station I’ll describe in my next column).
The results are stored once per minute
in the BFC memory for later retrieval.
The digital channels switch on the camera, take a picture, and then shut off the
camera. This was necessary because
the camera has an automatic shut-off
feature. It’s my contention that the
automatic shut-off built into cameras
these days is a design fault and not a
feature (as if I can’t determine for myself
when to shut off a camera).
Note that the digital channels are
not limited to cameras. There are
several types of sensors that output a
digital signal rather than an analog
voltage that can use the digital channels. Even servos can be operated
through the digital channels. As far as
I know, no BalloonSat has ever actively participated in its mission by making
something happen, they’ve only
TRANSISTOR EXPERIMENT WINNER
Earlier this summer, I asked read- occurred to most technical people to
ers to look at a transistor experiment even consider the situation.
I had run while trying to find the The resistance curves are proba-
minimum amount of current I needed bly correct. You have demonstrated a
to saturate a transistor. I received a few phenomenon which was exploited in
replies to my request and I’ve printed the early days of transistor usage.
Mr. Tom Patrick’s response here. Tom, Someone saw the same thing you
I’m sending you a patch that was sent saw, and added a bit of insight, in that
intonear space. Thanks for responding. this was a common connection for use
in precision analog switches. With
transistor switches biased in the
normal manner, the Vce saturation
voltages were typically 100 mV, they
varied with temperature, and caused
great errors in circuits which needed to
switch small voltages. With the transistor operated in the reverse direction,
the Vce saturation voltage was an
order of magnitude lower, allowing the
switch to be more accurate.
The first operational amplifiers
were made with discrete components.
A common configuration for best
accuracy (initially with vacuum tubes)
is a circuit which converts a DC signal
into an AC signal by chopping it up
with a switch, then amplifies the AC
signal to the desired voltage level.
Finally, a second switch, synchronized
to the first one, puts the signal back
together. This synchronous modulator-demodulator provided better stability,
back when a hot transistor had a
beta of 20 to 50, and manufacturing
processes could not guarantee
repeatability between components.
To summarize, the inverted connection is a circuit which had its time in
the sun, and is not used any more (or is
it?). The physics of transistors can be
explored in more detail to understand
why the inverted connection has lower
on-resistance, but with the advent of
D-MOSFETs, and the newer MESFETs,
the on resistance is so low that this
circuit has mostly been retired.
Sorry that this is so long-winded.
Thanks again, for an interesting article.
Your article in the July ‘06 Nuts &
Volts Magazine (and experiment with
measuring on-resistance) provide an
interesting question for your readers.
Your data, so nicely graphed, also illustrate a point of transistor usage which
may have been partly lost due to the
passing of time and development of
newer devices for switching purposes.
To answer your questions, consider the following:
The ohmmeter cannot directly
measure resistance. It must use Ohm’s
law with voltages and currents, and
provide an answer based on R = V/I.
In other words, the ohmmeter provides a known current, and measures
the voltage developed as the current
passes through the resistor. The
voltage is proportional to resistance
(V = I*R), so the measured voltage
reading is converted to resistance if
the current is fixed.
When you turn on the base-to-emitter junction, you create the
possibility of passing electrons from
the emitter to the collector, in normal
usage of the NPN transistor. The
barrier to electron flow is removed,
and there is no reason to expect that
you could not also pass them backwards, which is what you do when you
set up the meter with reverse polarity
leads. Nothing says that you cannot
use the transistor in an Inverted mode,
since you can read “NPN” the same
way, forwards or backwards! This point
is not made in most semiconductor
theory books, because it has not
— Tom Patrick
passively recorded data. Adding a
servo to a BalloonSat can change that.
After recovering my BalloonSat, I
needed to download its data (also
available on the Nuts & Volts website,
named “download.txt”). I use a terminal program and a downloading cable
to retrieve the data. In place of the terminal program and cable, you should
be able to use Stamp Plot Lite and a
BS-1 programming cable. However, I
didn’t have time to test Stamp Plot Lite
in time for this column, so I’ll have to
give you a report on it next time.
Note that there’s one variable to
change in the download code: the
variable NoADC (number of ADC
channels used to collect sensor data).
This variable formats the data downloaded into the text file. I made my
download cable with a three pin male
header, two wires, a female DB- 9
connector, a DB- 9 housing, thin heat
shrink, and some hot glue (next to
duct tape, hot glue is the most important item in near space!). Cut and strip
the ends of two lengths of wire and
slide one piece of heat shrink over
each wire. Use one wire to connect
the ground pin of the male header to
pin 5 of the DB- 9. Use the second wire
to connect the I/O pin of the male
header to pin 2 of the DB- 9. You’ll
have two wires soldered to the two
outside pins of a three pin header. Use
a black wire to distinguish the ground
pin of the header from the I/O pin and
there’ll be less of a chance of plugging
the header into the BFC backwards.
After soldering the connections,
cover the pins of the male header with
heat shrink. Then begin filling the
bottom half of a DB- 9 housing with hot
glue and place the soldered DB- 9 connector into the shell. Put some more hot
glue into the other half of the DB- 9 shell
and close the shell halves. Bolt the two
halves together and back-fill the opening of the housing (where the cable
exits the housing) with more hot glue.
To use the cable, plug the male
header of the download cable into an
open digital port (either P6 or P7) and
program the BFC to download its data
(in the downloading code above, data
is downloaded over P7). So that
there’s time to switch the serial cable
from programming the BFC to down-