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.
2 solid state pressure switch systems
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.
3 solid state pressure switch systems
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
4 solid state pressure switch systems
Appendix ......................................................................................................................................... a
Appendix A: List of components used in the circuit ................................................................... a
Appendix B: Schematic of the project ........................................................................................ b
References ....................................................................................................................................... c
5 solid state pressure switch systems
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
6 solid state pressure switch systems
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
7 solid state pressure switch systems
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:
β’ The error in the value displayed on the seven segment screen must be no more than
Β±1% of the transducers full scale voltage output (100mV)
β’ The system must operate within the Extended Industrial temperature range (-40 to 85
degrees Celsius).
8 solid state pressure switch systems
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 Β±312ππ 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
β¬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 β¬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;
9 solid state pressure switch systems
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.
10 solid state pressure switch systems
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.
11 solid state pressure switch systems
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:
β’ Accuracy: The Keller 212R is accurate to Β±1% of the full scale pressure (10 Bar), this
means that at 10 Bar the output voltage will be 100ππππ Β± 1ππππ.
β’ 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 Ξπ π is consistent with temperature [5].
β’ 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.
β’ 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.
12 solid state pressure switch systems
Parameter Min. Typ. Max. Unit
Pressure Range 0(0) - 10(145) Bar(PSI)
Supply Current - - 2 mA
Supply Voltage 8 10 28 πππ·π·π·π·
Span Sensitivity - 0.01 - ππππππππππππ
Bridge
Resistance (π π π΅π΅)
- 3.5 - ππΞ©
Operating
Temperature
-40 - 85 Degrees Celsius
Linearity and
Hysteresis Error
- Β±0.2 Β±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 (ππππππππ1 = ππππππππ2).
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
13 solid state pressure switch systems
the voltage output of each individual pin is measured with respect to ground. This voltage can
be altered by adjusting the supply voltage πππ΅π΅. Initially the transducer was run off a +12ππ single
ended supply and this created an offset of 3.9ππ DC at each of the ππππππππ pins however due to
common voltage considerations at the MAX140 this was changed to a bipolar source of
+6ππ,β4ππ to give a 0ππ 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.
RΒ² = 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
14 solid state pressure switch systems
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 45ππΞ© 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 ππππ is the output voltage
from the transducer, π π π΅π΅ is the bridge voltage from the transducer and π π ππ 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:
ππ =π π ππ
π π π΅π΅ + π π ππππππ
Dividing top and bottom by π π ππ gives the following equation:
ππ =1
1 + π π π΅π΅π π ππ
ππππ
It is clear from this equation that if π π ππ is significantly larger than π π π΅π΅, 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.
15 solid state pressure switch systems
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.
ππ1
ππ2 ππππ
16 solid state pressure switch systems
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 ππ1 and ππ2. From above:
ππππππππ1 = βπ π 2π π 1ππ1
If the input contribution from ππ1 is then neglected (ie if ππ1 is tied to ground) it can be seen that
the voltage output from ππ2 is:
ππππππππ2 =π π 1 + π π 2π π 1
ππ+
Where ππ+ is the voltage measured at the output of the potential divider:
ππ+ =π π 4 + π π 5
π π 3 + π π 4 + π π 5ππ2
Substituting this into the original equation for ππππππππ2:
ππππππππ2 =(π π 1 + π π 2)
π π 1οΏ½
π π 4 + π π 5π π 3 + π π 4 + π π 5
οΏ½ππ2
The output is the combined contribution from ππππππππ1 and ππππππππ2:
ππππππππ = ππππππππ1 + ππππππππ2 = (π π 1 + π π 2)
π π 1οΏ½
π π 4 + π π 5π π 3 + π π 4 + π π 5
οΏ½ππ2 βπ π 2π π 1ππ1
17 solid state pressure switch systems
The values for the resistors are chosen such that π π 1 = π π 2 = π π 3 = π π 4 + π π 5 in order to achieve
unity gain. The final voltage output from the differential amplifier is therefore:
ππππππππ =π π 2π π 1
(ππ2 β ππ1) =π π 4 + π π 5π π 3
(ππ2 β ππ1)
The differential gain of the amp is therefore:
π΄π΄ππ =π π 2π π 1
=π π 4 + π π 5π π 3
The Common Mode Rejection ratio of the amplifier is dependent on the mismatch in the
resistors[7]:
πΆπΆπππ π π π Ξπ π = οΏ½1 + π π 2π π 1οΏ½ οΏ½ 1
4Ξπ π οΏ½
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 π π 1(π π 4 + π π 5) = π π 3π π 2. In addition to this the
resistor values are set such that π π 1 = π π 2 = π π 3 = π π 4 . In the previous design the value of these
resistors was chosen to be 1ππΞ© however in order to ensure that the transducer was not getting
loaded by the instrumentation amplifier, the values of resistors π π 1,π π 2,π π 3 were increased
to 12ππΞ© and resistor π π 4 was chosen to be 10ππΞ©.
