Variometer

Selbstbau eines ~15 € Variometers, wie ich erfahren habe, nicht sehr schnell der Drucksensor und Co, aber völlig ausreichend. Für den ambitionierten Segler  ->L1D vom Trappy.


***** Pixelproc Anleitung leider nicht mehr online (hier noch im Webarchiv), Nachfolger, das Coinvario leider auch nicht. *****


Anleitung auf http://www.pixelproc.net, geht nicht mehr!


Noch gefunden auf Spanisch: Anleitung als pdf.


Text von Pixelproc ohne Links:

A cheap, sensitive and tiny audio-only vario

This vario uses the Microchip PIC24HJ12GP201 16bit microcontroller, a Bosch BMP085 pressure sensor and Kingstate KPEG006 piezo speaker.


Schematic


Please note that the capacitors C5 and C6 are actually two capacitors in parallel, the values (approx 10uF) as shown, plus a 0.1uF. The 0.1uF caps are important as they have low impedance in the MHz clock frequency range. First solder the 0.1uF ceramic smd caps, then stack the larger values (ceramic or tantalum) on top and solder them.

Ceramic or tantalum caps are a must as they have adequately low ESR. Electrolytic caps will compromise the performance of the sensor.

The circuit current draw is 3mA at 3V, withe PIC24 MCU running off the internal FRC oscillator at an effective clock of 460kHz. You have a variety of options for the power source :

1. A small coin cell 3.7V Li-ion rechargeable coin cell battery e.g. the PD2450 with a capacity of 200mAH, available from Sparkfun. Note that the usual Lithium coin cell batteries CR2032 etc. are not suitable. They have a nominal voltage of only 3V and are not rechargeable.
2. A single AA/AAA battery with a DC-DC converter circuit using for example the MAX1678.
3. Build the whole unit into another existing instrument like a commercial gps, and leech off the power supply.


The photo shows a prototype of the audio vario with some unorthodox free-wiring on a PCB ground plane. The only schematic component missing from this unit is the 3V LDO linear regulator. That's because it's meant to be inserted in a commercial gps, to convert it into an el-cheapo gps-vario. The size of this prototype is 12mm x 17mm - thats all the space I had !

This vario can be used as a stand-alone audio vario, which powers up with default settings for sinktone threshold and climbtone threshold. Of course you can always re-compile the code with different parameters.

You can also use the unit as a "vario co-processor" in a flight instrument. It will respond to commands on the SPI bus to set the sinktone and climbtone thresholds, turn the audio on and off, or return data such as altitude, temperature, climbrate etc.

Be careful soldering the BMP085. Flux residue and excessive solder trapped under the sensor can result in low-resistance paths, manifested as abnormally high temperature readings and circuit current (nominal current draw is about 3mA at 3V regulator output). Another pilot experimenter reported the same problem and fixed it the same way I did, by re-soldering and cleaning with alcohol.

Make sure the pressure sensor opening is taped shut during assembly and cleaning. Preferably use a "no-clean" flux pen.

I have had success with the following procedure :
1. Tin the pressure sensor pads and PCB footprint pads with solder.
2. Remove the excess solder using a desoldering wick. If you have solder bumps on the sensor pads or PCB footprint, it will be hard to align the sensor on the footprint without it sliding around.
3. Mark the position of the centre pads on each side of the sensor with a black marker. This will help with alignment of the sensor on the pcb footprint.
4. Tin a strip of copper foil (about the width of the sensor side) liberally with solder on both sides. Add some "no-clean" flux on both sides of the foil.
5. Align the sensor on the PCB footprint.
6. Slide the tinned foil a few mm under one side of the sensor and heat the foil outside the sensor with a soldering iron. Once the solder melts, hold the iron for a second, then slowly slide the foil out, keeping the sensor in place. Repeat for the four sides.

If you do this correctly, the solder will wick between the sensor pads and PCB footprint pads as you draw the foil out. You will end up with a small air gap between the sensor and board, as recommended by the BMP085 sensor datasheet. This also makes it easier to clean any flux residue.

Gerber Files for PCB

  This pcb design uses the SOIC-20 surface mount version of the PIC24HJ12GP201 - (thanks to Carsten Halpick for finding a track bridge error in the top layer - now fixed). The board is 19mm x 18mm, double-sided, through-plated with components on both sides. Passes BatchPCB online DRC tests. If you order from them, the cost of a PCB with solder masks and silkscreens is USD2.50 + shipping.


Source code

Compiled with the free MicroChip C30 compiler suite. Flashed with the Microchip PICKit2 USB programmer.





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BMP085 Drucksensor







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