𝐶2 ≈ 10𝐶𝐶2

𝐶𝐶1 was chosen to be 1𝜇𝜇𝜇𝜇 arbitrarily and from this 𝐶𝐶2 was then chosen to be 100𝑛𝑛𝜇𝜇.


With 𝑓𝑓𝑓𝑓 = 3𝐻𝐻𝐻𝐻 the resistor values could be worked out and the equation became:

3𝐻𝐻𝐻𝐻 =1

2𝜋𝜋�𝑅𝑅1𝑅𝑅2 × 100𝑓𝑓𝜇𝜇

Rearranging the equation:

𝑅𝑅1𝑅𝑅2 =1

4𝜋𝜋2 × 100𝑓𝑓𝜇𝜇 × 9𝐻𝐻𝐻𝐻= 28.14477 × 109Ω

The values for 𝑅𝑅1 and 𝑅𝑅2 were then chosen to be 270𝑉𝑉Ω and 100𝑉𝑉Ω as:

270𝑉𝑉Ω × 100𝑉𝑉Ω = 27 × 109Ω

This is sufficiently close to the desired value.

3.2: User Interface and Unit Control

3.2.1 The A/D Converter & Seven Segment Display

The ADC used in this project was the MAXIM Max140 chip. Essentially the MAX140 acts

as a voltmeter on chip and has 22 digital outputs which are connected to each segment of the

LED Displays.

Figure 9: MAX140 Operating circuit [10]


The key difference between the Operating circuit shown in Figure 8 and the operating

circuit decided upon in this project is the supply voltage to the chip. In order to bias the

Common Voltage pin at approximately 0V the Common Pin was connected to the low reference

voltage and the supply voltage was changed from +5V referenced to ground to +3V, -2V. This

meant that the IN LO pin could be connected to ground and the voltage from the unit

Conversion circuit could be taken single-ended at the IN HI pin. The process involved with

deciding on the supply voltages will be discussed in the debugging section of this report.

The seven segment displays used in this project were common anode, meaning that the

segment would illuminate when the corresponding pin was connected to ground. This element

of the display was exploited in the Unit conversion stage to allow for the shifting of the decimal

place depending on whether the display was to show pressure in Bars or PSI.

3.2.2 Unit Control

Figure 10: Unit Control Circuit


The unit control circuit consists of an OPAMP with switchable gain. In the position shown in

figure 10 the system is in PSI mode, meaning that the display will show the pressure and alarms

in PSI. The transducer outputs 10mV per bar of pressure so if the displayed pressure is to be in

PSI, the voltage at the Unit control stage will need to be altered to show PSI on screen. The

equation for converting from Bar to PSI is:

1𝐵𝐵𝑉𝑉𝐵𝐵 ≈ 14.5𝑉𝑉𝑃𝑃𝑃𝑃

When the unit conversion switch is moved from Bar the PSI, the following two events occur:

• The Unit Control Op Amp gain is made equal to 1.45

• The decimal point on the leftmost screen is disconnected from ground and the decimal

point connected to the middle screen is connected to ground, thus illuminating the

middle decimal point.

𝑅𝑅10 and 𝑅𝑅11 are connected to ground and 𝑅𝑅9 is in the feedback loop. This means that the gain

of the amplifier is:

𝐴𝐴𝑑𝑑 = 1 + �𝑅𝑅9

𝑅𝑅10 + 𝑅𝑅11�

In order to obtain a gain of 1.45 resistor values were chosen to be:

𝑅𝑅9 = 10𝑉𝑉Ω,𝑅𝑅10 = 22𝑉𝑉Ω,𝑅𝑅11 = 2𝑉𝑉Ω.

The potentiometer 𝑅𝑅11 allows for the gain to be set to exactly 1.45 and removes any

errors that arise due to resistor tolerances.

It can be seen in Figure 10 that the gain of the OP Amp depends on the position of the

DPDT switch. In the configuration shown, there are resistors in the feedback loop which give

the gain of 1.45, however when the switch is in the Bars position, the resistors are removed

from the feedback loop and as a result the op amp is one of unity gain.

Resistor 𝑅𝑅8 is used to prevent noise that might occur during the switching action and

corrupt the displayed signal. This interference is stopped by setting 𝑅𝑅8 to be a very high value of

10𝑀𝑀Ω.


3.3 Pump Activation

3.3.1 Pressure Limits

The switching system present in the system has been modified from the original design

based on the availability of components in the lab. In the original design a three-way switch was

to be used to switch between Pressure, High alarm and Low alarm. In the modified circuit, this

has been replaced with two SPST switches, one to switch between Pressure and Alarms and the

other to select the High or Low Alarm.

Figure 11: High and Low Limit setting circuit

The Pressure limits in the system determine when the pump is turned on and off. A High

pressure limit and a low pressure limit can be set by the user by adjusting the value of the

potentiometers 𝑅𝑅19 and 𝑅𝑅21 as shown in figure 11 above. The modified circuit was initially

designed around limits that were chosen for the high and low pressure ranges chosen by the

student last year. However at a later stage, these were changed to reflect more realistic values

for the high and low pressure thresholds of a water storage tank.

Bar PSI

High Limit Range 1-7 14.5-101

Low Limit Range 0.5-4 7.25-58

𝑉𝑉𝑅𝑅𝑅𝑅𝑅𝑅


The circuit in Figure 11 also shows that the High Limit can never be lower than the low

limit as the low limit is essentially the high limit passed through an additional potential divider.

