I devised a very cheap and simple-to-make electric nylon line cutter that could be used to electronically cut the cord that fastens the helium balloon to the payload in high altitude flights.
This line cutter cuts through nylon cord using heat generated by an electric current (from two AA batteries) passing along some nichrome resistance wire wound around the nylon cord.
This 12 second video demonstrates my homemade electric nylon line cutter in action.
In the video, 1.3 mm diameter nylon cord is employed (this nylon cord comes from a garden string trimmer, or strimmer). Nichrome resistance wire (rated at 19 ohms per meter) is tightly wrapped in a spiral around the nylon cord, as can be seen in the video (note that the wire does not touch itself, but is spaced out in the spiral, to avoid short circuit).
Connecting this resistance wire to 3 volts supplied by two alkaline AA batteries results in a current of around 1.5 amps flowing, which generates enough heat to melt the nylon and sever the cord. You can see this electric current monitored on the multimeter in the video.
Once the current is turned on, it takes around 6 seconds for the resistance wire to heat up and sever the nylon line.
Faster cutting speeds should be obtainable using thinner nylon fishing line. I used thick 1.3 mm diameter nylon cord in this demonstration just because I happened to have that in stock. But for high altitude balloons, thinner nylon fishing line will be preferable. Nylon is a good material for this purpose, because it has a lowish melting point of around 220ºC or so (depending on the particular type of nylon).
By using a high current transistor to switch on the current to the resistance wire, this will enable an electronic circuit or a computer (such as a Raspberry Pi or Arduino) to control the line cutter.
This electric nylon line cutter could be controlled remotely from the ground using two-way radio communications systems such as LoRa or LoRaWAN. If you can remotely sever the balloon fastening line from the ground, this will enable you to use less helium in the balloon, so that the balloon does not automatically burst at altitude, and instead continues to a greater height, like a floater balloon. Then once the maximum altitude is achieved, you can send a radio signal from the ground to sever the balloon fastening line, so that the payload parachutes back down to Earth.
It may be a good idea to add one of these remotely-controlled line cutters to the parachute fastening cord as well. That way, if your parachute gets caught in a tree as it returns to Earth, you could remotely activate the line cutter to sever the parachute, so that your payload can fall to the ground out of the tree.