Onwards and Upwards, Your near space guide Overview of the NearSys Weather Station Figure 1. The NearSys Weather Station Parts List

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1 The NearSys Weather Station is a temperature, pressure, and relative humidity sensor soldering kit. The weather station is not a stand-alone kit it must be plugged into the analog ports of a flight computer. The flight computer then provides the power to operate the station and the memory to record the output from all three sensors. The NearSys Weather Station permits BalloonSats to log environmental data during their mission. Then after recovery, BalloonSat teams can produce charts of environmental conditions as a function of altitude by processing the weather station s data with a spreadsheet. Onwards and Upwards, Your near space guide Overview of the NearSys Weather Station There are nine parts in the weather station to solder together. The resulting weather station is capable of recording environmental conditions from the ground to near space. The heart of the NearSys Weather Station is its three sensors, a temperature, pressure, and relative humidity sensor. Each sensor produces a voltage of a value that depends on the environmental condition it is measuring. After plugging Figure 1. The NearSys Weather Station the station s cable into a flight computer s Easy-Plug I/O port, it begins digitizing the voltages and records them for later analysis. The weight of the weather station is only 28 grams, leaving a lot of weight free for other BalloonSat experiments. Parts List Look at the top of the printed circuit board (PCB) and you ll see white lettering indicating the placement and orientation of individual electronic components. Each component has a unique and meaningful reference consisting of a letter followed by a digit. All resistors for example, have a reference beginning with the letter R. Below is a list of the components in the NearSys Weather Station kit and their references you ll find on the PCB. 1

2 C1 0.1 uf capacitor R1 1k ohm resistor (brown, black, red, gold) U1 HIH-4000 relative humidity sensor U2 LM335 temperature sensor U2 SM5812 pressure sensor The remaining items are required to complete the NearSys Weather Station, but they do not have a reference on the PCB. One by five male header Two zip ties 2-1/2 long PVC pipe Wire (#24 AWG) Heat shrink tubing Weather Station printed circuit board Easy-Plug I/O Adapter printed circuit board Component Pictorials The following pictures illustrate the physical appearance of the components you ll find in the kit. Figure 2. Capacitors Figure 3. Resistor Figure 4. Weather station printed circuit board Figure 5. Temperature sensor 2

3 Figure 6. Pressure sensor Figure 7. Relative humidity sensor Figure 8. One by five right angle header Figure 9. Heat shrink tubing Figure 10. Easy-Plug I/O adapter printed circuit board 3

4 Theory of Operation Figure 11. NearSpace Weather Station Schematic The PCB of the NearSys Weather Station supports its three sensors in producing environmentally sensitive output. Below is a brief description of each component and how it supports the mission of the weather station. The temperature sensor (LM335) produces a voltage that varies linearly with changes in the temperature. Ideally, it produces 0.01 (or 1/100 th ) volts for every kelvin and would theoretically produce zero volts at Absolute Zero. The Kelvin scale is an absolute temperature scales frequently used in physics. A single kelvin is equal to a degree Celsius, but the Kelvin scale begins at absolute zero, or -273 O degrees Celsius. At room temperature, or around 293 kelvins, the LM335 produces a voltage of 2.93 volts. The temperature rating of the LM335 does not extend to temperatures as cold as they get in near space, but it seems to produce a valid temperature regardless. The HIH-4000 is a relative humidity sensor that also produces a voltage output that depends linearly on the relative humidity around it. Unlike the LM335, the HIH has an outset voltage meaning it produces a voltage at 0% relative humidity (of about 0.8 volts). This makes the equation for converting HIH-4000 voltage into relative humidity a little more complex than the equation for the LM335, but it s still simple for the spreadsheet that will perform the data processing. The SM5812 produces a voltage that varies linearly with changes in the air pressure. Like the HIH-4000, the pressure sensor also has an offset voltage (of 0.5 volts) at zero pressure. The maximum voltage output from the SM5812 is 4.5 volts, which may be a problem for ADCs with a maximum input voltage of volts. However, this is only a 4

