[islandlabs] Balloon Cameras Info
toolfox at gmail.com
Sun Mar 6 23:13:23 UTC 2011
Today I started digging into the keycams that Bill and I were tinkering
with last fall. One camera was wired up with four external connections
from the two switches, so I dismantled it and made notes on the
The four wires are paired red/black for the power switch and
white/green for the shutter/mode switch.
With the unit off, there is (with a full charge) 3.26 v across the
red/black pair (red positive) and 0 v across the white/green pair.
Cross-checking the other combinations (red being the only apparent live
wire), I got 3.00 v across red/white and 3.28 v across red/green.
There is no direct continuity between any two pairs in any combination,
indicating that there is no common ground amongst these wires.
Shorting the red/black wires naturally turns the system on to standby
mode. By extension, the shorted wires will obviously be 0 v across them.
Once on, the red/black wires remain with the same 3.26 v potential
across them. However, the white/green combo now has 3.27 v (white
positive), indicating that the system is on.
So, for a microcontroller-based control system, the white wires must be
sensed for positive voltage or a logic "1" to indicate that the camera
is up and running. This is true whether the camera is taking still
photos or video. Of course, the white/green combination needs to be
activated for each still picture and can't be sensed while "pressing"
For complete disclosure, the red/white combo yielded 0 v, and the
red/green read 3.28 v (red positive) with the camera powered up.
I have no connections for the camera's LED. If we bring this signal out
as well, we might be better able to deduct the camera's state at any
time. But let's see what we can with what we have.
To power up initially, we need to short the red/black wires
momentarily. No other useful information on camera state can be
gleaned from these wires, so perhaps a simple
transistor-based optoisolator will do. This will use one I/O pin and
doesn't need to worry about electrical reference grounds between the two
Similarly, the white/green wires can use a second optoisolator so
starting the video camera or taking a photo with the still camera will
consume a second I/O pin.
To sense a failure mode of the camera shutting off, we need to see a
difference signal. One choice is the white/green pair with either using
green as a ground reference and sensing voltage on the white (positive
voltage=camera on; no voltage=camera off) or connecting yet another
optoisolator across the white/green wires in an inverse pattern.
Using a third optoisolator avoids struggling with ground references (I
keep mentioning this for a reason that I'll get to later), but might
interfere with operation of the camera's switch input due to the
increased current load across the wires from the optoisolator's inputs.
This needs more experimentation.
At this point, we've used three pins: Power switch output, shutter
switch output, and shutter voltage input. Running two cameras off one
controller will use 6 pins, which on an ATTiny can theoretically be
If, on the other hand, we combine the two shutter switch optoisolators
in some way that they don't feedback on each other, we can reduce the
the I/O pin count to 4: 2 for each camera.
I'm not coming up with any solutions on this approach. Personally, I'm
leaning toward the 3-pin design.
This method of sensing camera state should be fine for the still
camera, but what about the video camera? If the video camera stops
recording and goes back to standby mode, the signals we're monitoring
will not show that anything went wrong!
The only indicator of coming out of video mode is the camera's LED:
It's out during recording and on during standby. Similarly, the
still-photo mode turns the LED off during photo capture and turns back
on after the image has been red from the CCD.
We want this system to be as robust as possible: we can't allow for
errors at 98,000 feet.
I think we should look to monitoring the LED as well. This means one
I/O for the power switch, one for the shutter/mode switch, one for
verifying that the camera is turned on, and one for watching that the
camera is properly collecting visual data.
We've overrun our allotment of I/O pins.
We're going to need two ATTiny controllers or a larger controller
(An ATMega? One of the PICs we have floating around?).
If we place an order with Jameco Electronics, it'll be a week for the
package to arrive on the East Coast. We could also look into ordering
with Mouser; shipping from Texas will be here in three business days.
Bill, I know you were finding some of these error modes with the camera
shutting off. What approach were you using when you were breadboarding
I'd like to get this system ready for stress-testing by next weekend if
possible. That will bring us into the second week of March.
Time's moving on...
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