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Arduino altimeter

by David Sariñena

An altimeter made in an easy, compact, cheap and light way, suitable for small rockets with D or E type engines, made just with 3 components plus a battery and a perfboard. Size is 30mm x 105 mm, and weight, included a 1 cell LiPo battery, does not exceed 40gr. The EEPROM chip that appears in the second picture is not required for this project.



Components
•    Arduino FIO board.
•    A 1cell LiPo battery, with a JST-PH 2mm connector (not the usual JST-XH 2.54mm connector)
•    A BMP085 altimeter sensor built over a small breakout board. Caution: you will find in the market different boards for the BMP085 sensor, with different quantity of pins and in different positions. My schema uses the Sparkfun breakout board.
•    An acoustic device which allows to get the maximum height without connecting the device to any computer.
Additionally, it is required a USB-FTDI cable to load the program code, or a couple of xbee modules for wi-fi communication.

Wiring

There are no connections (red marks) in the right side of the design. That’s because there won’t be components in this part of the device. The idea was to develop an upgrade using other components such an EEPROM memory, and I designed the wiring taking into account this, so it can be simplified to:



Noise filtering
The BMP085 has 4 working modes, and in the quickest one the r.m.s can be up to 2 meters. Also you can find non desirable peaks, so I made in my code a couple of noise filtering procedures.
Not to use any sample which supposes a change of meters equivalent to a Mach 2 speed (all D & E rockets cannot reach those speeds).
For the rest of measures, use a smoother function that doesn't require much memory. The selected one was a Double Exponential Smoothing function.

Accoustic Protocol
The last issue to define as part of the design is the tone protocol for the acoustic device, and it will be as reviewed in this table:

Concept

Tone qty

Duration

Frequency

Ref. pressure calculated

1 tone

2 s

440 Hz

 

 

 

 

MEASURE = 5 dígits

 

 

 

Start measure signal

3 tones

0.5s detached by 0.25s and 2s silence at the end

440 Hz

Number 0

1 tone

1 s

600 Hz

Number 1 to 9

1 a 9 tones

0,25 s detached by 0,25s.

600 Hz

Gap between digits

 

0,75s of silence

 

End of protocol

 

2s of silence

 


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