Poorman's Space Program

Designing Near Space Experiments (Part 2)

Paul Verhage

In Part 1, we discussed data loggers suitable for a BalloonSat. Now it's time to discuss sensors.

3.1.7 Temperature Sensor

Measuring temperature is among the oldest of the various scientific measurements made from high altitude balloons.

Figure 3.5.  A temperature sensor based on the voltage divider circuit.

The temperature sensing element in this sensor is a thermistor, or temperature sensitive resistor.  Power to operate the temperature sensor comes from the Hobo data logger, so no additional batteries are required to record temperature.  The mathematical equations that convert these sensor readings into temperature will be described in Chapter 5. 

Table 3.1.  Parts List for Temperature Sensor

2.5 mm (3/32 inch) stereo plug

#24 AWG stranded wire (red, green, and white suggested)

Fixed resistor (1/8 or ¼ watt)

Thermistor

Small diameter heat-shrink tubing

Determine the best value for the fixed resistor (Rf) as follows:

□ Measure the resistance (Rc) of the thermistor at dry ice temperature or get the resistance of the thermistor at -78^O C (-100^O F) from its data sheet.

□ Measure the resistance (Rh) of the thermistor at 48^O C (100^O F) or get the resistance of the thermistor at 48^O C (100^O F) from its data sheet.

□ Multiply Rc by Rh and take the square root of the value.

□ The result is the best value for the fixed resistor, Rf.

Note: The temperatures of -78^O C (-100^O F) and 48^O C (100^O F) where chosen because they approximate the minimum and maximum temperatures the temperature sensor will ever experience in a near space flight. The square root of their product is the geometric mean. A voltage divider circuit with a fixed resistor equal to the geometric mean of the variable resistor produces the largest voltage change in the voltage divider circuit. And the greater the voltage change, the smaller the temperature change that can be detected.  

The 2.5 mm stereo plug provides power to the sensor, voltage from the sensor, and a ground connection. The three connections in a stereo plug are tip, ring, and base and illustrated below.

Follow these directions to make a thermistor based temperature sensor for the Hobo:

□ Cut a green and a white wire as long as the completed temperature sensor.

□ Cut a red wire three inches long.

□ Strip 3 mm (1/8 inch) of insulation from one end of all three wires.

□ Lightly tin each stripped end.

□ Open the housing on the 2.5 mm (3/32 inch) stereo plug.

Note: The stereo plug has three solder pads for its tip, ring, and base sections.  Identify the solder pad for which section of the plug.

□ Lightly tin each solder pad of the stereo plug.

□ Solder the ends of the three wires to the following pads.

See Fig. 3.7 while carrying out the next set of directions:

□ Strip 6 mm (¼ inch) of insulation from the free end of the red wire and tin it.

□ Cut both leads of the fixed resistor to 6 mm (¼ inch) long.

□ Tin both leads of the resistor.

□ Slide a 12-mm (½ inch) long piece of 3.5 mm (1/8 inch) diameter heat shrink tubing over the red wire.

□ Place one lead of the resistor in contact with the tinned red wire.

□ Heat the red wire and resistor lead with a soldering iron and solder the two together.

□ After the joint cools, slide the heat shrink over the soldered connection and shrink.

□ Lay the resistor against the white wire.

□ Mark where the remaining lead of the resistor lays against the insulation of the white wire.

□ Carefully strip insulation from the white wire at the marked locations.

□ Twist the lead of the resistor once around the white wire and solder them together.

□ Slide 2.5 mm (3/32 inch) diameter heat shrink over the soldered resistor and white wire.

□ Trim the white and green wire to the same length.

□ Strip 6 mm (¼ inch) of insulation from the ends of the white and green wires.

□ Tin the ends of the white and green wire.

□ Cut two pieces of 3.5 mm (1/8 inch) diameter heat shrink tubing.

□ Slide the heat shrink over the white and green wires.

□ Trim the leads of the thermistor to 6 mm (¼ inch) long and tin.

□ Place a lead of the thermistor against the tinned white wire.

□ Heat the wire and thermistor lead with a soldering iron and solder them together.

□ Repeat this process with the green wire and the other lead of the thermistor.

□ Slide the heat shrink over the solder thermistor leads and shrink.

□ Twist the wires of the temperature sensor together.

3.1.8 Light Sensor

A photocell can function as the variable resistor in a voltage divider circuit to create a simple light sensor for a Hobo. 

A common cadmium sulfide (CdS) photocell has a spectral sensitivity similar to that of the human eye. The following components are needed to make a CdS photocell-based light sensor for a Hobo data logger.

Table 3.3.  Parts List for a Light Sensor

2.5 mm (3/32 inch) stereo plug

#24 AWG stranded wire (red, green, and white suggested)

Fixed resistor (1/8 or ¼ watt)

Cadmium sulfide (CdS) photocell

Small diameter heat shrink tubing

Determine the best value for the fixed resistor (Rf) as follows:

□ Measure the resistance of the photocell in the dark (Rd) or get its dark resistance from its data sheet.

□ Measure the resistance of the photocell in bright sunlight (Rl) or get its light resistance from its data sheet.

□ Multiply Rd by Rl and take the square root of the value.

□ The result is the best value for the fixed resistor, Rf.

Now follow the same directions for assembling the temperature sensor, but replace the thermistor with the CdS photocell.

3.1.9 LED Photometer

A device that measures the intensity of a band of wavelengths of light is a photometer. Here we will build a photometer that uses light-emitting diodes (LEDs) to detect sunlight.

