This micro altimeter was developed by request of a friend (vega) in order to assist the competitors from the Bulgarian National Association of Rocketry (FAI Spacemodeling) during their training rocket launches. This altimeter is extremely light – about 0.35gr and with the battery it weighs less than 1 gram. The BOM for the altimeter is about 4$ and the BOM for the USB reader is 8$. I also designed this altimeter to be fully compatible with the requirements of the FAI association, however please note that this is not a commercial product but free to use and copy for non-profit use under the GNU General Public License.
The main characteristics of this altimeter are:
- Size: 8mm x 10mm
- Weight: 0.35gr without battery, 0.85gr with battery
- Power supply: 2.6v to 4.2v and 1.5mA average consumption
- Built-in hardware and software serial number
- Unique digital 1min time-altitude signature
- Sampling speed is either 20 or 40 samples per second
- Total record time 800 or 400 seconds
- Adjustable start-preparation time between 1 and 60 minutes
- Protection from reversed battery polarity
- Capable of working with a single CR927 – 3v battery
The MAV altimeter uses the newer and inexpensive BMP280 sensor which is controlled by the Microchip’s PIC12F1840 microcontroller and information is recorded in a 512Kb I2C EEPROM memory bank. BTN as indicated in the schematic is not an actual button but two bare pads on the PCB which have to be brought in contact. D2 is there to protect the device from reversed polarity when the battery is connected. Considering the very restricted power supply capabilities of a small 3V battery, this diode should have as low Vf (forward voltage) as possible.
The MAV altimeter is set-up and the information can be downloaded through the USB reader where the MAV altimeter is plugged to. This USB reader has a built in 3.3v power supply for the MAV altimeter and a 4.2v Li-Po charger which is being preset for 50mA charging current which should be suitable for Li-Po batteries with capacity no less than 25mAh.
I made the software for this altimeter of two different applications. The USER application is the main application which is normally used. This application allows you to format the memory (not the Digital time-altitude signature), download and visualize the recorded data. You can also setup the recording speed between 40 and 20 samples per second, and the pre-flight setup time, however you cannot change the software ID number.
The SERVICE application is for device maintenance purposes only. Beside the normal USER settings, it allows you to change the software ID number and to adjust the sampling clock. Because the MAV altimeter is using the PIC18F1240 internal clock which is normally accurate within 1%, in some cases it might need adjustment if the accuracy is worsen for whatever reason.
And finally the flow chart for the MICRO ALTIMETER’s internal logic organization and LED indications.
TECHNICAL AND USER FILES:
Firmware – HEX
PCB – CADCAM
USER AND SERVICE APPLICATION
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