5 problem for very low elevation launches. Launches from elevations of around 1,000 feet seem to begin at a low enough air pressure for readings though out the mission. The weather station terminates in a 3 by 3 header. The first row of pins contains the voltages from each sensor. The second row has a single wire soldered to it to provide the needed five volts to operate the weather station. The third row also has a single wire soldered to it and provides ground for the weather station. Assembling the NearSys Weather Station The diagram below illustrates the placement of the components you will solder to the weather station PCB. Check off each step below as you complete it. Figure 12. Parts Layout for the NearSys Weather Station R1 1 k-ohms (brown, black, red, gold) C1 0.1 uf capacitor (104) U1 HIH-4000 Note: The open face of the sensor faces towards the pressure sensor U2 LM335 Note: Orient the temperature sensor as illustrated in the diagram above U3 SM5812 Note: Orient the pressure sensor as illustrated in the diagram above (note the position of the barb) Cable Cut the wire into five pieces, one foot long each Strip ¼ of insulation from both ends of each wire Thread each wire through a large strain relief hole and back into the PCB pad as shown below 5

6 Figure 13. A strain relieved wire. This wire is threaded from the underside of the PCB and through a large near the edge of the board. Then it is bent soldered into a pad inside the strain relief hole. Note that the insulation is intact where it passes through the strain relief hole. Repeat for the other four wires Solder all five wires and trim Making the Easy-Plug I/O Adapter Insert the 1 by 5 header into the Easy-Plug I/O Plug printed circuit board and solder Note: The short pins must be soldered to the printed circuit board. Leave the long pins free so they will plug into the flight computer s I/O port. Cut the five wires to the same length and strip ¼ inch of insulation from their ends Insert the wires into the Easy-Plug printed circuit board the same way they are inserted into the Weather Station printed circuit board Note: Follow the same color scheme with the following wires soldering to these pads Red wire solders to the +5V pad Press wire solders to the A2 pad RH wire solders to the A1 pad Temp wire solders to the A0 pad Green wire solders to the GND pad Figure 14. The completed Easy-Plug I/O Adapter 6

7 The Easy-Plug I/O Adapter plugs into the Easy-Plug Port on the flight computer. Checking Your Work That completes the assembly of the NearSys Weather Station. However, don t plug the weather station into a working flight computer just yet. That s because if there is an error in the assembly, the weather station could be damaged, or damage a flight computer when powered up. Now perform these five checks. 1. Check the Soldering Check the underside of the PCB looking for blobs of solder that may bridge across two pads. If there appears to be such a bridge, briefly apply some heat to the pads with your soldering iron and pull the molten solder into two separate cones. Or you could lay solder wick across the solder and try to wick up the excess solder. Do these actions quickly as too much heat can damage copper traces on the PCB. 2. Check for Shorts Set the multimeter to the continuity setting and tap the test leads together. The multimeter will ring or beep to indicate there is a short between the test leads. Now perform the test for real by applying the +5V and GND pins of the header. There should be no ringing. If there is, then there s a short in the PCB that needs to be located and fixed. Pretty much the only way a short can exist in the PCB is through a solder bridge. So look over the underside of the PCB again, for a solder connection that has overflowed its pad. Then test pairs of signal pins in the header (Tenp, Press, and RH). The DMM shouldn t ring when any combination of two are tapped. Finally, tap between each signal pin and ground and each signal pin and +5V. Again, there should be no ringing in the DMM. 3. Check signal voltages Insert the weather station Easy-Plug I/O header into a flight computer and then power up the flight computer. Write the following program in the BalloonSat Mini (one without the camera relay) and download Weather: readadc 1,b0 readadc 2,b1 readadc 4,b2 debug goto weather For the BalloonSat Easy, use this code Weather: readadc 0,b0 readadc 1,b1 readadc 2,b2 debug goto weather 7