Another modification that was performed on the original circuit was the addition of the
potentiometer π π 5 which allows for the correction of any offsets present in the instrumentation
amplifier stage due to resistor mismatches. The value for π π 4 was chosen to be less than the
other resistors in case the offset was to be corrected with a value of resistance less than 12ππΞ©
and the potentiometer in series allows for a maximum resistance of 14.7ππΞ©. The variation in
resistance is 4.7ππΞ© 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.
18 solid state pressure switch systems
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, πππ·π· 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:
ππππ = 2ππππππ = 1οΏ½π π 6π π 7π·π·1π·π·2
For a second order filter, the Q factor is 1.414 and in order to have unity gain:
πΆπΆ1 β₯ 4ππ2πΆπΆ2 β 10πΆπΆ2
πΆπΆ1 was chosen to be 1ππππ arbitrarily and from this πΆπΆ2 was then chosen to be 100ππππ.
19 solid state pressure switch systems
With ππππ = 3π»π»π»π» the resistor values could be worked out and the equation became:
3π»π»π»π» =1
2πποΏ½π π 1π π 2 Γ 100ππππ
Rearranging the equation:
π π 1π π 2 =1
4ππ2 Γ 100ππππ Γ 9π»π»π»π»= 28.14477 Γ 109Ξ©
The values for π π 1 and π π 2 were then chosen to be 270ππΞ© and 100ππΞ© as:
270ππΞ© Γ 100ππΞ© = 27 Γ 109Ξ©
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]
20 solid state pressure switch systems
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
21 solid state pressure switch systems
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:
1π΅π΅πππ΅π΅ β 14.5ππππππ
When the unit conversion switch is moved from Bar the PSI, the following two events occur:
β’ The Unit Control Op Amp gain is made equal to 1.45
β’ 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.
π π 10 and π π 11 are connected to ground and π π 9 is in the feedback loop. This means that the gain
of the amplifier is:
π΄π΄ππ = 1 + οΏ½π π 9
π π 10 + π π 11οΏ½
In order to obtain a gain of 1.45 resistor values were chosen to be:
π π 9 = 10ππΞ©,π π 10 = 22ππΞ©,π π 11 = 2ππΞ©.
The potentiometer π π 11 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 π π 8 is used to prevent noise that might occur during the switching action and
corrupt the displayed signal. This interference is stopped by setting π π 8 to be a very high value of
10ππΞ©.
22 solid state pressure switch systems
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 π π 19 and π π 21 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
πππ π π π π π
23 solid state pressure switch systems
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 π π 16 which
was chosen to generate enough current to send the diode into reverse breakdown mode. When
in reverse breakdown, the voltage at πππ π π π π π 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 π π 19. The values for resistors π π 18,π π 19 and π π 26 were calculated using
simultaneous equations:
Equation 1:
10ππππ =π π 26
π π 18 + π π 26πππ π π π π π
Equation 2:
70ππππ =(π π 26 + π π 19)
π π 18 + π π 26 + π π 19πππ π π π π π
Rearranging Equation 1:
10ππππ Γ π π 18 = (πππ π π π π π β 10ππππ)π π 26
Equation 3:
π π 18 = 122.45π π 26
Substituting Equation 2:
π π 26 + π π 19 =70πππππππ π π π π π
(π π 18 + π π 26 + π π 19)
24 solid state pressure switch systems
Moving all of the π π 26 + π π 19 terms to one side:
οΏ½1 β70πππππππ π π π π π
οΏ½ (π π 26 + π π 19) =70πππππππ π π π π π
π π 18
π π 18 = 16.6357(π π 26 + π π 19)
Substituting from Equation 3 above:
122.45π π 26 = 16.6357(π π 26 + π π 19)
π π 19 β 6π π 26
π π 18 = 122.2π π 26
Using standard resistor values:
π π 18 = 100ππ
π π 26 = 1ππ
π π 19 = 4.7ππ
In the same way, simultaneous equations were used to calculate the values for the
resistors in the low pressure threshold. The chosen values were:
π π 20 = 470Ξ©
π π 22 = 470Ξ©
π π 21 = 2.5ππΞ©
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
25 solid state pressure switch systems
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
26 solid state pressure switch systems
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.
27 solid state pressure switch systems
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.
ππππππππππππππ π»π»π»π»π»π»π»π»π»π»π΅π΅π»π»π»π»πππ»π» =10
2 Γ 105= 50ππππ
However the datasheet of the OP77FZ Op Amp states the maximum noise input is 65ππππ
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
28 solid state pressure switch systems
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]:
π π 23 = 470Ξ©
π π 24 = 10ππΞ©
π π 25 = 10ππΞ©
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
29 solid state pressure switch systems
ππππ (ππ) ππππ (ππ) ππ ππ ππππππππππππ
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 βheldβ 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.
30 solid state pressure switch systems
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
31 solid state pressure switch systems
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. ππππππππππππ = 0.6πππ·π·π·π·
where the supply voltage is 6 Volts. The transistor has a πππ΅π΅π π ππππππ = 0.75ππ and πππ·π·π π ππππππ = 0.25ππ
The minimum π½π½ππ= 110. If πππ π = 30πππ΄π΄ (corresponding to 100mcd) then the forward voltage drop
across the diode can be found to be 1.5V [17].
