Near Space Project


October 2015

Decoding a little further

I’ve been waiting a few weeks for a number of items to arrive in the mail, one of which is a MT8870 DTMF Decoder. dtmf

The idea is to allow the payload to be controlled with a DTMF tone. Mainly so I can initiate an emergency cut down if required.

I am using a Radiometrix RX2A for the receiver and have connected its output to one of the audio input channels of the DTMF Decoder.


I modified some code found here which takes the DTMF tone and allows it to be decoded via the arduino.

Initially the tones that are being received are displayed numerically on the screen just for a test. But I still need to allow a command to run when the right combination is received.

A few tests on the work bench, and it looks to be working. I don’t have much confidence in the receiver over a longer distance, so some testing will need to occur.

Tracker 2 Progress with Transmitter

Well, things are starting to heat up here in good old Sydney Town, both weather wise and in the shack. I’ve been playing with the 2nd tracker and attempting to get it transmitting some RTTY, and finally tonight after several nights of learning I finally have it working.20151006_223648

First of all I’m still using the FlexTrack code with a Radiometrix TX2EH but have modified the code slightly to work for my purpose. The code appears to use FSK for a 3v model whilst my transmitter is 5volts. For what ever reason I didn’t have any real success with it. I had to tune my radio to USB to hear any tones, and for some reason I was only getting a single tone.
I wanted to have the ability to decode using a handheld and my handheld doesn’t have SSB, so I looked into the AFSK option. Instead of varying the frequency of the transmitter I generate an audio tone and transmit tha20151006_230925_LLSt instead. Tested the receive with my Yaesu FT-857 and the frequency looks a little off from what I would expect, but this may just need some further tweaking of the code.

Consulting a AFSK tone table for RTTY on the KLOTH.NET website, I chose a 425 Hz shift and selected a 2295 Hz high tone and a 1870 Hz low tone. I then exchanged some lines of code that would normally cause the transmitter to frequency shift with the Arduino tone command. After some mucking around, I finally had some success and was able to successfully decode the RTTY

After setting the correct RTTY decode options in dl-fldigi a big smilefldigi came across my face as I was decoding the output from the tracker. The example screenshot is with the GPS disconnected, but with the GPS connected, the zero’s quickly filled with the GPS coordinates.

I’ve still got some tweaking to do, as I’m not happy with the transmitter, it seems unstable with frequency drift and you can tell from the waterfall image it is a little noisy. Hopefully I just need to tweak the code to get this right.

But at least I’ve managed to get it working, as I can now look at experimenting with other digital modes. I know RTTY is fairly old these days and other protocols do exist, but RTTY just seems simple and if it works from near space, then I’m happy.  Happy to hear from anyone who can suggest any improvements.

Next I want to de-key the transmitter between transmissions so that a receiver can be used to receive commands from the ground. May have to look at a DTMF decoder or something.

Looking at something like thisan Solu DTMF MT8870 Decoder Module for Arduino.

Totex – 1500g Kaymont Balloon

My Totex – Kaymont 1500g balloon has arrived, so its getting closer to having the critical parts needed for launch.

I decided upon the Totex balloon after reading reports of other balloon launches having success with them but also because there are limited sources for weather style balloons down under. The 1500g should be suitable for my payload which is already at 1.2 Kg’s and hopefully reach a good altitude.

You can get similar balloons via  Amazon 30ft dia. Professional Weather Balloon, 1200g but I purchased from Kaymont via Ebay

High Altitude Science Filling System

I’m still looking into a filling attachment for the balloon and have seen systems available from High Altitude Science but may try and make my own or find a local option.

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