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# Wireless weather station shows the principle

2016-11-25 04:29
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Next we show wireless weather station works. The wireless weather station consists of a remote station and a base station components. Remote station is solar and wake up again one minute to collect and transmit data. Base station receives and buffers the input data, and then connected to the computer via an RS232 transfer for processing. In each station is a dedicated circuit board and a separate, RF circuit cards. Here begin our discussion with the remote station design.
Remote station consists of four functional components: sensors,  microcontroller , RF circuit PIC16F873, power supply. The schematic shown in Figure 2, which shows the field of sensors and microprocessors. Humidity sensing, I chose Humirel HS1101 capacitive sensor. This device, when combined with CMOS 555 timer operates as an astable multivibrator, generates a signal with a frequency dependent on humidity. Reduce the temperature effects, it is important to use the device in this design Texas Instruments TLC555. Reference Data Sheet HS1101 If another device. Please also note that you must pay attention to the HS1101 and 555 nodes. Stray capacitance values ??will cause errors and unpredictable measurements. I chose this node above solder circuit boards. betw1een output frequency, the relative humidity of 555 can be seen in Figure 1. A first-order equation is the frequency and relative humidity: Right = 565.1 - 0.0767 * f a second order equations can be used to improve the accuracy: R = -6.4790 e - 06 * f2 1.0047 e 02 * f 2.7567 e 02
Temperature sensing is very simple LM335. This output device is equal to the absolute temperature in Kelvin divided by 100, or: Output Voltage = temperature (K) / 100 to determine the temperature in degrees Celsius, using the equations: oC = 100 * Output Voltage - 273 to determine the temperature in degrees Fahrenheit using the equations: = 1.8 * oC 32.2 = 1.8 * (100 * Output Voltage - 273) 32.2 * Output voltage = 180 - 459.2 At room temperature, the device output is approximately 3 volts. Pressure sensing by Motorola MPX5100A, operating from 0 to 16 PSI. However, we are interested in only a small part of this range. Barometric pressure reading drops betw1een 28 and 32 inches of mercury. This is equivalent to 13.75 到 15.72 PSI. Increase the dynamic range of output, I added an amplifier circuit (happy), minus about 3.7 volts output from the sensor, then you can add different 4. Because MPX5100 may take up to 10 horses, Q1 is added to provide microprocessor control switch. I chose microchip PIC16F873 because it combines well program and data memory, 10 a / D, and three timers. Timer2 humidity signal for the measurement cycle. The A / D is used to measure temperature and pressure sensors and monitoring the battery voltage. To maximize accuracy I use an external reference from 4.096 volts 0.1% National Semiconductor. And 10 A / D, which provides a solution 4 mv / count. This interface includes a radio link to the wire and a data output. Because the transmitter circuit in 3.3 V, the analog switch I use five volt output from the microprocessor. Although not shown, the pin 14 74 hc4066 need to be connected to 5 volts though pin 7 needs to be grounded. JP1 is the head, five feet connected to the RF circuit board.

The power supply for the Remote Station is shown in Figure 3. When the sun is shining on the solar panel, enough power is generated to drive the 50mA current source formed by Q1, U1, and R1. This current acts as a trickle charger for three AA NiCAD batteries. The batteries power U2 – a switchmode regulator that provides the 5 volts for the microprocessor and sensors.This is followed by U3, which is a linear regulator that provides the 3.3 volts for the RF circuitry. L2 and C5 were added to reduce the switching noise from U3. D2 is used to isolate the solar panel from the rest of the circuit when it is dark. Note that C3 and C5 should be 6.3V low ESR capacitors.

The RF section of this design, shown in Figure 4, was built around the TX5002 and RX5002 chips from RF Monolithics. Due to the footprint of these devices, it was necessary to design a small printed circuit board. Fortunately, the chips have a pin-out that allowed a single board to be used for both the transmitter and the receiver. The completed layout can be seen in Figure 5.

It is necessary to use solder paste to connect the RFM modules to the PCB. Carefully apply the paste to the pads of U1 on the board. Position the module on board and heat with a hot air gun until the solder melts. Except for C2, the remaining components are in 0603 SMT packages. C2 is a tantalum capacitor in an “A” package.

I was able to get all the traces on a single layer so fabrication was greatly simplified. The four corner holes were drilled out to allow mounting with 4-40 screws. The antenna is connected via a BNC jack on the left side. However, a length of coax can be connected to this pad in order to use a panel-mount connector.

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