The high reference threshold is set using a potential divider network connected to a

reference voltage source. In this case, the reference voltage source is a bandgap diode voltage

reference. The LT1004 bandgap diode used in the project provides excellent accuracy and

stability over a large temperature range. The diode is fed current from the resistor 𝑅𝑅16 which

was chosen to generate enough current to send the diode into reverse breakdown mode. When

in reverse breakdown, the voltage at 𝑉𝑉𝑅𝑅𝑅𝑅𝑅𝑅 will be equal to 1.23V, this voltage needs to be

dropped again so that the high pressure limit can be swept from 10mV to 70mV by adjusting

the potentiometer 𝑅𝑅19. The values for resistors 𝑅𝑅18,𝑅𝑅19 and 𝑅𝑅26 were calculated using

simultaneous equations:

Equation 1:

10𝑚𝑚𝑉𝑉 =𝑅𝑅26

𝑅𝑅18 + 𝑅𝑅26𝑉𝑉𝑅𝑅𝑅𝑅𝑅𝑅

Equation 2:

70𝑚𝑚𝑉𝑉 =(𝑅𝑅26 + 𝑅𝑅19)

𝑅𝑅18 + 𝑅𝑅26 + 𝑅𝑅19𝑉𝑉𝑅𝑅𝑅𝑅𝑅𝑅

Rearranging Equation 1:

10𝑚𝑚𝑉𝑉 × 𝑅𝑅18 = (𝑉𝑉𝑅𝑅𝑅𝑅𝑅𝑅 − 10𝑚𝑚𝑉𝑉)𝑅𝑅26

Equation 3:

𝑅𝑅18 = 122.45𝑅𝑅26

Substituting Equation 2:

𝑅𝑅26 + 𝑅𝑅19 =70𝑚𝑚𝑉𝑉𝑉𝑉𝑅𝑅𝑅𝑅𝑅𝑅

(𝑅𝑅18 + 𝑅𝑅26 + 𝑅𝑅19)


Moving all of the 𝑅𝑅26 + 𝑅𝑅19 terms to one side:

�1 −70𝑚𝑚𝑉𝑉𝑉𝑉𝑅𝑅𝑅𝑅𝑅𝑅

� (𝑅𝑅26 + 𝑅𝑅19) =70𝑚𝑚𝑉𝑉𝑉𝑉𝑅𝑅𝑅𝑅𝑅𝑅

𝑅𝑅18

𝑅𝑅18 = 16.6357(𝑅𝑅26 + 𝑅𝑅19)

Substituting from Equation 3 above:

122.45𝑅𝑅26 = 16.6357(𝑅𝑅26 + 𝑅𝑅19)

𝑅𝑅19 ≈ 6𝑅𝑅26

𝑅𝑅18 = 122.2𝑅𝑅26

Using standard resistor values:

𝑅𝑅18 = 100𝑉𝑉

𝑅𝑅26 = 1𝑉𝑉

𝑅𝑅19 = 4.7𝑉𝑉

In the same way, simultaneous equations were used to calculate the values for the

resistors in the low pressure threshold. The chosen values were:

𝑅𝑅20 = 470Ω

𝑅𝑅22 = 470Ω

𝑅𝑅21 = 2.5𝑉𝑉Ω

Using these configurations the thresholds changed slightly due to using standard resistor

values. This unfortunately lead to undesirable effects in the low limit range particularly, where

there was found to be a tradeoff between using a low minimum threshold and having an upper

minimum threshold that was much lower than the high limit threshold. The high and low limit

ranges with these resistor values were found to be:

Low Limit Range High Limit Range

0.5-6.2 Bar 1.1-6.4 Bar


3.3.2 Threshold Detection

Figure 12: Threshold Detection Circuit

A window detector circuit was used to determine whether the pump would be activated

or deactivated based on the pressure, P coming from the signal processing stage of this circuit.

A window detector circuit is one which checks if an unknown voltage lies between two

threshold values [10], by using two Op Amps as comparators. In the case of the pressure switch,

the pressure is being compared to the user defined high and low pressure thresholds.

In the window detector configuration in Figure 12, the output of the Op Amp U7 will be

high if the pressure input P exceeds the current high voltage threshold. The output of the Op

Amp U8 will be high if the pressure input is lower than the current low voltage threshold.

One of the primary concerns associated with the window detector and comparator

circuits is small oscillation around the threshold value caused by noise which results in the rapid


turning on and off of the switch. This problem is shown in Figure 13 in the case of the High

threshold comparator Op Amp. Obviously this also occurs at the low threshold comparator as

well. In order to fix this problem, hysteresis is added to the window detector by adding a

resistor in the positive feedback loop of each of the comparator op amps.

Figure 13: Effect of threshold level oscillation on the output of the comparator [11]

Adding hysteresis to the window detection means that there are now 2 threshold limits

per comparator which come into effect depending on which way the input is transitioning. This

solves the problem of rapid output switching as the threshold value is reduced when the

threshold limit is passed by an increasing input signal and increased when the threshold limit is

passed by a decreasing input. This effect can be seen clearly in figure 14.


Figure 14: Threshold detector with added hysteresis [12]

The minimum value for the hysteresis voltage can be calculated by taking the ratio of

the amplifier supply voltage to the open loop gain of the amplifier.

