Digital Altimeter / Project Number DK113 - abstract
Easy and cheap altimeter with included thermometer and barometer
is described in this article. It operates on physical law that atmospheric
pressure falls with altitude. This altimeter could be use for trips, hiking,
biking and other hobbies. Altimeter could show you also ambient temperature
and actual pressure. Moreover it has memory for 10 hours and when you will be
back from trip you can download data to PC and see profile of your trip.
The resolution of altimeter is around 1m. Altimeter recognizes if you lift it
from floor to your desk! This short time resolution is better then with GPS and
in addition it doesn't need signal from satellite. One principal disadvantage is
dependence on pressure change during trip. Practical experiences show that this
doesn't annoy at all.
The block schematic is shown in attached picture. The heart of altimeter
is microcontroller P89C51 which collects and processes data from all other
devices. Absolute pressure sensor MPX4115 provides output voltage proportional
to atmospheric pressure. This voltage is amplified by factor of 2 twice,
filtered and connected to one input of 8 channel 12bits Analog to Digital
converter AD7888 which communicates with microcontroller.
Programmable voltage reference TL431 provides 2.5V output which is
amplified to 4.6V and use as accurate reference for AD converter.
Measured data are displayed on 1x16 matrix LCD display. There are
5 variables available - temperature, altitude, actual atmospheric pressure,
free memory and trip duration in minutes. Dependence of altitude on pressure
change is defined by formula with logarithm. It was main reason for using
C language with compiler for writing of control program. The resolution of
used AD converter is increased by 256 averaging of measured value.
Temperature and actual pressure needs calibration because of variation
of parameters. There is calibration menu available in altimeter where you
can setup correct value of temperature and pressure. Calibration data are
then stored in EEPROM memory and reloaded after each start. You don't need
any special programmer and you can adjust altimeter for your purpose yourself.
This EEPROM memory is also use for saving of measured altitude.
There is space for about 10 hours trip. Data are sent do PC through standard
serial port. Available free space is also shown on display. Easy infrared
wireless port is available, the only wire you need is for charging battery.
Whole altimeter is powered from 2 NiMH batteries in AAA size.
DC to DC converter MC33463 is used for converting this low voltage to +5V.
Recommended fast and easy charger is also described. The basic power
management of complete altimeter is used. Microcontroller uses idle mode
when it waits for pressed buttons. Analog part of all altimeter is powered
only during measurement. The system also watches battery voltage and it
shows warning message if it is under limit.
Altimeter is using only 3 small SMD devices. The rest uses common
components, is easy to build and reproducibility is without any problems.