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La ACES Student Ballooning Course
Programming Unit
Activity P7 – Measuring Temperature with BalloonSat
Summary:
Students will use the BalloonSat development board to measure temperature with the on-board
temperature sensor. Students will measure the voltages at test points to verify proper operation of
this subsystem.
Materials:
Each student team should have the following materials, equipment, and supplies:
1. An assembled and tested BalloonSat development board.
2. Bench power supply capable of supplying 9 to 15 VDC at 100 mA of current.
3. Personal computer with Parallax BASIC Stamp software installed.
4. Standard 9-pin serial cable.
5. BalloonSat schematic diagram. (BalloonSat Assembly Manual)
6. Manufacturer’s datasheet for the Analog Devices AD780 Precision Voltage Reference.
Procedure:
1. Setup the BalloonSat development board, appropriate DC power supply and host
computer. Verify communication with BASIC Stamp micro-controller.
2. Referring to the BalloonSat schematic diagram, identify the components related to the
temperature measurement subsystem. Locate IC’s U4, U5, U6 and associated
components.
3. Refer to the component datasheet for the AD780. The circuit used on BalloonSat is
similar to the example shown in Figure 9 of the AD780 datasheet. Your BalloonSat may
not have resistors installed at locations RV1, R5 or R6. These resistors set the gain of
amplifier U6 according to the relationship
Gain = 1 + R6/(RV1 + R5).
What is the gain of the circuit of Figure 9 of the AD780 datasheet? Install available
resistors on the BalloonSat to set a gain of approximately 2. For example if you install 1K
resistors at R5 and R6 and set variable resistor to near zero resistance, the gain will be
about 2. The ratio of values is more important than the absolute value.
4. What is the expected voltage that you would measure at U5 pin 3 (temperature transducer
output)? Use a multimeter to measure the voltage at that point being careful not to
inadvertently touch other pins of the IC. That is your first test point. Is the reading
consistent with the ambient temperature?
LSU v06/04/2007
Programming Unit - Activity P7
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La ACES Student Ballooning Course
Programming Unit
Activity P7 – Measuring Temperature with BalloonSat
5. The next test point to measure is the voltage at U6 pin 3. This is the input to the amplifier
stage and should be the same value as at the previous testpoint (assuming the temperature
hasn’t changed).
6. The output of the amplifier is the next test point to measure. What do you expect to see
with the multimeter? This level should be the voltage at the input of the amplifier multiplied
by the gain of the amplifier.
7. Now measure the voltage at U4 pin 6. This is the amplifier output and is also the input to
the analog-to-digital converter. This test point’s voltage should be the same as measured in
step 6 above. If not, be sure that JMP3 has been installed in the proper position to route the
amplified temperature signal to the ADC. If necessary install a jumper at JMP3 and repeat
the measurement.
8. Modify the ADC program you wrote in a previous activity to digitize the temperature
signal at CH3. Compare the value reported by your program to the value measured with the
multimeter. Compare to the expected value given the current temperature and voltage gain
established by opamp U6.
Expected Outcome:
Each student should have configured BalloonSat to measure temperature using the on-board
transducer of U5. Students should have traced the temperature sensor’s signal from U5, through
intermediate stages, to the ADC chip and measured voltage levels at logical test points.
References:
BalloonSat Assembly Manual (Includes schematic, parts list, and layout drawing)
Parallax Programming Handbook (www.parallax.com)
BASIC Stamp Editor software and Help files
Manufacturer’s datasheet for AD780 Precision Voltage Reference.
LSU v06/04/2007
Programming Unit - Activity P7
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