until recovery. Each ADC input has its
own +5V and ground pin, so it’s easy
to plug sensors to the PFC.
Even though the PICAXE-08M is a
small micro, the PFC PCB is divided
into three levels in order to fit inside a
ping pong ball. Each level has its
particular function and is connected to
the other levels through a pair of headers and receptacles (called the side
receptacles). The headers and receptacles allow all three levels to be stacked
and destacked at will; so the levels are
not permanently connected together.
The first level (Figure 3) is the
interface level and it’s exposed when
the PongSat is opened. This is the level
where sensors are attached, the PFC is
programmed, and the mission data is
downloaded. The two resistors on this
level are the pull down and current limiting resistors used for programming the
PICAXE. The programming header is a
1x3 male header and the I/O ports are
2x3 female receptacles. Since the PFC
is made with a single sided PCB (
printed circuit board), the two side receptacles (which connect the first level to the
second level) are lifted above the PCB
by 1/10” so a soldering iron can fit
beneath and solder them to the PCB.
The first level PCB is 3/4” in diameter.
The second level (Figure 4) is the
logic level. The PICAXE-08M is mounted on this level between two rows of
header pins. The longest pins of the
headers are soldered pointing down.
This leaves the header pins long
enough to connect to the receptacles
in the first and third levels. The PCB for
the second level is 1/2” in diameter.
The third level (Figure 5) is the
power level. There’s a low dropout
voltage regulator and capacitor soldered to this level. The PFC’s power
cable passes through a strain relief hole
in this PCB before being soldered to
the power and ground pads. Currently,
my power cable terminates in a two
pin header while I experiment with
power options for the PFC. The PCB
for the third level is 1/2” in diameter.
PFC is built with small PCBs, there are
a couple of things to be careful with
while assembling it.
■ FIGURE 3
Solder the I/O receptacles and the
programming header first. Then flip the
PCB over. Unlike the headers and
receptacles that were just soldered, the
two side receptacles (which are used
to connect the PFC levels together) are
soldered to the copper side of the PCB
(in other words, to the bottom of the
PCB). Raise them 1/10” above the PCB
to create enough of a gap for the
soldering iron to reach the receptacle
pins and their PCB pads. Now flip the
PCB over and solder the two resistors.
■ FIGURE 4
This level is easier to assemble than
the first level. Begin by soldering the
eight pin DIP socket or PICAXE (if you’re
comfortable with your soldering ability).
The side headers are soldered to the PCB
with their longest pins passing through
the PCB. If you solder the headers upside
down, then the header pins on the bottom will be too short to properly connect
to the third level side receptacles.
■ FIGURE 5
Begin assembling this level by
soldering two thin gauge wires to the
PCB power pads. Then fold the wires
over and pass them through the relief
holes in the PCB. Since this is the power
cable, use a red colored wire for the
positive pad and a green or black
colored wire for the ground pad. Solder
the LM2940 and 22 µF cap in place.
Lastly, solder the two side receptacles.
■ FIGURE 6. Mask for copper layer of PCB.
■ FIGURE 7. The placement of parts.
BUILDING THE PONGSAT
Refer to Figures 6 and 7. Since the
July 2007 23