𝑀𝑀𝑀𝑀𝑛𝑛𝑀𝑀𝑚𝑚𝑀𝑀𝑚𝑚 𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐵𝐵𝐻𝐻𝐻𝐻𝑀𝑀𝐻𝐻 =10

2 × 105= 50𝜇𝜇𝑉𝑉

However the datasheet of the OP77FZ Op Amp states the maximum noise input is 65𝜇𝜇𝑉𝑉

so the hysteresis voltage was chosen to be 2.5mV [12]. This voltage is not so large as to

potentially change the logic HI or LO output and it is a convenient value to pick as it simplifies

the calculation of the resistor values required in the circuit.

On examination of the circuit it was found that the configuration of the resistors in the

window detector could be modeled as potential dividers. There are 2 possible equivalent

potential dividers per comparator; the first is when the output is registered as high, and the

second is when the output will be registers as low. In the case of the High threshold detection

circuit, the output will be high when the input exceeds the high pressure threshold limit and in

the case of the Low threshold detection circuit, the output will be high when the input pressure


is less than the low pressure threshold limit. The values for these resistors had been calculated

by Sarah Rouviere last year and upon testing the hysteresis component of the circuit, it was

found that the window detector was working correctly. The resistor values were calculated to

be [13]:

𝑅𝑅23 = 470Ω

𝑅𝑅24 = 10𝑀𝑀Ω

𝑅𝑅25 = 10𝑀𝑀Ω

The outputs of the window detector were passed through Schmitt triggers in order to define

the high and low states of the window detector.

3.3.3 Pump Activation

Two NOR gates were used in a NOR Latch configuration to control the activation and

deactivation of the pump. Figure 15 shows the logic block circuitry of the NOR latch. In its

implementation in the circuit, the HI output from the window detector is entering the latch at

the R terminal and the LOW output of the window detector is entering the latch at the S

terminal.

Figure 15: SR Latch [15]

The truth table for the NOR latch is shown in Figure 16


𝐇𝐇𝐇𝐇 (𝐑𝐑) 𝐋𝐋𝐋𝐋 (𝐒𝐒) 𝐐𝐐 𝐐𝐐 𝐀𝐀𝐀𝐀𝐀𝐀𝐀𝐀𝐀𝐀𝐀𝐀

0 0 Q 0 Hold state

0 1 1 0 Set

1 0 0 1 Reset

1 1 0 Q Not Allowed Figure 16: NOR Latch Truth Table

There are three different combinations that will occur in the circuit; the first is when the

pressure falls below the low threshold and the pump must be turned on, the second is when

the pressure in the tank exceeds the high threshold and the pump must be turned off and the

third is when the pressure in the tank is neither above the high pressure threshold or below the

low pressure threshold in which the motor should be ‘held’ in its current state. From the values

in the truth table above, it makes the most sense to take the output Q as the pump controller.

The truth table for the three pump action scenarios can be seen in figure 17.

HI LO Q Pump Action

P<LO 0 1 1 ON

LO<P<HI 0 0 Q Hold

P>HI 1 0 0 OFF Figure 17: Table detailing the operation of the pump

Figure 18 shows the overall pump operation. It can be seen that when B is HI the pump is

turned on. When both A and B are LO the pump will hold its current state until either A or B

changes. Finally when A is HI the pump will be turned off.


Figure 18: Pump Control Operation

01234567

0 20 40 60 80 100 120

Pres

sure

(bar

)

Time

Pressure

Pressure

-2

0

2

4

6

LO

HI

Time

A

A

-2

0

2

4

6

LO

HI

Time

B

B

-2

0

2

4

6

Pum

p O

n/O

ff

Time

Pump ON/OFF

pump on

PUMP ON PUMP ON PUMP OFF


3.3.4 LED Buffer The initial aim of the design was to have a TRIAC soft start a motor which would suck water up

from the underground well. However due to time constraints on the project an LED Buffer was

designed instead to indicate if the circuit was functioning or not. The LED Buffer would

illuminate if the pump was to be activated and turn off when the pump was to be deactivated.

Figure 19: LED Buffer fed from a logic gate [16]

The transistor used in the LED buffer was a BC107 Bipolar Junction transistor. 𝑉𝑉𝑂𝑂𝑂𝑂𝑂𝑂𝑖𝑖𝑂𝑂 = 0.6𝑉𝑉𝐷𝐷𝐷𝐷

where the supply voltage is 6 Volts. The transistor has a 𝑉𝑉𝐵𝐵𝑅𝑅𝑆𝑆𝑆𝑆𝑆𝑆 = 0.75𝑉𝑉 and 𝑉𝑉𝐷𝐷𝑅𝑅𝑆𝑆𝑆𝑆𝑆𝑆 = 0.25𝑉𝑉

The minimum 𝛽𝛽𝑓𝑓= 110. If 𝑃𝑃𝑅𝑅 = 30𝑚𝑚𝐴𝐴 (corresponding to 100mcd) then the forward voltage drop

across the diode can be found to be 1.5V [17].

The voltage drop across 𝑅𝑅𝐷𝐷 can be calculated as following:

𝑉𝑉𝑅𝑅𝐷𝐷 = 𝑉𝑉𝐷𝐷𝐷𝐷 − 𝑉𝑉𝐿𝐿𝑅𝑅𝐷𝐷 − 𝑉𝑉𝐷𝐷𝑅𝑅𝑆𝑆𝑆𝑆𝑆𝑆 = 6𝑉𝑉 − 1.5𝑉𝑉 − 0.25𝑉𝑉 = 4.25𝑉𝑉

𝑅𝑅𝐷𝐷 =𝑉𝑉𝑅𝑅𝐶𝐶𝑃𝑃𝑅𝑅

=4.25

30𝑚𝑚𝐴𝐴= 141Ω


We will use a 150Ω resistor in order to keep the values standard. The power rating of the

resistor would be:

𝑉𝑉𝑅𝑅𝐶𝐶 = 𝑉𝑉𝑅𝑅𝐶𝐶 × 𝑃𝑃𝐷𝐷 = 4.25 × 30𝑚𝑚𝐴𝐴 = 127.5𝑚𝑚𝑚𝑚

So a 250mW resistor would be used.

