Just to the left of the LEDs is header J19, which is
used to connect a cable for optional offboard LEDs. This
might be helpful if you need to see the status LEDs after
the board is packed into a sealed enclosure. If you
choose to use J19, connect all cathodes to the common
ground provided at the connector, and connect individual
anodes to J19-2 to J19-5. Jumper JP3 is used to select the
onboard or external LEDs.
Line Cutter Circuit
The upper left quarter of the board includes the
circuit that can autonomously cut the cord between the
balloon and payload. The actual cut is made using an
electrically heated nichrome wire held loosely around the
cord. The circuit itself is standard stuff. The
microcontroller initiates the cut by driving the base of a
transistor which energizes a relay coil which actuates the
relay. A red LED (LED5) mounted near the relay indicates
the status of the relay.
It is on when the relay secondary is closed, providing
a visual warning that a cut is imminent. The purpose is to
verify that the cutter is off at launch. This has been useful
several times before the payload was sent on its way.
Our relay has a 12A rated contact — plenty for this
use — and the cutter draws about 1A to 2A for a short
June 2017 31
■ FIGURE 6. Schematic diagram.
■ FIGURE 5. Sensor board Version 1.