Figure 3.10. A completed NearSys LED photometer.

A light-emitting diode (LED) emits light having a narrow band of wavelengths determined by the composition of the semiconductor chip inside the LED. Not only does a LED produce light of a specific color, but, as described in various papers and books by Forrest M. Mims III, it's also sensitive to the same color of light. Therefore, when the proper color of light shines on an LED, it functions as what Mims describes as a "spectrally selective photodiode."

LEDs, like solar cells, produce a current proportional to the amount of light shining on them. If the intensity of the light doubles, so does the amount of current produced. However, for this to be useful to a data logger, the current must be converted into a voltage. There are several ways to make this conversion, and this photometer does so by passing the current through a resistor and measuring the voltage drop across it.  According to Ohm's Law, the voltage drop produced across a resistor is equal to the resistance of the resistor times the current flowing through it. A change in the resistor's voltage drop is what the data logger measures when the intensity of the properly colored light intensity changes.

Building the LED Photometer

To reduce its directional sensitivity, this LED photometer uses eight diffuse, epoxy-encapsulated LEDs that point outwards at 45-degree intervals. Therefore, as the BalloonSat rotates around its vertical axis, the combined LEDs should measure the same total light intensity around the horizon. In addition, combining the currents from all the LEDs makes the photometer more sensitive because of the increased current. Figure 3.11 is the circuit diagram of the photometer.

Table 3.4 lists the parts required to assemble the photometer.  

Eight diffuse (not water clear) epoxy-encapsulated LEDs of the same color

Resistor (see Table 3-5 below)

Wire

2.5mm (3/32 inch) stereo plug

A NearSys LED photometer printed circuit board (PCB)
Foamcore

Note: The PCB is available from NearSys@gmail.com .

Table 3.5 lists the recommended values for the photometer. You can experiment with these values to obtain optimum results.

Table 3.5.  Recommended Resistor Values for the LED Photometer.

LED            Resistor

Red               4.7 M             

Yellow           470 k              

Green            4.7 M             

Infrared         10 M  

Assembly Directions

For the photometer to work, the LEDs must be soldered in the proper orientation. A single backward LED blocks the flow of current from the other LEDs and the photometer produces no output. Look carefully at an LED and you’ll notice that its rim is flat next to one of the leads, usually the shorter lead (as illustrated by Fig. 3-13). The flat rim and short lead marks the cathode of the LED. While assembling the photometer, keep in mind that Fig. 3-12 marks this lead of the LED as the LED flat side.  

Follow these steps to assemble the photometer:

□ Cut two wires long enough to reach between the Hobo and the outside of the BalloonSat where the photometer will be mounted.

□ Bend the LED leads at a 90 degree angle so they lay properly on the PCB.

□ Solder the LEDs as Figure 3-12 illustrates.

□ Bend the leads of the resistor and solder.

□ Strip 6 mm (¼ inch) of the insulation off one end of both wires.

□ Solder the stripped ends of the wires into the PCB as illustrated in Fig. 3-12.

□ Trim the soldered wires on the PCB.

□ Open the housing of the 2.5 mm (3/32 inch) stereo plug.

□ Slide the plastic housing over the two wires.

□ Identify the tip and base pads of the stereo plug.

□ Strip 4 mm (1/8 inch) of insulation from the remaining ends of the two wires.

□ Tin the wires and solder to the stereo plug.

□ Apply hot glue to the soldered pads and screw the housing back over the stereo plug.

Mounting the Photometer

There’s no one correct way to mount the photometer onto the BalloonSat. So what follows are three suggestions:

Suggestion 1

□ Attach the photometer to the outside of the BalloonSat with hot glue.

□ Pass the photometer’s cable through a hole drilled in the BalloonSat’s airframe.

□ Restrict the photometer’s field of view to a narrow band along the horizon by sandwiching it between the airframe and a sheet of foamcore.

Suggestion 2

□ Mount the photometer to the outside of the BalloonSat airframe.

□ Pass the photometer’s cable through a hole drilled in BalloonSat’s airframe.

□ Cover the photometer with a translucent plastic cap (like half a plastic baseball) to diffuse the light shining on the photometer.

Suggestion 3

□ Mount the photometer inside the BalloonSat.

□ Cut openings through the airframe to admit light.

□ Cover the openings with white tape to diffuse external light.

Output from the LED-based Photometer

Since the current output from the LEDs is proportional to light intensity, the photometer voltage recorded by the Hobo is also proportional to light intensity. However, determining the absolute intensity of sunlight before launch is beyond the scope of the BalloonSat Principia. Therefore, charts of the photometer voltages can only show the relative changes in light intensity as a function of time.

If you’d like to experiment with a more sophisticated LED photometer, then check out the GLOBE Sun Photometer web site recommended by Forrest M. Mims III (which uses an LED-based sun photometer based on his design) at http://www.mcs.drexel.edu/~dbrooks/globe/globe_work.html. You can read more at Parallax, www.parallax.com by checking out their Earth Measurements lab book for its photometer exercise.

Note that LEDs used as spectrally selective photodiodes are more temperature sensitive than conventional silicon photodiodes. If you want data that accurately indicates the sunlight intensity throughout a flight, which, as noted above, is beyond the scope of the BalloonSat Principia, it will be necessary to determine the temperature coefficient of the LEDs. You can then use the temperature measured by the BalloonSat to apply a correction to the sunlight data. Editor.

Part 3 of "Designing Near Space Experiments" will be published in the August 2008 installment of The Citizen Scientist.