Programmable balloon cutdown


This board was originally designed for the University Of Tennessee Amature Radio Club for use in their Trans-Atlantic balloon flights and recovery of development payloads. It has never failed to cutdown a payload in flight. Description: This circuit board is designed to fly between the balloon and payload parachute and will sever the nylon cord connecting them after a preset time using a hot nichrome wire. The unit is self contained and powered by a 2CR5 type lithium camera battery. A new battery should last at least 4 flights. Total weight including battery is just 3 OZ ! An onboard led give status information to ensure proper operation. An on/off switch allows the unit to be prepared in advance and activated just before launch. After the set time has elapsed a surge of current on the order of 4 Amps heats the nichrome wire red hot and melts the nylon cord passed thru it. The weight of the payload pulling down and the lift of the balloon pulling up tear the melted cord to shreads and release the balloon to continue it's journey to eventual burst while the parachute carries the cutdown board and payload safely to the ground.


The circuit board requires that the battery be glued to the foil side of the pc board with a drop of RTV type silicone sealer. This is non conductive and very resistant to temperature and vibration yet can easily be removed to install a new battery. The black and red wires are directly soldered to the battery terminals to avoid voltage loss thru a battery holder.

Setting the Time

The dip switch pack contains 4 switches labeled S1-S4. Each switch corresponds to a set time interval when set to the on positon. The switches are additive in that the sum of time for all switches turned on will be set into the microprocessor timer.


When the power slide switch is set to the down position power is applied to the timer circuit and the led will flash 5 short blinks to indicate start of timming sequence. The led will flash briefly every 10 seconds to indicate correct operation and a visual indicator of “all is well”. This ensures that the timming sequence has been started before launch. After the programmed time has passed the cpu connects the battery to the nichrome wire via a high current fet switch and holds current on for 15 seconds. This will melt just about any line that will pass thru the loops of nichrome wire. After the 15 second heating time the cpu turns off into sleep mode to save battery life. The nichrome wire is very long lived and can be used for many flights. Additional nichrome wire is provided in the event that you wish to replace it.

The timming of the switch settings is shown below.
	S4= 1   Minute
	S3= 15 Minutes
	S2= 30 Minutes
	S1= 60 minutes

Since the times add for all switches turned on it is obvious that the maximum time is 106 minutes while the minimum time is 1 minute.

Important!!! When switching the unit from on to off and back to on you must wait at least 10 seconds in the off position to give the onboard power capacitor time to discharge. This resets the cpu. Proper operation will be indicated by the 5 short flashes of the led. If you switch the power on but do not see the 5 flashes within 10 seconds you need to turn the unit off wait 10 seconds and try again. The power capacitors job is to supply 5 volts to the cpu durring high current cutdown because the battery voltage drops under such a stiff load.


The software for the cpu is provided in the file named cutdown.pbp. This is written in Pic Basic Pro for the 12F675 cpu. The chip has a 1 K byte limit but a lot could be done if you wish to improve it.

By Carl Lyster WA4ADG
ctlyster at

hardware/cutdown/timerv1.txt · Last modified: 2018/07/29 17:58 (external edit)
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