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La ACES Student Ballooning Course
Electronics Unit
Activity E7. A/D Conversion using BalloonSat
Summary:
Students will investigate the operation of analog-to-digital and/or digital-to-analog converters
using the functionality of the BalloonSat.
Materials:
This activity will be done by the team as a whole. The team should have available the following
materials, equipment, and supplies:
1. Assembled and tested BalloonSat flight computer (supplied by La ACES and assembled in
a separate activity during the Programming Unit)
2. Basic Stamp development suite. (supplied by La ACES)
3. Sensor assortment (supplied by La ACES)
5. DC power supply for BalloonSat - 5 volts DC at 1 amp.
6. Digital multimeter
Optional extra equipment
7. Function generator capable of 1 Hz to 10 kHz sine, square, triangle wave, with DC offset.
8. Oscilloscope
The laboratory should also be equipped with the following:
1. Flat work tables sufficient to seat all students with plenty of work space
2. Computer (PC) with Basic Stamp development suite software installed. Also a data
analysis program such as Vernier's Graphical Analysis or even simply Excel should be
available.
Procedure:10 K
In a previous activity a Basic Stamp routine had been designed to read the BalloonSat A/D
converter and display and/or store the result. That program will be employed during this activity,
so it should be loaded into the BalloonSat and ready to run.
Prepare a variable resistor (10 K potentiometer) and battery assembly according to the circuit
diagram below. The connection to BalloonSat is made by soldering the lead from the
potentiometer wiper to one of the Channel Inputs and the common lead to a ground pad.
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Electronics Unit - Activity E7
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La ACES Student Ballooning Course
Electronics Unit
Activity E7. A/D Conversion using BalloonSat
Connect the potentiometer circuit to the A/D input of the BalloonSat and start the program that
reads the analog input. Also connect a digital multimeter (DMM) to measure the same signal
being supplied to BalloonSat.
Rotate the potentiometer to different settings and record both the BalloonSat measured value and
the reading on the DMM. Enter the pairs of values into the data analysis program. Record at least
20 data points spread out over the whole range of the A/D input. Include two or three points that
are beyond the range of the A/D. (greater than 2.5 V)
Plot the BalloonSat reading (dependent axis) vs DMM reading (independent axis).
Optional additional measurements
Adjust the function generator output amplitude and DC offset to deliver a signal whose minimum
and maximum instantaneous values fall within the input range of BalloonSat's A/D converter. It
may be helpful to connect an oscilloscope (if available) to monitor the output of the function
generator.
A program will have to be prepared for BalloonSat to read the A/D input as rapidly as possible
and store the measure valued into memory for later recovery and display. It is anticipated that the
maximum sample rate that can be realized, because of programming overhead, will be on the
order of 100 samples per second. Configure the program to record nor more than 2 or 3 seconds
of data.
Connect the function generator to BalloonSat's analog input. Set the frequency to 1 or 2 Hz. Let
BalloonSat record data, then recover the stored values and transfer them to the data analysis
program for plotting.
Compare a plot of the saved data values vs time to the appearance of the oscilloscope display of
the function generator signal.
Repeat the process for other frequencies. Choose several frequencies from a few Hz up to several
hundred Hz. Try to assess when the sampled data are no longer an adequate representation of
the real signal. Compare the signal frequency to the sample rate.
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Electronics Unit - Activity E7
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La ACES Student Ballooning Course
Electronics Unit
Activity E7. A/D Conversion using BalloonSat
Expected Outcomes:
For the dc measurements made using the potentiometer signal, students should notice that the
input to the A/D can change over small ranges without changing the digitized result. The graph
of digitized vs DMM values should reveal reasonably good linearity.
In the case of the time varying signals, students should be able to recognize when the sampling
rate of BalloonSat is inadequate to faithfully reproduce the input signal. This will give them an
idea of the fastest signal variations they can expect to measure in a flight payload employing
BalloonSat.
Issues:
The program to read and store data in BalloonSat is reasonably complex. Enough time, both
within formal sessions and as individual work, should be pre-allocated. In particular, use of the
on board EEPROM memory will possibly be a challenge.
References:
Basic Stamp Reference Manual
ADC083X ADC family manufacturer's datasheet
(included in Programming unit References folder)
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