Near Space Project


Cut Down System

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.


GPS Arrived for backup tracker

Just received in the mail a GPS Arduino Module with a Ublox NEO-7M chip which will be built into a 2nd stand alone tracker for the payload.

Ublox 7M
Ublox 7M

The backup tracker is also waiting for its own transmitter and receiver module to enable communications with the payload mid flight.

This GPS will also run the cut down system where I want to enable the termination of the flight if a certain latitude is passed.

Got the iron heated up to add some pins so the experimenting can begin whilst I wait the other bits to arrive.

Glowing Red Hot

Ran a couple of additional tests for my cut down circuit and I have decided to change the batteries.

Initially I was trailing two banks of 4 x AA’s in parallel giving about 6 volts. This seemed to be enough to heat the wire. However I hoped for a better solution.

Firstly I would have needed 8 fresh batteries which adds extra weight, and I still wasn’t getting the wire glowing red.

So tonight after stupidly dropping the AA battery holder and cracking the case, I decided to run a test with a couple of 9 Volt Batteries and wow, so glad I did.9vbattery

With two 9 volt batteries in parallel the wire glows bright red and cut through my test twine like butter.  So I think I’m moving in the right direction,

I should probably also run a test with a single 9v to see if I get a similar result. Happy for anyone to offer other suggestions for a cheap simple battery source to get that Nichrome wire red hot.

So far the cutdown system is partly fixed to a PCB

So far I’m happy with the circuit. The relay is soldered in place, and I still need affix a few of the other components but its looking good. Relay also seems to hold up to the current.

Thinking about getting the lithium version, as it may be better for the cold temperature.

Balloon Cut Down System

Reading several blogs about High Altitude Balloon launches in Australia, it appears I may need a way to cut the payload free from the balloon mid flight.

This will ensure that the payload doesn’t travel into a unrecoverable location such as the ocean or dense scrub, but more importantly it can be terminated if the payload looks like it may land into a densely populated area.

I plan to build a cut down circuit and control it via a 2nd Arduino. I’m hoping to be able to transmit a tone such as DTMF to the Arduino and have it trigger a current through some nichrome wire.

I’ve found a cut down circuit on Bovine Aerospace and have built a trial system.

Cut Down Circuit
Circuit from Bovine Aerospace

I have tested some AA’s in parallel to get approx 6v’s with a higher current, which seemed to be enough to heat the wire. However the current was over 2 Amperes and I only had a small relay capable of 2 Amperes and it became damaged. I have since upgraded to a relay that supports more Amperes and everything seems to be working again.

Cut Down Testing
Cut Down Testing

The first test I ran was using some baler twine with a milk bottle on the end. The milk bottle was approx 3 Kg for this test. I’m still yet to confirm if the AA’s are the best batteries for the job, maybe I need something more compact or more current to get the wire glowing red hot.

This is already being activated by the Arduino using a digital output but I am still yet to implement a receiver based activation. I also still need to determine the best way to trigger it.

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