8 The debug window will open after the program downloads into the flight computer. Look at variables B0, B1, and B2 and values similar to these should be displayed. The exact values depend on the current environmental conditions. B0: 128 (for relative humidity) B1: 149 (for temperature) B2: 196 (for pressure) The value in memory byte B0 will vary the most. As long as the values displayed are anywhere close to these, the weather station is working. Be concerned when the values are close to 0 or 255. A bad value is possible for broken connections, shorts, or reversed sensors. Completing the Near Space Weather Station In order to protect weather station readings from sunlight, the weather station mounts inside of a white PVC tube. Exposure of the temperature and relative humidity sensors to sunlight creates invalid readings. In addition to shielding sensors from sun exposure, the tube also protects the weather station on landing. Clean up the edges of the PVC tube with medium sand paper Remove the printing on the PVC tube with sand fine paper Place the tube along a door jam to draw a straight line along its length Mark four points on the centered on the tube with the following dimensions Figure 15. Placement in the PVC tube of the weather station PCB mounting holes Drill the holes with a 1/8 inch diameter bit Place the weather station inside the PVC tube and use the plastic zip ties to hold it in place. 8

9 Figure 16. Near Space Weather Station mounted inside a PVC tube. The weather station is sufficiently lightweight tat it can be mounted to the outside of an airframe with rubber bands. Figure 17. Completed Near Space Weather Station attached to an antenna boom for exposure to near space conditions. Post Mission Data Processing After BalloonSat recovery and data download, you should have a file similar to this one for processing (this example was generated with a 10-bit ADC). 553,602, ,601, ,600, ,599, ,597, ,593, ,590, ,588, ,583,512 9

10 Data collected with the READADC (8-bit) command has smaller numbers than data collected with the READADC10 (10-bit) command. Regardless, the first step is to convert the numbers from ADC counts to volts. This is done by dividing the number by the maximum value of the READADC command and multiplying by five. For READADC10 voltage = (count/1024) * 5 For READADC voltage = (count/256) * 5 Relative Humidity Conversion The relative humidity sensor is linear but has an offset of 0.8 volts. In other words, the HIH-4000 produces a 0.8 volt output at 0% relative humidity (RH). A change of volts is a change of 1% RH. %RH = voltage 0.8) / More Accurate Relative Humidity Conversions With your HIH-4000 relative humidity sensor, there came a calibration sheet. The sheet gives the voltage of the sensor at 0% RH and 75.3% RH. There is also an equation for calculating the relative humidity with your particular sensor. The equation will look something like this Sensor RH: (Vout-0.799)/0.032 (as can be seen in the figure below). Figure 15. The datasheet from a HIH-4000 relative humidity sensor. 10

11 Temperature Conversion The voltage of the temperature sensor is in units of 100 kelvins. So multiplying the voltage by 100 converts it to a temperature reading of kelvins. Since kelvins is common unit for most people, convert the units of kelvins into units of Celsius by subtracting 273. O C = (voltage * 100) 273 Celsius can be converted to Fahrenheit two ways. The neatest way takes advantage of the fact that there is 1.8 Fahrenheit degrees for every 1.0 Celsius degrees and that they intersect at -40 degrees. O F = (( O C + 40) * 1.8)-40 Pressure Conversion The SM5812 pressure sensor has a bottom value of 0.5 volts at 0 PSI and a top value of 4.5 volts at 14.7 PSI (for a range of 4.0 volts). PSI = ((voltage 0.5) / 4) * 14.7 Alternatively, the sensor voltage can be converted in pressure in units of millibars. mb = ((voltage 0.5) / 4) * 1013 SM5812 I2C Data Bus See the help for the READI2C command and the PICAXE I2C app notes on using I2C for PICAXE-18X microcontrollers. This means a BalloonSat Mini (with the PICAXE- 08M which does not have I2C) can only use its built-in ADC to access pressure data. In addition, the NearSys.com website has an app note for the I2C bus. A sample Excel spreadsheet is available at the NearSys website at the Weather Station webpage. PICAXE Links You ll find PICAXE resources like the program editor and datasheets at the PICAXE website: The Program Editor s Help menu contains the BASIC command references for the PICAXE. 11

12 Figure 18. Example chart of air pressure data Figure 19. Example chart of relative humidity data 12

13 Figure 20. Example chart of temperature data 27 June