The base current can then be calculated as the collector current divided by the current gain:

𝑃𝑃𝐵𝐵𝐸𝐸𝐸𝐸𝑆𝑆 =𝑃𝑃𝐷𝐷𝛽𝛽𝑅𝑅

=𝑃𝑃𝑅𝑅𝛽𝛽𝑅𝑅

=30𝑚𝑚𝐴𝐴

110= 0.27𝑚𝑚𝐴𝐴

To account for temperature variations the overdrive factor is set to 2 so:

𝑃𝑃𝐵𝐵 = 𝜎𝜎𝐿𝐿𝑃𝑃𝐵𝐵𝐸𝐸𝐸𝐸𝑆𝑆 = 2 × 0.27𝑚𝑚𝐴𝐴 = 0.51𝑚𝑚𝐴𝐴

Now 𝑅𝑅𝐵𝐵 can be determined:

𝑅𝑅𝐵𝐵 =𝑉𝑉𝑅𝑅𝐵𝐵𝑃𝑃𝐵𝐵

=𝑉𝑉𝑂𝑂𝑂𝑂𝑀𝑀𝑀𝑀𝑀𝑀 − 𝑉𝑉𝐵𝐵𝑅𝑅𝑆𝑆𝑆𝑆𝑆𝑆

𝑃𝑃𝐵𝐵=

0.6 × 6 − 0.750.51𝑚𝑚𝐴𝐴

= 5.6𝑉𝑉Ω

In addition to this it is good practice to place a large value resistor on the Base Emitter Junction

so a 100𝑉𝑉Ω resistor will be placed in addition to 𝑅𝑅𝐵𝐵 and 𝑅𝑅𝐷𝐷.

Figure 20: LED Buffer Circuit


4. Experimentation & Debugging

“Debugging time increases as a square of the program’s size.”

— Chris Wenham

The main component of this project was the debugging of the circuit that was designed last

year. In this chapter, the circuit will be stepped through one module at a time and the bugs that

were found will be described. In addition to this, the corrective measures carried out on the

circuit will be outlined and the subsequent experimentation that was performed on the system

will be documented.


4.1 Pressure Measurement & Signal Processing

4.1.1 Pressure Transducer

The very first problem that was encountered with the circuit was that the transducer

itself was producing zero volts across the entire scale of pressures. The source of the

problem was isolated using the continuity tester on the multimeter and it was found that

one of the internal wires of the transducer had become severed. After this wire was

connected the transducer produced the expected differential voltage of 10mV per Bar of

pressure.

4.1.2 Instrumentation Amplifier

Figure 21: Graph showing 20.1 mV offset present in the instrumentation amplifier

The major problem encountered by the student last year was a 20.1mV offset that

occurred at the output of the instrumentation amplifier. This is a significant problem as 20.1

mV is over 20% of the full scale output of the transducer (100mV) meaning that the results

obtained will be very inaccurate. The graph in Figure 21 shows the constant 20.1mV offset

present at the output of the instrumentation amplifier stage. Numerous methods were

tried in an attempt to correct the offset including:

0

10

20

30

40

50

60

70

80

90

0 2 4 6 8

Out

put (

mV)

Pressure (Bar)

Output from Transducer and Output from Instrumentation Amplifier

Output From Transducer(mV)

Output fromInstrumentation Amplifier


• Replacing the three Op Amps that made up the instrumentation amplifier

• Increasing the resistor values in the Instrumentation amplifier from 1𝑉𝑉Ω to 12𝑉𝑉Ω

to prevent the circuit from loading the transducer with current

• Added a potentiometer in series with the grounded resistor on Op Amp U2 in

order to dial out the unexpected resistance which arises due to resistor

tolerances.

The offset was successfully removed by adding the potentiometer to the ground rail at Op Amp

U2. The problem of the offset arose as a result of the unaccounted for resistor tolerances

(±1%) changing the CMRR of the instrumentation amplifier.

Figure 22: Improved Instrumentation Amplifier (left), Original Instrumentation Amplifier (right)


4.1.3 Butterworth Filter

The Butterworth Filter was the next component of the circuit to be debugged. There were

numerous problems with the filter present on the board:

• The Op Amp overheated when power was connected to the circuit

• The Design on the board did not match the design in the schematic

• The capacitor values on the board did not match the values on the schematic

The Schematic design of the filter was a 2nd order low pass butterworth filter designed using the

Sallen-Key topology, according to the schematic [14]:

𝑅𝑅6 = 2.2𝑉𝑉Ω

𝑅𝑅7 = 3.3𝑉𝑉Ω

𝐶𝐶1 = 6.8𝑛𝑛𝜇𝜇

𝐶𝐶2 = 0.1𝜇𝜇𝜇𝜇

Figure 23: Sallen Key Topology 2nd Order Low Pass Filter, design from the Schematic [14]

However the design on the board was quite different to the schematic. There were

found to be resistors in the negative feedback loop which were not present in the schematic,

nor were they mentioned in the System Design process of the report. Figure 24 shows the

configuration that was soldered onto the strip board.


