output of approximately 500 mV between viewing
background sky and direct sunlight. This high impedance
signal is conditioned by a non-inverting BiCMOS amplifier
before being presented to an eight bit ADC channel. The
■ FIGURE 2. Acceleration,
altitude and roll interface
circuitry.
TS922IN dual op-amp is capable of rail-to-rail operation
down to a supply level of 2.7 volts (the second op-amp in
this package is configured as a voltage-follower to drive
the REF input of the MAX 4194 instrumentation amplifier).
An Introduction to Pressure
Sensing
Many different units are used to measure pressure,
depending on the country and target application (to
convert between three of the most commonly used units,
you need to know that 1 psi equals 6.8947 kilopascals
[kPa] or 68.947 mbars). Mean atmospheric pressure is
defined as 1013.25 mbar ( 14. 69 psi) at sea level. However,
the actual value can fluctuate by ± 20 mbars due to normal
weather variations. If we take into account sensor errors
and normal variations in atmospheric pressure and air
temperature, the apparent altitude can easily deviate by
hundreds of meters — not an insignificant error
compared with the maximum altitudes achieved by model
rockets. Near sea level air pressure decreases almost
linearly with increasing altitude but the relationship
becomes increasingly non-linear above a few kilometers.
For a detailed demonstration of the relationship between
pressure, altitude, and temperature, visit the NASA
interactive atmosphere simulator ( www.grc.nasa.gov/
www/K-12/airplane/ atmosi.html).
Integrated circuit pressure sensors employ piezo-resistive technology to convert pressure to an electrical
signal. The main element used is a silicon chip, which has
36
January 2009
been micro-machined to produce a flexible diaphragm,
around which four identical resistors are diffused to form
a bridge. The force of pressure on the diaphragm causes
the balance of the bridge to change, creating a differential
voltage output proportional to the applied pressure. This
technology can be used to assemble the following three
types of pressure sensor.
• Gauge: Measured pressure relative to ambient pressure
• Differential: Measured pressure relative to another pressure
• Absolute: Measured pressure relative to a vacuum
Sensors are available for a variety of pressure ranges
and outputs. The most basic pressure sensors only
provide mV level differential outputs direct from a
bridge, while more expensive versions have onboard
amplification and volt level outputs. A recent trend is
towards including an integral ADC that enables direct
2
connection to a microcontroller I C or SPI interface.
The appropriate pressure sensor for measuring altitude is
an absolute type with a maximum rating of 1,013 mbars.
For model rocketry, the sensor would only be used over a
narrow part of this pressure range and there would be
little advantage in using an expensive type that offered
general-purpose signal conditioning.
