Previously we posted about a GPS/arduino based cutdown. This post describes a simpler balloon cutdown which uses only a countdown stopwatch. A prototype of the timer based cutdown is pictured below:
The circuitry resides in the tupperware container, while the nichrome wire is encased in the foam block above the tupperware. A nylon cord runs vertically through the tupperware and the nichrome wire coil. When the countdown timer hits zero, the nichrome wire is heated by the current passing through it and melts the nylon cord. The foam block is a safety device to prevent the nichrome wire from starting a fire if the cutdown is inadvertently triggered on the ground. The cord is severed above the cutdown container, so the cutdown device must be attached to a parachute or something else below. Our plan is to use this device to invert a solar balloon during short test flights. The inverted balloon should act as a large streamer to slow the descent of the payload after the cutdown is triggered.
A circuit diagram created with xcircuit and a more detailed view of the circuit are shown below. Basically we hooked up the wires originally attached to a piezo buzzer in the countdown timer to a 555 timer IC, which is configured to produce a pulse of +5 V for about 10s when it receives a signal from the countdown timer. The signal from the 555 timer triggers the reed relay, which closes a circuit containing only the nichrome wire and 4 AA batteries, heating the nichrome wire and severing the nylon cord. If you make this circuit be sure to get the values of R1 and C1 correct as these determine the length of the pulse emitted by the 555 timer.
In theory any cheap countdown timer should work for this as long as you are able to access the wires leading to its internal piezo buzzer. One caveat: most stopwatches make a beep every time a button is pressed, so it also may be necessary to install a switch somewhere in the circuit to stop the cutdown from triggering when you start the timer.