Figure 24: On-board layout of the Low Pass filter

Before testing the on-board filter the Overheating problem was addressed. In most

cases, overheating occurs when the output of the Op Amp is connected to ground. By using the

continuity tester on the multimeter it was discovered that two of the horizontal strips on the

strip board were connected by a piece of stray solder thus connecting the output of the filter to

ground. This was amended by using a soldering iron to melt the stray solder and a solder sucker

to remove the solder from the board. Although this solved the problem of the OP Amp

overheating, the frequency response of this part of the system showed that it was not working

as a low pass filter; instead the output was 0V at all frequencies. At this stage the decision was

made to redesign the filter using the Sallen-Key topology. The design of this filter is covered in

Chapter 3 of this report. The frequency response of the filter showed a cutoff frequency of just

over 3Hz, the frequency response in Figure 25 is a simulated version of the filter which is now a

part of the signal processing system.


Figure 25: Simulated Frequency response of the system (simulated using MATLAB's fdatool)

4.2 User Interface and Unit Control

4.2.1 MAX140 & Seven Segment Display

The MAX140 chip was the first part of the User Interface to be debugged and experimented

on. In order to do this, the Unit control stage was disconnected and the input of the MAX140

was wired to the output of the signal processing stage. On initial inspection there was no

indication present that the chip was functioning at all, the main symptom of this problem was

the absence of activity on the seven segment displays. Before testing any of the features of the

MAX140, the seven segment displays were tested by connecting each of the pins to ground

when the power of the system was on. As the seven segment displays are common cathode,

connecting the pins to ground illuminates the segment associated with that pin. The schematic

for the common anode seven segment display is shown in Figure 26.

Figure 26: Schematic for a common anode display [18]


After it was verified that the seven segment displays were functional, the pins on the

MAX140 chip were tested using the multimeter. The first problem that was addressed was that

the 𝑉𝑉+ pin on the MAX140 was not receiving a voltage when it should have been receiving 5V.

Through using the continuity tester on the multimeter it was found that the wire connecting

the 5V rail to the pin of the MAX140 had become disconnected, after re-connecting this wire

and referencing the ground of the 5V supply to the ground of the bipolar voltage source that

provides power to the transducer and Op Amps, this problem was solved and the screen began

to display random numbers.

While checking the other pins on the MAX140 a major discontinuity was found between

the wiring schematic on the datasheet and the wiring that was on the board. The error lies

within the Buffer and A/Z pins on the MAX140. Figure 27 shows that the buffer pin should

connected to a resistor which is then connected to the capacitors on the A/Z pin and the

integrator pin. However the wiring on the board had the resistor connected in series with the

capacitor on the A/Z pin. When this wiring issue was amended the seven segment display

began to show numbers that in some way resembled the input voltage from the output of the

signal processing stage.

Figure 27: MAX140 with A/Z and Buffer pin wiring error (left), MAX140 wired according to datasheet (Right)

When this wiring problem was solved the display on the screen began to show pressure

values that were approximately the same as the ones on the reference pressure monitor,

however it was found that as the pressure was increased the pressure displayed on screen

became un-synced from the reference pressure monitor reading. The readings from the display

are shown in Figure 28.


Figure 28: Differential voltage measured at MAX140 input pins and displayed pressure

As is shown in Figure 28, the display on screen is similar to the differential input voltage

measured at the IN HI and IN LO pins of the max 140 at low pressures, but as the pressure

increases, the results on the seven segment display lose linearity. At 5.6 Bar, the displayed

pressure was only 4.5 Bar; an inaccuracy of nearly 20%.

The first attempt to solve the problem involved checking all of the components attached

to the MAX140 chip. Over the course of this process it was found that the potentiometer 𝑅𝑅14

was faulty as changing the position of the sweeper did not affect the resistance of the

potentiometer and therefore did not change the value for REF HI. On replacing the

potentiometer, the nonlinear characteristics of the display persisted meaning that the source

problem lay elsewhere.

After consulting the MAX140 datasheet [9] it was found that the IN HI and IN LO pins of

the MAX140 chip are designed for a differential voltage input as opposed to a single ended

input. In order to test whether this would fix the problem the output from the differential stage

0 1 2 3 4 5 6

Mea

sure

d Vo

ltage

(mV)

Pressure (Bars)

Output Voltage measured differentially and Output voltage shown on Max140 Display

Display Differential

60

50

40

30

20

10


of the instrumentation amplifier was directly connected to the input of the MAX140 as shown

in Figure 29.

Figure 29: Connecting the MAX140 to the differential output of the Instrumentation amplifier

Although connecting the OP Amp to the MAX140 differentially did not solve the non-

linearity issue, it was an active factor in finding the solution to the problem. On consulting the

datasheet once again, it was found that the maximum Analog Common Voltage was 3.15V, this

was much smaller than the voltage that was being input to IN HI and IN LO which was 3.9𝑉𝑉 at 0

Bar and increased 10mV with every bar. In order to tackle the nonlinearity problem with the

display, new supply voltages would have to be selected so that the voltage output from the

transducer would be reduced. The aim was to select a supply voltage that would cause the

output of the transducer to be approximately 2.5𝑉𝑉 at 0 Bar as opposed to 3.9𝑉𝑉. The supply

voltages chosen were +6𝑉𝑉 and −4𝑉𝑉. With these supply voltages in place the configuration

outlined in Figure 29 functioned correctly and pressure from the transducer was shown in Bars

on the seven segment displays.