The voltage drop across π π π·π· can be calculated as following:
πππ π π·π· = πππ·π·π·π· β πππΏπΏπ π π·π· β πππ·π·π π ππππππ = 6ππ β 1.5ππ β 0.25ππ = 4.25ππ
π π π·π· =πππ π πΆπΆπππ π
=4.25
30πππ΄π΄= 141Ξ©
32 solid state pressure switch systems
We will use a 150Ξ© resistor in order to keep the values standard. The power rating of the
resistor would be:
πππ π πΆπΆ = πππ π πΆπΆ Γ πππ·π· = 4.25 Γ 30πππ΄π΄ = 127.5ππππ
So a 250mW resistor would be used.
The base current can then be calculated as the collector current divided by the current gain:
πππ΅π΅πΈπΈπΈπΈππ =πππ·π·π½π½π π
=πππ π π½π½π π
=30πππ΄π΄
110= 0.27πππ΄π΄
To account for temperature variations the overdrive factor is set to 2 so:
πππ΅π΅ = πππΏπΏπππ΅π΅πΈπΈπΈπΈππ = 2 Γ 0.27πππ΄π΄ = 0.51πππ΄π΄
Now π π π΅π΅ can be determined:
π π π΅π΅ =πππ π π΅π΅πππ΅π΅
=ππππππππππππ β πππ΅π΅π π ππππππ
πππ΅π΅=
0.6 Γ 6 β 0.750.51πππ΄π΄
= 5.6ππΞ©
In addition to this it is good practice to place a large value resistor on the Base Emitter Junction
so a 100ππΞ© resistor will be placed in addition to π π π΅π΅ and π π π·π·.
Figure 20: LED Buffer Circuit
33 solid state pressure switch systems
4. Experimentation & Debugging
βDebugging time increases as a square of the programβs size.β
β 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.
34 solid state pressure switch systems
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
35 solid state pressure switch systems
β’ Replacing the three Op Amps that made up the instrumentation amplifier
β’ Increasing the resistor values in the Instrumentation amplifier from 1ππΞ© to 12ππΞ©
to prevent the circuit from loading the transducer with current
β’ 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
(Β±1%) changing the CMRR of the instrumentation amplifier.
Figure 22: Improved Instrumentation Amplifier (left), Original Instrumentation Amplifier (right)
36 solid state pressure switch systems
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:
β’ The Op Amp overheated when power was connected to the circuit
β’ The Design on the board did not match the design in the schematic
β’ 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]:
π π 6 = 2.2ππΞ©
π π 7 = 3.3ππΞ©
πΆπΆ1 = 6.8ππππ
πΆπΆ2 = 0.1ππππ
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.
37 solid state pressure switch systems
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.
38 solid state pressure switch systems
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]
39 solid state pressure switch systems
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 ππ+ 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.
40 solid state pressure switch systems
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 π π 14
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
41 solid state pressure switch systems
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 3.9ππ 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 2.5ππ at 0 Bar as opposed to 3.9ππ. The supply
voltages chosen were +6ππ and β4ππ. 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.
42 solid state pressure switch systems
However this configuration caused 3 more problems:
β’ The unit switching stage was now disconnected so a new method for switching
units would have to be designed
β’ The low pass filter which removed AC noise above 3Hz is no longer acting in the
circuit
β’ 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.
43 solid state pressure switch systems
This was accomplished by changing the supply voltage of the chip from +5ππ with respect to
Ground to +3ππ,β2ππ. 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)
44 solid state pressure switch systems
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
45 solid state pressure switch systems
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 β 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 3.5ππ 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.
46 solid state pressure switch systems
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 +6ππ,β4ππ) and the Schmitt Trigger (πππππππ»π»π΅π΅π»π»ππ πππ»π» +
3,β2ππ) 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.
47 solid state pressure switch systems
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 Rouviereβs 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
Itβs 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.
48 solid state pressure switch systems
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 = ππππππππ
R4 = ππππππππ
R5 = ππ.ππππππ
R6 = ππππππππππ
R7 = ππππππππππ
R8 = ππππππ
R9 =ππππππππ
R10 = ππππππππ
R11 = ππππππ
R12 = ππππππ
R13 = ππππππππππ
R14 = ππππππππ
R15 = ππππππππππ
R16 = ππππππ
R18 = ππππππππππ
R19 = ππππππ
R26 = ππππππππ
R20 = R22 = ππππππ = ππππππππ
R21 = ππ.ππππππ
R24 = R25 = ππππππππ
R34= R35 = ππππππππ
R36 = ππππππππ
R37 = ππ.ππππππ
R38 = ππππππππππ
C1 = C7 = C8 = ππππππ
C2 = C3 = C6 =ππ.ππππππ
C4=ππ.ππππππππ
C5 = ππ.ππππππππππ
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].
Top Related