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Chapter 3- Hardware

3.1 System Block Diagram

The wireless weather station consists of a base station and remote station that include a temperature sensor to take measurement of the weather conditions. The remote station collect and transfer the data to the transmitter module circuit through the Mitsubishi M16 microcontroller. The base station receives the incoming data and transfers it via an RS-232 connection to the RS-232 transceiver and then to the host PC. This is shown in the System Block Diagram in Figure 3.1
The complete system design of the wireless weather station project is as shown in Photo 3.1.

The complete system design of the wireless weather station project is as shown in Photo 3.1


Figure 3.1 : System Block Diagram for Wireless Weather Station


Photo 3.1 :M16 Boardwith Receiver, Transmitter Board, Temperature Sensor and Digital Thermometer.

3.1.1 Base Station

The base station shown in Figure 3.2 consists of a RF receiver circuit , RS-232 transceiver (MAX202), power supply and a host PC. The MAX202 transceiver will convert the +5V input to the +/-10V needed for RS-232 output levels. RS-232 buffering and level translation is accomplished through the MAX202. A standard DB9 connects to the PC provides connectivity to the RF receiver board.

The transceiver circuit was designed to support both transmitter and receiver modules.
The receiver selected is a QM (Quasi AM/FM) receiver module, QMR1-434 that operates at a frequency of 433.92 Mhz with optimal range 200m.The data rates up to 10Kbits/s.It operate on single 5V supply. The data sheet can be found at www.rfsolutions.co.uk.

The antenna is used to receive an electrical signal through the airwaves. The size of the antenna determines the length of broadcast.


Figure 3.2 : System Block Diagram for Base Station


3.1.2 Circuit Design for Receiver with RS-232 Transceiver


Figure 3.3 : Circuit Diagram for Receiver with RS-232 Transceiver

3.1.3 Remote Station

The remote station shown in Figure 3.4 consists of the temperature sensor, M16 microcontroller, RF transmitter circuit and the power supply.
The transmitter selected is a radio transmitter module, QFMT1-434 that operates at a frequency of 433.92 MHz with optimal range 200m. The data rates up to 10Kbits/s. It can operate from 3V to 9V supply. I selected this transmitter as it can transmit more than 10 meter require for this project and it can operate at 5V. The data sheet can be found at www.rfsolutions.co.uk.

The antenna is used to transmit an electrical signal through the airwaves.
The size of the antenna determines the length of broadcast. The MCU periodically scans the sensors, calibrates and compensates their data and communicates the resulting information to the transmitter.


Figure 3.4 : System Block Diagram for Remote Station

3.1.4 Circuit Design for Temperature Sensor with Transmitter

Figure 3.5 : Circuit Diagram for LM35C Temperature Sensor with Transmitter

The circuit diagram for the transmitter circuit and the temperature sensor is shown in Figure 3.5.

We have added the LED to indicate that there is a 5V supply to the board.
As the software is not yet written, I use docklight evaluation software to test the receiver circuit.

It is able to receive and display the data at the base station PC.

 

3.2 Temperature Sensor

From the literature review on the comparison of temperature sensors, I will select the integrated circuit temperature sensor for this wireless weather station project.

When choosing a precision integrated circuit temperature sensor for this project, the accuracy, operating temperature range, supply voltage and availability of part are the key factors.

The accuracy is defined as the error between the output voltage and 10mV/°C times the device’s case temperature at specified conditions of voltage, current and temperature.

3.2.1 Comparison of Integrated Circuit Temperature Sensor

Figure 3.6 : Accuracy versus Temperature (Guaranteed)

As shown in Figure 3.6, the accuracy versus temperature performance for LM35A is +/- 0.5 at TA= 25°C. The accuracy of LM35A is the best by comparing with LM35C and LM35D which is +/- 1.0.

I did not select LM35A due to availability of part and our specifications set for the temperature range is from -20°C to +62°C. Moreover it is more costly than LM35C and LM35D.

The accuracy of LM35D is +/- 1.5 and is not so good if compared with LM35C which is +/- 1.0.

As the operating temperature for LM35D range from 0°C to 100°C, hence I selected LM35C for the temperature sensor.

