This is two instruments using the same hardware. Both applications could not be coded in the limited menory of the 14M. It will be used at home as a barometer, then re-loaded to go on a RV trip as an altimeter. For this latter application, it is designed to operate from a car battery, and to measure that voltage. In our RV, the refrigerator runs off the “house” batteries, which last about two days with no recharge, hence this latter facility. In both applications means are provided to set the offset, either sea-level barometric pressure or current altitude, respectively.
The design uses the Freescale MPX4115A sensor. A biassed amplifier restricts the range to –700 to 13000 feet, adequate for an RV on paved roads in the Western USA. A two-range approximation is used to convert pressure to altitude, being the most that would fit in the code space. Up to 5500’ the output of READADC10 is accumulated 13 times, and deducted from 12250 to give the altitude in feet. Above that the multiplier is 16 and the offset 13500. There is a + or – 200’ hysterisis to prevent jitter errors. Provision is made for properly displaying altitude below sea level. The approximation error is between –160 and +129 feet.
The barometer code is simpler. The count is acumulated 32 times, divided by 25 and 1774 added to give the pressure in 0.01” of Hg. In this case there was enough code space to store and adjust the offset in EEPROM. This approximation error is –.0022 to + .0012” from 25 to 30.5”.
The voltmeter also has a biassed amplifier to cover only the range 5 to 15V, displaying in 10 mv steps. The measured error is less than 0.3% from 7 to 15V.
The display uses a MC14489 serial LCD controller. However, a military-surplus six-digit 7-segment incandescent display was used, which is clearly readable in full sun. This required isolating diodes in each segment and boosters for both the segment and digit drives. Full details are on http://www.worsleyassociates.com/Projects/Altimeter/index.html
This now includes(2/23/2008) some observations on a road trip ranging from 100 ft below sea level to 5000 ft.
The design uses the Freescale MPX4115A sensor. A biassed amplifier restricts the range to –700 to 13000 feet, adequate for an RV on paved roads in the Western USA. A two-range approximation is used to convert pressure to altitude, being the most that would fit in the code space. Up to 5500’ the output of READADC10 is accumulated 13 times, and deducted from 12250 to give the altitude in feet. Above that the multiplier is 16 and the offset 13500. There is a + or – 200’ hysterisis to prevent jitter errors. Provision is made for properly displaying altitude below sea level. The approximation error is between –160 and +129 feet.
The barometer code is simpler. The count is acumulated 32 times, divided by 25 and 1774 added to give the pressure in 0.01” of Hg. In this case there was enough code space to store and adjust the offset in EEPROM. This approximation error is –.0022 to + .0012” from 25 to 30.5”.
The voltmeter also has a biassed amplifier to cover only the range 5 to 15V, displaying in 10 mv steps. The measured error is less than 0.3% from 7 to 15V.
The display uses a MC14489 serial LCD controller. However, a military-surplus six-digit 7-segment incandescent display was used, which is clearly readable in full sun. This required isolating diodes in each segment and boosters for both the segment and digit drives. Full details are on http://www.worsleyassociates.com/Projects/Altimeter/index.html
This now includes(2/23/2008) some observations on a road trip ranging from 100 ft below sea level to 5000 ft.
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