APPLICATIONS
The MGTA has many possible
applications, ranging from whimsical
to serious engineering uses. On the
fun side of things, an MGTA could be
the basis for an indoor wind chime. It
is very sensitive to normal room air
currents. The MGTA voltage output
can be digitized in as few as eight
bits to generate digital values
representative of the micro gusts and
breezes in a room. Many microcom-puters include internal eight or 10 bit
analog-to-digital converters (ADCs).
These values could be used by a
microcontroller to play wind chime-
like sounds and tones, musical notes,
or melodies and songs.
An MGTA can also be used to
make a sensitive leak detector for
finding hidden air leaks around doors
or windows. The circuit can also
monitor air currents on a lab table top
for identifying the effects of small
winds (breezes) on circuits under test.
It could also serve as a wind velocity
indicator in a low velocity wind tunnel
for paper air planes, or possibly as an
air speed indicator for slow moving
model gliders. An MGTA can also be
useful to study the thermally insulating properties of materials including
small insulators and coffee cups.
several seconds (as dominated by the
LM399 oven time constant).
Slow Start Instructions
(Troubleshooting)
op-amps).
For the recommended LM6132BIN,
that’s around 0.5V, for the OPA2277 or
OP-297 zero wind is around 2.5V. If
you are unable to set the zero wind
value, power down and read the circuit
description and troubleshooting
sections below. If the output voltage
looks good, take off the coffee cup. The
output voltage should jump up by
about one or two volts, depending on
the micro winds near the LM399. If the
MGTA is still checking out okay, blow a
tiny puff of air over the LM399. You
should see a noticeable change in the
output voltage above 1V. Higher
breezes cause changes over several
volts. The instrument reaches full scale
(saturation) in a strong breeze.
A digital oscilloscope display (or a
chart recorder) can show the time
response of the MGTA. With a Styrofoam
coffee cup rested over the LM399 (but
still leaving the trimpot exposed), the
output voltage should be at the zero
wind set point. With the coffee cup
removed, the MGTA measures the micro
gusts over the work table surface. And,
after a puff of air is blown at the LM399,
there can be an output voltage rise ranging from mV to several volts over less
than one second, followed by a decay of
Apply power to the circuit. Set the
regulated power supply voltage to
within about 100 mV of 15V ( 14.9V to
15.1V). You should be able to set
about 6. 9 V (±0.5V) at pin 3 of U2A
using the trimpot.
Next, with no wind (coffee cup
over LM399), check the voltage
between the heater (+) connection
and common. At no wind (coffee cup
installed), you should see about 13V
(±0.5V). At full wind (blow harder),
this voltage drops to about 9.2V
(±0.5V). If you feel an urge to touch
the can, don’t (ITISHOT). Instead, use
something like the top swivel end of a
jeweler’s screw driver to force the oven
heater full on (but only for a moment
so as not to damage the LM399!).
Once the LM399 front end is
working, move to pin 1 of the op-amp.
This first stage output should vary from
roughly 0.1V (higher with non rail-to-rail op-amps) to 4.5V as measured to
common from no wind to a full scale
breeze. And finally, pin 7 of the dual
op-amp should range from about 0.5V
no wind (higher with non
rail-to-rail op-amps) to about
14V at a full scale.
■ The formal schematic.
Circuit
Description
R1 can serve dual roles
as both the LM399H heater
current sense resistor and as a
current limiter to prevent a
dangerous overheating condition in the event of a short in
the LM399. The voltage at the
node of R1 and the LM399
heater typically ranges from
13V at no wind (with the coffee
cup over the LM399) to 9V full
scale (a very small breeze
indeed!). R1, the 100 ohm
sense resistor, causes a
voltage drop from the 15V rail
proportional to the LM399
42
February 2006
