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 Wheat- Stone bridge Diff- Amp A/D Converter -20 to 40°c Ω Ω mV V

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.

Output voltage = Va – Vb = – = 190 mV Minimum Temperature at -20°c Maximum Temperature at 40°c Output voltage = Va – Vb = – = 100 mV