The pressure sensor is a MOTOROLA MPX2100A. Temperature is measured with a DALLAS DS1820. The temperature value is used to compensate the pressure sensor drift. The analog to digital conversion is made with the high precision delta-sigma converter Analog Devices AN7714AN-5. Accuracy of measurement is 0.01% on a full scale of 20mV, which give the resolution of 20 microvolts necessary to reach 1m altitude resolution.
Altitude calculation from the atmospheric pressure value is made by a MICROCHIP PIC16F84-10 with a second degree polynomial approximation, sufficient to have a good accuracy between 0 and 3500m. The PIC microcontroller makes the conversion into a character dot matrix and directly generates pixel and synchro signal in accordance with the composite video standard. To do all this job in real time, the PIC is overclocked from 10MHz to 16MHz (there is no guarantee from Microchip, but that works all right). People familiar with PIC assembler will be able to look at the assembler code and notice that it is not trivial. Complexity is due do the fact that the video signal production requires a very short and tidy sequencing.
A pushbutton is used to reset the altitude when on ground.
The board wiring can be done by anyone who has some experience with a soldering iron.
You can download here the PIC programming file.

Electronic board anemometer sensor

Specifications: Altimeter: 0 to 3500m, 1m resolution, 2.5% accuracy Anemometer: 0 to 30m/s, 0.1m/s resolution Voltmeter: 2 channels (cradle and video), 0 to 18V range,     10mV resolution Thermometer: -25 to +70°C, 1°C resolution Output: composite video signal 625/50 PAL standard Power requirement: 7V to 18V, 17mA Size: 53mm x 38mm x 12mm Weight: 20g

Major difference between the Basic altimeter and the Altimeter-anemometer is the Microchip PIC16F876 microcontroller in place of the PIC16F84. This more powerful microcontroller give the opportunity to display 6 lines of 14 characters, and to add a very useful voltmeter function thanks to the integrated analog to digital converter. The anemometer sensor can be purchased from Radiospares. Sensor rotation speed is transmitted through magnetic coupling, which give a very accurate measurement. Calibration of the sensor is adjusted with a potentiometer placed on the electronic board. A special calibration picture displays the anemometer rate in m/s/Hz and the atmospheric pressure in hPa.

Calibration picture

Magnetic sensor

Specification: Compass function: 0 to 360° display, 1° resolution, 8 cardinal points display Calibration: calibration mode to improve accuracy and compensate magnetic interference Sensor size: Ø 12mm x 12mm Weight: 3g

Magnetic sensor is distributed by the Swiss company PEWATRON. It also exists under the brand DINSMORE. This sensor is made up of a mobile magnet whose position is measured by hall effect sensors. 2 version are available : analog version: 2 hall effect sensors give 2 DC voltage representative of the cosine and sine of the direction. digital version: 4 hall effect sensors give 4 boolean signal coding 8 directions (N, NE, E, SE, S, SW, W, NW). The analog sensor is used here. The 2 outputs are converted by the 10 bits analog to digital converter of the PIC16F876. The microcontroller makes the division and the arc-tangent calculation necessary to obtain the angle. To get a good accuracy, a sensor calibration is necessary to measure and store in EEPROM the maximum and minimum values of sine and cosine outputs. This calibration procedure also give the opportunity to cancel parasitic magnetic effects of metal parts in the cradle. This sensor has been set up by BN Chagny on his cradle. Accuracy proved to be very good, provided that the sensor is maintained perfectly vertical. If not, the magnet inside the sensor is no more mobile and the accuracy is affected.

Calibration picture