Ava High Altitude Balloon Project

With the delivery of the Styrofoam sheets for the payload container I decided to make a hot wire cutter. Utilising an old DC PSU from some thing and after getting some proper Nichrome wire (note fuse wire doesn’t work!) I constructed the following hot wire cutter out of some old wood and a workmate. Using some work working techniques I decided to make a guide for the Styrofoam rather than marking a line and cutting down the line. By placing a block of wood marking the exact distance between the wire and the edge of the Styrofoam I was able to get some near perfect cuts. I would strongly recommend a set square to check the edges are 90′ but for my first attempt I was really chuffed.

The on/off switch was a paper clip 🙂

I’m going to construct a second rig to make notches in the pieces so they all fit snugly together but for the mean time here it is :

Knocked up a quick antenna using a 25cms 50 ohm SMA patch and a SMA to N-Type convertor.

The radials and aerial are 164mm in size to make a 1/4 wave dipole antenna ( See http://wb5mrn.com/ham/antenna.htm ) and are made from the centre core of some old TV Coax. I’ve bolted them on to a N-Type panel mount socket with some small screws and some Nyloc nuts. Also splashed a little solder on to hopefully keep it all together. Biggest problem is how to store it without bending all the radials 🙂

Firstly a massive thanks to my mate Anthony aka John. John has supplied a A560 and an A710 camera for the project. Cheers mate appreciated.

After some messing with CHDK on the A710 I have made a very quick time lapse video :

I’m struggling to get the A710 to work with a 8Gb card but sure I’ll work it out.

In other news my pink gaffer tape turned up today :

Why pink ? Long story I lost a bet. But it’s probably not a bad choice of colour for standing out when locating it. Finally my Styrofoam has arrived as well, I need to look to making a hot wire cutter next.

So things to do :

1/ Make a payload container.

2/ Test CHDK script + make it boot from 8Gb card

3/ Design a PCB for the flight computer.

The one final issue with my design, the intermittent operation of the GPS has been fixed. Looked like a dry joint on the VCC for the antenna. After a repair job (see picture below) the GPS started spewing forth location information as intended. Additionally I did my first untethered battery powered test which worked perfectly. Its worth noting at this point even if you are using one of Inventek’s active patch antennas you must still power pin1. I know I’m not the only person to make this mistake, Inventek recommend an inductor/capacitor filter/protection on this pin as well so I’ve included this in my design (note the surface mount inductor on breadboard!)

Click for bigger!

Onwards with the design of a PCB!

My NTX2 (434.075) was wired up to the rest of the flight computer. Using Rob Harrisons code at http://www.pegasushabproject.org.uk/wiki/doku.php/ideas:notes?s[]=ntx2 it was a quick job to insert this into my code and pass the telemetry string too it. This was the easy bit 🙂 The theory is fairly simple.

By adjusting the voltage on the TXD pin between two values (“MARK” and a “SPACE”) you can effectively make the 1’s and 0’s data which can then be decoded by DL-FLDIGI. Normally the carrier shift should be about 425Hz. The NTX2 has a range of 6000Hz over the input voltage range of 0 to 3V. So in theory 425Hz should be a modulation of about 0.2125v between the MARK and SPACE.

The sample circuit above was using 3.3V however on my Arduino the digital high voltage is 5V so I had to amend the voltage divider circuit. After alot of experimentation and alot of silly errors I managed to get the desired carrier shift of 425Hz. One thing to note is make sure you set your pinMode to output or you will get silly errors ( if you don’t set pinmode to output the low voltage isn’t 0v it’s 2v ish).

This is what I’ve come up with. Having tested it with my shiny new FT-817 I can confirm it’s all working 🙂

As a side project to the AVA High Altitude Balloon we are also looking at the feasibility of a low cost tracking aerial. This project is in the very early stages and is being look at by Matt from my work. Currently it consists of a digital compass and a pair of servos :

Work in progress.. 🙂

Current state of the AVA breadboard less the NTX2.

Well firstly I fixed the issue with the lack of GPS signal. A miss read of the datasheet, the active aerial does need power imagine that. Anyway once I’d powered that up and provided power to pin2 on the ISM300 ( the battery circuit is “optional” but in the small print it says “you must hold pin 2 over 1.65v to function”.  Todays lesson is always read the datasheets carefully again and again.

Once the GPS was giving location data a few hours coding on the Arduino was decrypting the NMEA string nicely. After a little erroneous mathematics the latitude and longditude are now converting into the correct format. One thing that threw me was when you Serial.print a float the default is it only does it to two decimal places. I thought I was loosing accuracy but the actual float does still contain the accuracy.

Once the telemetry information was working I copied in the temperature sensor code and constructed the UKHAS telemetry string :

$$AVA,1223,11:45:22,53.1111,1.2234,250,6,23,22*002A

Where

$$Call Sign,TICKER,UTC TIME,LATITUDE,LONGDITUDE,ALTITUDE,SATELLITES,TEMP INT,TEMP EXT CHECKSUM

Also made the LED blink when there is no GPS lock which is currently always as the active aerial doesn’t seem to be working again, dodgy wiring probably.

Need to get the NTX2 working now, on the plus side someone can loan me a radio to test it with!

Well I soldered the chip to the breakout board and set it all up. I’m getting out put but it doesn’t seem to “see” any satellites. Must be something obvious will keep at it!