However this configuration caused 3 more problems:

• The unit switching stage was now disconnected so a new method for switching

units would have to be designed

• The low pass filter which removed AC noise above 3Hz is no longer acting in the

circuit

• The entire design for the alarm system would have to be redesigned to work

with the new differential input

Solving the first problem was relatively simple, the differential stage of the instrumentation

amplifier could be modified in order to have switchable gain, much like the single ended version

that was on the circuit prior to the modification. In order to do this resistors would have to be

added to the negative feedback loop of the instrumentation amplifier. An example of this

configuration can be seen in Figure 30. It is clear that moving the switch connects/disconnects

the gain resistor to the negative feedback loop.

Figure 30: Unit Conversion using switchable gain at the differential stage

Although this was a valid solution to the first problem, the second two problems proved

to be more difficult so the idea of taking the voltage single endedly with respect to ground was

revisited. In order to make this solution work, the supply voltages for the MAX140 chip would

have to be changed in order to set the analog common voltage to be as close to 0V as possible.


This was accomplished by changing the supply voltage of the chip from +5𝑉𝑉 with respect to

Ground to +3𝑉𝑉,−2𝑉𝑉. Referencing the Analogue Common Voltage to 0V means that a

differential voltage can be sent in on the IN HI pin with a 0V DC offset and the IN LO pin can be

connected to ground.

4.2.2 Unit Control

The first trivial problem with the Unit selection stage was that the op amp itself was

wired upside down, meaning that instead of negative feedback, it has positive feedback and the

pressure signal entered through the inverting input of the amp. Once this minor problem was

addressed the wiring of the Unit Control system was examined.

The main problem with the unit control stage was the absence of the switching

mechanism. According to the schematic a single pole single throw switch should have been

present at the ground connection of the OP Amp to give a gain of 1.45, however this switch was

missing. After the installation of the switch there was still problems that arose from the resistor

in the negative feedback loop. Figure 31 shows the schematic design against the circuit which

was found on the board.

Figure 31: On Board Circuit (left), Circuit in the schematic (right)


When the S1 switch was disconnected the gain of the amp should have become unity,

however the output of the amp instead displayed -3.71V over the entire range of pressure. It

was found that directly connecting the negative input pin to the output of the amp fixed this

problem so a Double Throw Double Throw switch was used instead to connect the negative

input pin to the output of the OP Amp when the resistors were disconnected from ground. This

solution worked and the Op amp has a gain that is switchable between 1.45 and unity.

The other problem that was noticed with this part of the system was that the decimal

places on the seven segment displays were not connected. As the Seven Segment displays are

common Anode [19] the task becomes simply a matter of connecting the middle decimal place

and leftmost decimal place pins of the seven segment displays to the pre-existing DPDT switch

as shown in figure 32. When this configuration was implemented, the pressure displayed on the

seven segment displays in either Bar or PSI depending on the position of the DPDT switch. At

this point the pressure sensing and signal processing stage was working correctly, as well as the

unit control and user interface.

Figure 32: Unit Control Switch with connections to Decimal points on the Seven Segment displays


4.3.1 Threshold Detection & Alarms In original schematic design a pair of OP Amps were used to buffer the Voltage from the

REF HI pin on the MAX140 chip to use as a reference voltage for the high and low pressure

alarms. However a dedicated bandgap reference diode was used instead to create a reference

voltage so these two Op Amps were removed. One of the initial problems with removing these

Op Amps was that the power supply connections for some of the other Op Amps were directly

dependent on the original reference voltage op amps being in place so the power supply wiring

had to be completely redone for the three Op Amps in the Threshold Detection and Alarms

section.

Following the rewiring of the OP Amps it was found that the alarm values were slightly

different to the expected however these values are not sufficiently far away from the design

values to impede the regular operation of the pressure switch.

Pressure Ranges High Threshold Low Threshold

Design 1 Bar-7 Bar 0.5 Bar-4 Bar

Actual 1.1 Bar – 6.8 Bar 0.8 Bar-4.8 Bar Figure 33: Design and Experimental Threshold Values

After the rewiring of the power supply connections had been completed it was noticed

that the Low Pressure alarm value was not displaying on the seven segment display when the

option was selected, instead the display jumped randomly, when the input pins of the MAX140

were tested using the multimeter it was found that a large 3.5𝑉𝑉 input voltage was found at the

IN HI pin. The maximum voltage at this pin should be no more than 100mV. Numerous attempts

were made to fix this problem, firstly the OP Amp was replaced and it was noted that the

problem persisted. Following this, the Op Amp was removed and the large 3.5V voltage

disappeared from both the input of the MAX140 and the output pin of the amplifier, suggesting

that the problem was not a result of any bad wiring on the board, but rather the amplifier itself.

On closer examination it was found that the offset pins of the amplifier had a potential

difference of 3.5 volts between them, this was the cause of the large voltage output.


In order to fix this problem the stripboard was drilled in such a way as to break any possible

connection to the offset pins meaning that there was no voltage flowing to the pins and as a

result the offset disappeared.

At this point the alarms were working and displaying correctly on the seven segment

display however there was a wiring problem with the window detector circuit. The resistors in

the positive feedback loop of the individual comparators which give the comparators hysteresis

were connected instead to the negative pin, meaning that the resistors were in the negative

feedback loop.

