■ FIGURE 4. Two channel
camera timer.
The tag is marked with text (like
Remove Before Launch) to remind
the launch crew to pull the shorting
block off the commit pins before the
balloon lifts off.
CONNECTING A
CAMERA TO THE PICAXE
CAMERA TIMER
The relay in the timer replaces a
camera’s shutter (and power switch in
the two channel version). Therefore,
these timers only work for
cameras with electrical
switches. Fortunately, this
means they’ll work for most
modern cameras.
You’ll need to open the
body of the camera to attach
the timer. Warning!! Part of
the camera’s flash circuit is
a high-voltage electrolytic
capacitor that’s capable of
delivering a nasty shock.
Under no circumstances
touch this capacitor, as it can
hold a hefty charge long after
the batteries have been
removed. Apply electrician’s
tape over the capacitor’s
exposed wires if you’re
concerned that you might
accidentally touch them
(would anybody intentionally
touch them?).
Open the camera only far
enough to expose the solder pads of
the shutter switch. After opening the
camera, it’s a good idea to test the
camera once again for damage by
loading batteries and pressing its
shutter switch. Remove the batteries
after testing and set a multimeter to
measure resistance or continuity.
Using the meter, identify which of the
two of the switch’s pads connect to
each other when the button is
pressed. If the multimeter leads are
touching the correct two pins, the
multimeter will indicate zero
ohms of resistance when the
switch is pressed.
Rather than try to
remove the shutter switch
from the camera, just solder
thin gauge wires to the two
pads and then run them out
of the camera case. It would
be a good idea to run the
wires through a hole or pair
of holes in the camera case
where they can be tied off for
strain relief. The free ends of
the wires are then soldered
to the timer.
Now, here’s a complicating factor. Some cameras
have three pins on the shutter switch
that are connected together when
the shutter switch is pressed.
Typically, the third pin triggers the
camera to focus (so you should only
see this happen with an autofocus or
AF camera). You can determine if the
camera has a focus pin in its shutter
switch by noting if a third pin shorts
when the button is only half pressed.
However, to do this, you first have to
locate the shutter switch pin that
connects to ground.
The switch’s ground pin is the pin
that the autofocus and shutter pins
connect to when the shutter switch is
pressed. After you identify the focus
and shutter pins in the switch, connect
both to one side of the relay and the
ground pin to the other side. Shorting
the focus and shutter pins together
like this causes the camera to focus
and take a picture together when the
relay is fired.
A possible alternative is to solder
a wire between the shutter switch
focus pin to the ground pin. This
keeps the camera constantly focusing
itself and should be okay as long as it
doesn’t drain the battery during a
typical two to three hour flight.
Connecting the two channel
timer to a camera is done the same
way except the second relay
connects to the camera’s power
switch. Follow the above procedures
to locate the correct pins to connect
to the relay.
FINISHING THE PICAXE
CAMERA TIMER
■ FIGURE 5
Now download the PICAXE
program and test the circuit. At power
up, the LED indictor will blink and the
timer will wait for the removal of the
shorting block. Once the block is
removed, the timer will begin taking
pictures and lighting its indicator LED.
If the timer checks out, cover the back
of the PCB with a piece of foam core
and hot glue to protect the traces on
the underside from shorting out. The
PICAXE BASIC code for the single
channel camera timer is available on
the Nuts & Volts website at www.nuts
volts.com.
92
January 2008
