computer will be installed in the rocket with its battery
end down (or its accelerometer daughterboard end up for
the BT- 60 version).
The accelerometer chip is turned 90 degrees on the
BT- 50 version relative to the other versions, so I swapped
the X and Y inputs to the PIC so the rocket’s X axis data is
routed to the same pin (RC1) in each version. For all
versions, upward acceleration under thrust will result in
vertical acceleration data values less than 512 (the
accelerometer outputs an analog signal of half its supply
voltage when under no acceleration,which the PIC’s ADC
converts to a number in the middle of its 10-bit range),
which will then switch to data values greater than 512
after motor burnout as the rocket begins to decelerate.
The data-parsing subroutines in the code assume these
orientation and acceleration conventions in order to
properly detect motor burnout and parachute
ejection, though you can omit or modify them
if you need to orient your computer differently
in your rocket.
Construction of the BT- 60 version of the
computer with through-hole parts is very
straightforward; the only challenge will be
choosing from the various options for the
power and I/O configuration described above.
Figures 4 and 5 show the top and bottom of
the completed BT- 60 computer. Construction
of the three surface-mount versions is more
challenging, though I deliberately picked
relatively “large” versions of the parts to ease
the process as much as possible. There are plenty of
resources on the Internet to teach you how to hand-solder
these parts including the accelerometer chip, which is only
available in a tiny LFCSP package. (Based on my
experience, LFCSP stands for “a Little Frustrating, but you
Can hand-Solder this Package.”)
However, I deliberately omitted a pad for the Z axis
output of the accelerometer as I was not confident of my
ability to solder it without bridging to the adjacent ground
traces. Since the Y and Z axes are pretty much
interchangeable in a rocket that may be spinning a bit
during its ascent, this is not a problem.
Before you solder the LFCSP to its place on the board,
you will need to remove a tiny bit of the inner parts of the
traces so they don’t short out through the central metal
pad on the bottom of the LFCSP. I had to build the pad
array for this chip from scratch as my version of
ExpressPCB didn’t have a pattern for it, and I wasn’t able
to position these pads any more precisely without
compromising their alignment with the pads on the
LFCSP. Figure 6 shows the pads trimmed prior to
soldering. I had good luck using a sharp hobby knife.
Figure 7 is a close-up of the LFCSP hand-soldered in
The battery installation methods for the three surface-mount computers are different. The BT- 50 version uses a
CR-2016, -2025, or -2032 lithium coin cell held in place by
a small metal plate and four screws (Figures 8 and 9)
since the coin cell holder used on the BT- 60 simply won’t
fit inside a BT- 50 tube when mounted to the PCB.
The BT- 20 version is designed to use two LR-44
button batteries — one above and one below the PCB —
connected and held in place via a stiff spring metal clip
that wraps around the end of the PCB. Insulate the center
part of the clip so it doesn’t short out the batteries, and
then use a large diameter piece of heat shrink to hold the
batteries and clip securely in place.
The BT- 5 version is designed to use either two LR-44
44 January 2015
■ FIGURE 8. Top of completed BT- 50 flight computer.
■ FIGURE 9. Bottom of completed BT- 50 flight
computer, showing metal plate and screws securing
the coin cell battery.
■ FIGURE 6. Close-up of pad
array for accelerometer LFCSP,
showing inner pad ends
■ FIGURE 7. Close-up of
accelerometer LFCSP, hand-soldered to the board.