Once this problem was fixed and it was verified that the circuit was wired as it was in the

schematic the power was connected to the circuit and it was discovered that the Op Amps in

the window detection circuit were overheating. Initially the cause was believed to be that the

output of the Op Amps were connected to ground and the continuity tester on the multimeter

verified this, but when the power was disconnected and the connection was tested again, the

continuity test failed, meaning that the output of the comparators were effectively connected

to ground only when the power was turned on. After consulting with Dr. Burke on the problem

it was clear that the overheating Op Amps were caused by different supply voltages being used

on the Window Detector (powered from +6𝑉𝑉,−4𝑉𝑉) and the Schmitt Trigger (𝑉𝑉𝑃𝑃𝑃𝑃𝐻𝐻𝐵𝐵𝐻𝐻𝑃𝑃 𝑏𝑏𝐻𝐻 +

3,−2𝑉𝑉) the two proposed solutions to the problem were:

1. Change the supply voltage of the Schmitt trigger to match that of the Window

detector

2. Place a resistor in between the output of the comparator Op Amps and the

Window Detector

Initially the first solution was explored but unfortunately the maximum supply voltage that

could be given to the SN74HC14 was 6V so the second solution was the only viable option. A

12K resistor was placed at the output of each of the comparator Op Amps and the overheating

problem was solved.


At this point it was discovered that the Schmitt Trigger chip was blown and it was not

inverting the output signals instead putting out a constant voltage of 1.8V, meaning that the

logic at the NOR latch fell apart and operation using the LED and buffer could not be tested.

However on reading Sarah Rouviere’s report it became clear that the Schmitt trigger was not

functioning and so it was removed and the output of the window detector was run straight to

the NOR latch instead [14]The performance of the pump control at the NOR latch was very

good as can be seen in the table in figure 34.

HI Latch Input (Volts) LO Latch Input (Volts) Q Output (Volts)

P<LO 0.07 3.81V 3.81V

LO<P<HI 0.07V 0.07V 0.07 if moving from HI

to LO, 3.81V if moving

from LO to HI

P>HI 3.81V 0.07V 0.07V Figure 34: NOR Latch Functionality

It’s clear that the logic HI voltage is 3.81V and the logic LO voltage is 0.07V. These values

will be sufficient as inputs to the buffer circuit.


5. Conclusion

Over the course of the project this pressure switch has moved from a non-functioning on-

stripboard model to a nearly fully functional switching system that allows a user to read

pressure in Bars or PSI from the transducer and control the amount of pressure allowed in their

storage tank. If another few weeks were available I am confident that this model could be

driven to completion.

Some major design changes were made to the initial circuit design over the course of the

project. Some of these decisions, like using a bandgap reference diode as a reference voltage

for the alarms worked very well and were kept in the final design, whereas some other design

ideas like taking the pressure differentially from the instrumentation amplifier were scrapped

due to being too impractical or for being not as good a solution as other proposed design ideas.

The selective nature of the design process was one of the most interesting parts of the project.

In addition to this, through designing new parts of the system I learned a lot about analogue

circuit design and electronics in general.

As well as the design skills that were picked up over the course of the project, my hands-on

electronics skills have improved immensely. My soldering skills have gotten a lot better since

the project began and in addition to this I have learned how to use some important pieces of

lab equipment like the spectrum analyzer and the Reference Pressure Monitor that I otherwise

would not have gotten to use.

Although primarily the work carried out was based on debugging the system that was

already on the board I have learned a huge amount about instrumentation, filters and analogue

circuit design. In addition to this the project has taught me a valuable lesson in my approach to

problem solving. I learned that time is saved in a project by having a good understanding of the

subject matter at hand and thinking carefully before making design decisions rather than

attempting to overcome the problems as quickly as possible which will rarely result in success.