Table 3.1 : Comparison Chart for Integrated Temperature Sensor


The LM35C is an integrated circuit sensor that can be used to measure temperature with an electrical output proportional to temperature (in°C). I selected LM35C because it can measure temperature more accurately than using a thermistor. The sensor circuitry is sealed and not subject to oxidation. The LM35C generates a higher output voltage than thermocouples and may not require the output voltage be amplified.

It has an output voltage that is proportional to Celcius temperature. The scale factor is 0.01V/ °C.
The LM35C does not require any external calibration or trimming and it maintains a typical accuracy of +/- 0.4°C at room temperature (TA = +25°C) and +/- 0.8 °C over a range of -40 °C to +110 °C.


3.2.2 Circuit Design for Temperature Sensor

Figure 3.7 : Circuit Diagram for LM35C Temperature Sensor

3.2.3 Circuit Operation

The circuit shown in Figure 3.7 monitors the temperature at the remote station with the LM35C precision integrated circuit temperature sensors whose output voltage is linearly proportional to the Celsius temperature. It is used with a single dc power supply operate at +5V. The 18K resistor connected to the output of LM35C is recommended in the data sheet for single supply temperature sensor.

The LMC6082 is a precision dual low offset voltage operational amplifier able to operate on single supply and it is use as an instrumentation amplifier in our circuit. This instrumentation amplifier can give rail to rail output swing voltage from 0 to 5V nearly hence it is easy for our design.
The output voltage of LMC6082 is connected to port AN1 of Mitsubishi microcontroller (M16C).
We connected the temperature sensor LM35C in series with 2 silicon diodes 1N4148 to ground so that we can obtain negative voltage at port AN2 in order for us to measure negative temperature. The specifications that I set for the temperature range from -20°C to +62°C with an accuracy of 0.1 °C.

I choose this temperature specification for my circuit design so that it can cover most of the countries actual temperature range.

I set the resolution to the maximum of 10 bits to achieve AD accuracy of +/-0.1 °C.
By connecting the temperature sensor directly to ground, the voltage at port AN2 will be zero hence I cannot measure negative temperature.

Due to the thermal characteristic of the diode, the temperature drift at the diodes 1N4148 will affect the voltage at port AN1 hence I connected the wire at point Y to port AN2 to act as negative ground compensation. We will write the ADC software to take care of the voltage difference at AN1 and AN2 to get the temperature reading (VAN1 – VAN2).

Assume the typical forward voltage drop (VF) for silicon diode is 0.6V

At room temperature of 25°C, the measure value for VAN2 = 1.23 volts

3.2.4 Circuit Analysis


Figure 3.8 : Schematic Diagram of Temperature Sensor

Let VTR be the voltage change at VT due to change in voltage at VR (virtual ground).
We performed an experiment to measure the voltage at VT, firstly with two diodes connected as shown in Figure 3.8 then follow by one diode, it was found that a change in VR can cause the voltage at VT to change accordingly. When VR increase, VT also increase hence DVR = DVT.R

Calculations :

All resistors used are of +/-5% tolerance.
I assume 1mA to flow in 1N4148.



Set range of temperature to measure : -20°C to +62°C
0 to 1023 steps equal to 1023 gap



I set the resolution to the maximum of 10 bits to achieve AD accuracy of +/-0.1°C.



To get 0.1°C accuracy, the AD step should be 820 equal voltage level of

Op-amp gain using LMC6082AIN is

Input swing = 10mV/°C, hence 82°C correspond 82°C x 10mV = 820mV
Output swing = 4V
The gain :-

Actual non inverter op-amp gain :-



If Rf= 3.9KΩ and Ri= 1KΩ then the

 

3.3 Communication

Radio frequency is selected as the wireless transfer medium as the project specification for the distance from the host to the remote system shall not be less than 30m.

The RS-232C is used to transfer digital information between the host computer and the receiver board.

Asynchronous communication was used in this project and the framing is set by the start bit. The timing will remain accurate enough throughout the limited duration of the character provided the clocks at the transmitter and receiver are reasonably close to the same speed. There is no set length of time between characters with asynchronous transmission. The receiver monitors the line until it receives a start bit. It counts bits, knowing the character length being employed and after the stop bit, it begins to monitor the line again, waiting for the next character.

 

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