Figure 1. A completed Temperature and Relative Humidity Sensor

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1 The NearSys Temperature and Relative Humidity allows a near space experiment to measure these two characteristics of the atmosphere. By recording the sensor s voltages on a regular basis and combining that data with GPS data, the experimenter can create a profile of the atmosphere. That profile is useful for determining the altitude of the stratosphere among other properties. Onwards and Upwards, Your near space guide Overview of the Temperature and Relative Humidity Sensor When supplied with five volts, the temperature and relative humidity sensors produce a voltage that is proportional to the property that they measure. Any analog to digital conversion command converts these sensor voltages into data that a flight computer can record. The data is best converted back into analog voltages and then temperature and percent relative humidity using a spreadsheet. Figure 1. A completed Temperature and Relative Humidity Sensor

2 Parts List Sensor Head C1 0.1 uf capacitor R1 1kΩ ¼ W resistor (brown, black, red, gold) U1 HIH relative humidity sensor U2 LM335 temperature sensor The remaining items are required to complete the NearSys Temperature and Relative Humidity Sensor, but they do not have a reference on the PCB. Wire (#24 AWG) Four-pin right angle header Four-channel Easy-Plug printed circuit board White plastic lid Figure 2. The PCBs from left to right are the Sensor Head and the 4-pin Easy-Plug. They connect together with #24 AWG wire. Theory of Operation Figure 3. NearSys Temperature and Relative Humidity Sensor Schematic.

3 Both sensors require five volts to operate. Their output is proportional to the characteristic they measure. The LM335 is a temperature controlled zener diode. Its breakdown voltage is varied base on its temperature. The 1k resistor is a current limiting resistor and in some ways, it acts like half of a voltage divider circuit for the LM335 voltage sensor. The HIH-4000 relative humidity sensor contains an exposed silicon die that reacts to the relative humidity. The rest of the IC produces a linear voltage in response to the relative humidity. The 0.1 uf capacitor grounds stray AC on the five volt supply line. Assembling the NearSys Temperature and Relative Humidity Sensor Solder the following parts R1 1 kω (brown, black, red, gold) C1 0.1 uf U1 HIH-4000 Figure 4. Parts layout for the T&RH sensor. Note: The open face of the relative humidity sensor faces the letters U1 on the top silk. U2 LM335 Note: Orient the temperature sensor according to the diagram in the white top silk. Cut four equal length wires and strip ¼ of insulation from one end Route one wire through each strain relief hole (the stripped end) Solder each wire to the PCB

4 Note: The strain relief holes are the four large holes near end of the PCB. The wires pass through the holes from the bottom of the PCB and then bend over so the bare ends can be soldered into the smaller holes near the strain relief holes. Using strain relief prevents normal use from breaking the wires off the PCB. Figure 5. An example of a wire and the strain relief hole. Notice that the wire remains insulated as it passes through the strain relief hole. The only place the wire is bare of insulation is where it is soldered to the inside soldering pad. Figure 6. The finished T&RH sensor head. Now the four wires in the sensor head will be soldered to the Four-channel Easy Plug. Figure four labels each of the four wires. Use those labels to solder the wires into their correct pads on the next PCB.

5 Figure 7. The 4-pin Easy Plug PCB. Wires from the sensor head are soldered to pads on the top and the 4-pin right angle header solders to the four pads at the bottom. Strip ¼ of insulation from the other one end of each wire Solder the ground (GND) wire to one of the side pads marked with a G Note: Route all the wires through their strain relief holes before soldering the wire to the Easy Plug. Note: Which G pad the wire is soldered to doesn t matter, since both are connected together in the Easy Plug. Solder the +5V wire to one of the pads marked with the +5 Solder the Temp wire to the pad marked with the 2 Solder the RH wire to the pad marked with the 4 Solder the four-pin right-angle header to the bottom of the Easy Plug Figure 8. Completed Four-channel Easy Plug.

6 Adding the Temperature and Relative Humidity Sensor to a BalloonSat The sensor head can be either be attached to the outside of the airframe or mounted inside near an opening in the airframe that allows air to reach the sensors. The sensor head has four holes that permits #2-56 bolts to attach the sensor head to the face of a BalloonSat. Alternatively, the sensor head can be attached to Styrofoam using hot glue. Both the temperature sensor and the relative humidity sensors are affected by exposure to direct sunlight. Therefore, it is important to limit their exposure to the sun if the most accurate readings are desired. The white plastic lid included in the sensor kit can be placed over the sensor head to limit exposure to the sun while still permitting air to flow around the sensors. The lid is easily drilled and can be bolted to the airframe. Using the Temperature and Relative Humidity Sensor The Four-channel Easy-Plug fits into the Easy-Port of the BalloonSat Mini flight computer. The pins are oriented properly, so verify the Easy-Plug s +5 pin is plugged into the +5V socket on the BalloonSat Mini. The Flight Computer needs code to digitize the two voltages produced by the Temperature and Relative Humidity Sensor. There are two BASIC commands that can do this, which is referred to as analog to digital conversion (ADC). The first converts a 0 to 5 volt signal to an 8-bit value between 0 and 255. READADC 2,B0 The second example converts a 0 to 5 volt signal to a 10-bit value between 0 and READADC10 2,B0 Which command you use is up to you. Just remember that the 10-bit conversion requires one word (two bytes) of memory to store (as opposed to only one byte of memory to store the 8-bit value). However, the greater resolution afforded by the 10-bit conversion is four times greater than the 8-bit version. With the configuration recommended in these directions, I/O channel 2 will report the temperature and I/O channel 4 will report the relative humidity. Converting a LM335 Reading into the Temperature One volt of the temperature sensor represents 100 kelvins. Use the following calculations to convert the result of the ADC of the temperature signal into the temperature. From the READADC Command Temperature (K) = (ADC Value / 256) X 500

7 From the READADC10 Command Temperature (K) = (ADC Value / 1024) X 500 Since the kelvin temperature scale is not a common unit for most people, convert the units of kelvins into units of Celsius by subtracting 273. O C = K 273 The Celsius scale can be converted to the Fahrenheit scale 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 the Celsius and Fahrenheit scales intersect at -40 degrees. With this in mind, the conversion is as follows. O F = (( O C + 40) * 1.8)-40 Converting a HIH-4000 Reading into the Relative Humidity The HIH-4000 produces a voltage proportional to the relative humidity. However, the voltage only spans 0.8 volts to 3.9 volts. Use either one of the following spreadsheet commands to convert the ADC reading into the percent relative humidity. From the READADC Command The following command will convert the RH reading in cell C3 into the percent relative humidity. = ((+C3*5/256)-0.8)/0.031 From the READADC10 Command The following command will convert the RH reading in cell C3 into the percent relative humidity. = ((+C3*5/1024)-0.8)/ December 2012