Altimeter and Barometer using a Picaxe14M2

This is an improved version of the altimeter made in 2008. The sole hardware change is to substitute a Picaxe 14M2 for the 14M. This permits better software.
It is used at home as a barometer, with readout in millibars ot inches of mercury then as an altimeter on trips. For this latter application, it is designed to operate from a car battery, and to measure that voltage.
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 our trips on paved roads in the USA and Canada.

The substitution of the M2 permits the better six-range approximation used on the Mobile Logger to convert pressure to altitude. The output of READADC10 is accumulated 12-17 times according to the range, and then an offset is deducted. Provision is made for properly displaying altitude below sea level and when in adjustment mode.
The barometer code is simpler. The count is acumulated then divided and a constant added to give the pressure in either 0.01 in of Hg or 0.1 millbar. The voltmeter also has a biassed amplifier to cover only the range 5 to 15V, displaying in 10 mv steps.

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.

This is the front of the Altimeter:

Here are the circuit diagrams

Here is the altimeter code