Transcript Presentation no 1 - Group5Weatherstation

We are Group5 Weatherstation
The team Members are :
Saran Jackson
Robert Howard
Robert Garvey
Gene Fitzgerald
Steven Dowling
Our First Variable Is
A Temperature Sensor.
We choose this mainly because of the
availability of components for the circuit.
The Temperature Sensor Consists Of 5
Components
•
•
•
•
•
A PT100 RTD
A Wheatstone Bridge
A Differential Amplifier (741)
Resistors
Dual Power Supply
The PT100 RTD (resistance temperature
detector) is a sensor that makes use of the
temperature dependence of a metals
resistance. In this instance it is Platinum, the
resistance of which is 100 ohms at 0°c. It
offers excellent accuracy over a wide
temperature range and this relationship is
approximately linear over a small range.
The Wheatstone bridge is a measurement
instrument used to measure an unknown
electrical resistance by balancing two legs
of a bridge circuit, one leg of which
includes the unknown component. It is an
extremely accurate way of measuring
resistances and has an unlimited array of
applications in industry.
We can connect signals to both of the inputs at the same time
producing an operational amplifier circuit called a Differential
Amplifier.
The Differential Amplifier circuit is a very useful op-amp
circuit and by adding more resistors in parallel with the input
resistors R1 and R3, the resultant circuit can be made to
either "Add" or "Subtract" the voltages applied to their
respective inputs. One of the most common ways of doing
this is to connect a "Resistive Bridge" commonly called a
Wheatstone Bridge to the input of the amplifier as shown
below.
• For the Wheatstone Bridge we have
selected a 15V DC power supply.
• For the Op-Amp we have also selected a
15V DC power supply.
PT100
-20 to 40°c
WheatStone
bridge
DiffAmp
A/D
Converter
92.16 Ω115.54Ω
100-190mV
2.8 - 5.32V
Sensitivity of Pt100
130
Resistance [ ohm ]
125
120
115
110
105
100
Resistance [ohm] vs Temperature [°C]
95
90
0
10
20
30
40
Temperature [ °C ]
50
60
• Low resistor ratio 1:1 = low output voltage
but high accuracy and linear relationship.
• High resistor ratio 1:10 = higher output
voltage but lower accuracy and slightly
curved relationship.
• Higher output voltage means a lower gain
op-amp is required.
Maximum Temperature at 40°c
R3
1*103
Va  (
)Vs.  (
)15  13.44Volts
R3  Rt
1*103  115.54
Vb  (
R4
1*103
)Vs.  (
)15  13.63Volts
R2  R4
100  1*103
Output voltage = Va – Vb = 13.63 – 13.44 = 190 mV
Minimum Temperature at -20°c
Va  (
R3
1*103
)Vs.  (
)15  13.73Volts
R3  Rt
1*103  92.16
Vb  (
R4
1*103
)Vs.  (
)15  13.63Volts
R2  R4
100  1*103
Output voltage = Va – Vb = 13.73 – 13.63 = 100 mV
Gain  1 
R2
R1
R 2  27.R1
R 2  15 K 
R1  570
 28  1 
R2
R1
 27 
R2
R1