Appendix

Appendix A: List of components used in the circuit Transducer: Keller 212R

Q1: BC107

U1, U2, U3, U4, U5, U6, U7, U8: Texas Instruments OP77

A/D Converter: Maxim MAX140EPL

NOR Latch (U11, U12): Texas Instruments SN7402

D1: LT1004, 1.125V Diode

R1, R2, R3 = 𝟏𝟏𝟏𝟏𝟏𝟏𝛀𝛀

R4 = 𝟏𝟏𝟏𝟏𝟏𝟏𝛀𝛀

R5 = 𝟒𝟒.𝟕𝟕𝟏𝟏𝛀𝛀

R6 = 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝛀𝛀

R7 = 𝟏𝟏𝟕𝟕𝟏𝟏𝟏𝟏𝛀𝛀

R8 = 𝟏𝟏𝟏𝟏𝛀𝛀

R9 =𝟏𝟏𝟏𝟏𝟏𝟏𝛀𝛀

R10 = 𝟏𝟏𝟏𝟏𝟏𝟏𝛀𝛀

R11 = 𝟏𝟏𝟏𝟏𝛀𝛀

R12 = 𝟏𝟏𝟏𝟏𝛀𝛀

R13 = 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝛀𝛀

R14 = 𝟏𝟏𝟏𝟏𝟏𝟏𝛀𝛀

R15 = 𝟏𝟏𝟒𝟒𝟏𝟏𝟏𝟏𝛀𝛀

R16 = 𝟏𝟏𝟏𝟏𝛀𝛀

R18 = 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝛀𝛀

R19 = 𝟓𝟓𝟏𝟏𝛀𝛀

R26 = 𝟑𝟑𝟑𝟑𝟏𝟏𝛀𝛀

R20 = R22 = 𝐑𝐑𝟏𝟏𝟑𝟑 = 𝟓𝟓𝟏𝟏𝟏𝟏𝛀𝛀

R21 = 𝟏𝟏.𝟓𝟓𝟏𝟏𝛀𝛀

R24 = R25 = 𝟏𝟏𝟏𝟏𝟏𝟏𝛀𝛀

R34= R35 = 𝟏𝟏𝟏𝟏𝟏𝟏𝛀𝛀

R36 = 𝟏𝟏𝟓𝟓𝟏𝟏𝛀𝛀

R37 = 𝟓𝟓.𝟓𝟓𝟏𝟏𝛀𝛀

R38 = 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝛀𝛀

C1 = C7 = C8 = 𝟏𝟏𝟏𝟏𝟏𝟏

C2 = C3 = C6 =𝟏𝟏.𝟏𝟏𝟏𝟏𝟏𝟏

C4=𝟏𝟏.𝟒𝟒𝟕𝟕𝟏𝟏𝟏𝟏

C5 = 𝟏𝟏.𝟏𝟏𝟒𝟒𝟕𝟕𝟏𝟏𝟏𝟏

Appendix B: Schematic of the project

References

[1] Picker Components, "Application Notes/RoHS: Picker Components," 9 April 2003. [Online]. Available: http://www.pickercomponents.com/pdf/application%20note/Contact_ARC_Phenomenon.pdf.

[2] PVL Ltd, "Press Release: Under pressure - choosing the right pressure switch," 20 January 2015. [Online]. Available: http://www.pvl.co.uk/choosing-the-right-pressure-switch.html.

[3] Instrumart, "Keller Econoline General Purpose Pressure Transmitter: Instrumart," 28 March 2015. [Online]. Available: https://www.instrumart.com/products/33976/keller-econoline-general-purpose-pressure-transmitter.

[4] Anderson Bolds, "Barksdale Solid State Pressure Switches: Anderson Bolds," Anderson Bolds, 28 March 2015. [Online]. Available: http://www.anderson-bolds.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=AB&Category_Code=PSSS. [Accessed 28 March 2015].

[5] Keller Druck, "Keller Series 21R/SR/MR Piezoresistive Transmitters Datasheet," 30 March 2015. [Online]. Available: http://www.keller-druck.com/picts/pdf/engl/21re.pdf. [Accessed 30 March 2015].

[6] D. M. J. Burke, "Strain Gauges and Pressure Transducers," in 4C13: Instrumentation Amplifiers and Transducers, Dublin, Trinity College Dublin: Dept Of Electronic and Electrical Engineering, 2015, pp. 10-12.

[7] D. M. J. Burke, "The Instrumentation Amplifier," in 4C13: Instrumentation Amplifiers and Transducers, Dublin, Trinity College Dublin: Dept Of Electronic and Electrical Engineering, 2015, pp. 1-12.

[8] Analog Devices, "All Datasheet: OP77FZ Datasheet," 31 March 2015. [Online]. Available: http://pdf1.alldatasheet.com/datasheet-pdf/view/49069/AD/OP77FZ.html. [Accessed 31 March 2015].

[9] MAXIM Integrated, "MAX138/139/140 Datasheet: MAXIM Integrated," 01 January 1995. [Online]. Available: http://datasheets.maximintegrated.com/en/ds/MAX138-MAX140.pdf.

d solid state pressure switch systems

[Accessed 5 April 2015].

[10]

P. T. Natarajan, "Basic Electronics Lecture Notes: Indian Institute of Technology, Madras," 5 April 2015. [Online]. Available: http://textofvideo.nptel.iitm.ac.in/122106025/lec33.pdf. [Accessed 5 April 2015].

[11]

D. M. J. Burke, "Non-Linear Applications I," in 4C13: Instrumentation Amplifiers and Transducers, Dublin, Trinity College Dublin: Dept Of Electronic and Electrical Engineering, 2015, p. 2.

[12]

D. M. J. Burke, "Non-Linear Applications I," in 4C13: Instrumentation Amplifiers and Transducers, Dublin, Trinity College Dublin: Dept Of Electronic and Electrical Engineering, 2015, p. 4.

[13]

Analog Devices, "All Datasheet: OP77FZ Datasheet," 31 March 2015. [Online]. Available: http://pdf1.alldatasheet.com/datasheet-pdf/view/49069/AD/OP77FZ.html. [Accessed 5 April 2015].

[14]

S. Rouviere, "A Solid State Pressure Transducer," Trinity College Dublin: Department of Electronic and Electrical Engineering, Dublin, 2014.

[15]

StartingElectronics.com, "SR Latch Tutorial: StartingElectronics.com," StartingElectronics.com, 16 January 2013. [Online]. Available: http://startingelectronics.com/software/VHDL-CPLD-course/tut9-SR-latch/S-R-latch-gates.png. [Accessed 5 April 2015].

[16]

D. M. J. Burke, "Transistor Inverter Applications I," in Digital Circuits Lecture Notes, Dublin, Trinity College Dublin, 2015, p. 1.

[17]

D. M. J. Burke, "Transistor Inverter Applications I," in Digital Circuits Lecture Notes, Dublin, Trinity College Dublin, 2015, p. 2.

[18]

L. Loflin, "Exploring the PICAXE Microcontroller: www.bristolwatch.com," [Online]. Available: http://www.bristolwatch.com/picaxe/images/8leds_common_anode.gif. [Accessed 5 April 2015].

[19]

Agilent Technologies, "Technical Data: 14.2mm Seven Segment Displays HDSP series," 7 September 2004. [Online]. Available: http://www.farnell.com/datasheets/57779.pdf. [Accessed 5 April 2015].

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