In the “HoHoHo 2” Flight i used a mechanical cutdown by using a servo. This did not work as i had a power failure. Also, using servo’s can be problematic if you want to use them with “arduino’s”. If you use those, be sure to use the old Servo library. The new servo library works with interrupts. Interrupts can be dangerous if you use multiple protocols that use interrupts, like the new “softwareserial” protocol of other accurately timed operations. Anyway, it looked like this, and probably would have worked:
[caption id="attachment_1905" align="aligncenter" width="300" caption="When the servo turned 90 degrees, a nylon cord looped around one of its arms would slip off"][/caption]
If servo’s get cold, the grease in them could freeze easily. Though, in a payload, generally it would not get cold, and if it does, the change is small that there is enough mass to get the grease to a solid state (freezing point). I can’t advise this method though, hot wire cutdowns are easier!
Hot Wire Cutdown
First of all, you are probably wanting to cut through something simple. Simple is foam, or nylon. The smaller the better. The following method was succesfully used in the “Space Camera Live 1
I had set a Latitude limit, a Longitude limit, and a “fall-sensing” limit – so after a 1000m fall it would cutdown the balloon if there was still any on the wire. This means that it would cut off the rests of the balloon, which can be substantial (~1.5kg!), so the payload will fall more slowly.
Although you can see from the images below my cutdown worked, but it did not let the wire “go” entirely. This is because there was so little of the balloon left, (<100g) that there was not enough force to pull out the broken wire!
[caption id="attachment_1901" align="aligncenter" width="300" caption="A payload with hotwire cutdown (this was the backup payload of Space Camera Live 1)"][/caption]
[caption id="attachment_1902" align="aligncenter" width="300" caption="Before launch (hotwire wrapped around nylon cord)"][/caption]
[caption id="attachment_1900" align="aligncenter" width="300" caption="After use (a 10 second burn)"][/caption]
Hot Wire Cutdown Electrical Schematic
I had a 7ohm/meter wire, and worked will if connected direcly to a battery of around 3~7 volts. That would result in a ~2A current. I did not have proper transistors to channel that, but i can advise relays for this anyway. I only had a 5V relay, while my system was 3V3. So what i did was: A low power 3v3 arduino pin drove a transistor, that would drive full 3v3 power into a 5V booster. The 5V booster would then switch the relay. To the other end of the relay, there was a battery, in series with the hot wire. If you have a 5V system, just use a 5V relais that drives the hotwire+battery instantly. Simple! Though, small relays cant take large currents for too long, of they could “keep itself shut” which can be dangerous and lead to flying fireballs in ours case.