MINI DIGITAL BAROMETRIC ALTIMETER

FRO_CROI found on Ebay very nice, small 12v 23A battery holders for PCB mounting style and decided to fit the SMALL DIGITAL BAROMETRIC ALTIMETER just on the back of the holder.

BAH_1 BAT

So basically this is the same schematic as the SMALL DIGITAL BAROMETRIC ALTIMETER, however I moved some of the connections to different MCU pins because of PCB space limitations.

mini altimeter sch

In spite of the small size the functional remains the same.

–    128Kb EEPROM memory which allows approximately 3.5 minutes of recording
–    40 record per second
–    Blink integer values up to 9999 meters
–    USB connector for memory download
–    13 bit ADC resolution trough oversampling
–    1m per bit resolution
–    On the field it will calculate and show maximum altitude
–    Temperature adjusted for 20 degrees Celsius
–    ICSP programming capability
–    Auto start detection

After that the PCB was a bit of a challenge with those small dimensions but I managed to fit everything on a 36×12 mm PCB.

PCBOnce I soldered all components the height was 5 mm.

TOPNHBOTNH

And after I mounted the battery holder the total height is 18 mm to give the final dimensions of 36x12x18 mm.

 

FRO_CROTOPSID

Of course the weight is as important as the size and normally both are related so the final weight was also reduced to 4gr without battery and 13 grams with the battery.

WNB WWB

Because this mini altimeter is intended for model/experimental rockets and thus high acceleration forces there were some concerns about how stable the battery will be held in the holder and whether the acceleration will not open the contacts and restart the device.

So taking into consideration the weight of the battery and the usual accelerations in the hobby rocketry I’ve calculated the force that the battery will apply on the metal spring. I’ve also measured the force necessary to bend the spring and there is considerable safety margin.

The only potential problem I see is that if the battery is subject somehow to horizontal acceleration it can be somehow ejected from the holder because of the spanned spring.  This is of course easily fixable with a piece of insulation tape.

Another particularity is the fact that there is no circuit breaker – my idea was to use a piece of paper or plastic as contact insulation (like when you buy new electrical toy and you have to pull a piece of paper) which will be pulled right before the flight in order to switch on the device.

TOS

There’s another important consideration – those small 12v batteries have also small capacity – around 50-60 mAh and this altimeter consumes about 35 mA. So one battery is good probably for 2-3 flights, depending on how fast you recover the rocket. But considering how cheap are these batteries on Ebay – less than a dollar the piece I don’t think this is a big problem.

The firmware is also the same one I wrote for the SMALL DIGITAL BAROMETRIC ALTIMETER and I just changed the MCU ports accordingly.

The work diagram for Firmware v2:

FLOW_CHART

The work diagram for Firmware v1:

ALT_SM_FC

And the MINI ALTIMETER is still using the same PC application to download and visualize the results.

ALTIMETER_M_2014_11_28_21_34_28

 

MCU FIRMWARE HID V2 (hex file)
EXROCKETS_SMALL_ALTIMETER_v_2_00 (ZIP file)
MCU FIRMWARE HID V1 (hex file)
EXROCKETS_SMALL_ALTIMETER_v_1_00 (ZIP file)
PCB MANUFACTURING FILES (Gerber files)

3 thoughts on “MINI DIGITAL BAROMETRIC ALTIMETER
  1. Great project. Did you used a professional service to produce your PCB? Looks awesome! I’m manufacturing my own PCBs, still need some developments to reach that quality 🙂

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