■ FIGURE 2. The schematic of the
single channel camera timer. The
two channel version has a second
relay and diode connected to
PICAXE-08 pin 4, or pin 3 on the IC.
camera. The two channel version
has full control over a camera
and is best for cameras with
power-save features that may
shut it down during a mission.
However, the two channel
camera timer is not limited to
operating a single camera. It
could operate two separate
cameras inside a BalloonSat or
a single camera along with a
second device that a relay can
operate.
THEORY OF
OPERATION
Upon powering up, the PICAXE
blinks its indicator LED (connected
to pin 1) five times showing its
readiness, then goes into a wait loop
until the shorting block is pulled off
the commit pin. The commit pin pulls
pin 3 to +5V and the shorting block
shorts it to ground. After detecting
the removal of the shorting block
(that is, when pin 3 goes high), the
PICAXE signals its detection of the
removal by blinking the indicator LED
five more times. Then, the PICAXE
enters an infinite loop where it
energizes the relay by setting pin 2
high and turning on the indicator LED
by setting pin 1 high. The LED is a
troubleshooting tool that indicates
when the PICAXE is triggering the
relay to take a picture.
After a one second pause, the
PICAXE sets pin 1 and pin 2 low
to open the relay and shut off the
indicator LED. When the PICAXE
turns off the relay, the magnetic
field contained within the relay’s coil
collapses and creates a potentially
damaging reverse current through
pin 2. To protect the PICAXE from
damage, the 1N4001 diode shorts
the relay’s coil to reverse current.
After a pause, the PICAXE again sets
pin 1 and pin 2 high to then take the
next photograph.
The two channel camera timer
operates the same way, except that
its two relays are fired in sequence.
The first relay attaches to the
camera’s power switch, allowing the
PICAXE to power up the camera.
After a short pause, the second relay
(which is connected to the camera’s
shutter switch) fires to take a picture.
After the camera has time to save
the image to memory, the first relay
is fired once more to shut down
the camera.
BUILDING THE ONE AND
TWO CHANNEL CAMERA
TIMER
version) are strain relief holes for the
timer. Wires connecting the timer to
the camera and battery pack pass
through the strain relief holes to
prevent normal use breaking the
soldered wires off of the PCB. The
battery holder for the timer can either
be a nine volt battery snap or a four
cell AAA holder.
The commit shorting block is a
short loop of wire terminated with
small crimps and mounted inside a
plastic housing. A cord tied to the
wire loop of the shorting block
exits the BalloonSat airframe and is
attached to a brightly colored tag.
Begin assembling the timer by soldering the lowest lying components
(the resistors and diodes) first. Solder
an eight pin IC socket to the PCB
rather than the PICAXE to protect
it from the heat of soldering. I
recommend using snappable headers
for the programming header and
commit pin (it’s cheaper to purchase a
long row of headers and snap them
off as needed). Watch the orientation
of the LM2940 voltage regulator and
the diode(s). If the diode is soldered
backwards, it will short out every time
the PICAXE fires the relay.
The large holes at the top and
bottom (and sides in the two channel
■ FIGURE 3. One channel camera timer.
January 2008 91