WARNING
A camera flash uses high voltage, and a large capacitor. Even if the camera has been sitting around for a “long time”, the capacitor can still hold enough juice to give a good shock. In order to avoid this, be sure to fully discharge the capacitor by shorting the 2 terminals with a screwdriver or something. (But not your nice screwdriver…the arc can actually pit the metal)
Procurement
I was about to buy a disposable camera just to dismantle it, but it seemed a waste of $10 (or whatever it cost). I remembered a couple people mentioning that sometimes you could get them for free from photo labs, so I figured I’d give that a shot first.
So I went to my local grocery store’s photo department, and asking if they had any “spent” disposable cameras. I felt a little foolish asking for what was essentially trash, so I described how I wanted the circuitry from them. I don’t really think it mattered, though. The guy behind the counter just brought the bin out for me and told me to have at it. Awesome!
I ended up with 2 Kodaks and 1 Fuji.
Reverse engineering
Since I had 2 of the Kodaks (i.e. one extra to screw up), I figured I’d start there.
The case pried apart easily enough, and it was easy to identify the main components of the circuit…Xenon tube, storage capacitor, battery, switches, etc. Most importantly, I found the contact that was made to trigger the flash. It was actually quite clever…the leaf shutter itself was part of the circuit, and when it was fully open, it would touch the other contact, closing the circuit and triggering the flash. I measured the voltage across this contact, and quickly realized why others that I’d found had used SCRs…it was about 300 volts!
My design
I had a couple requirements that I had in mind while designing my circuit:
- be able to turn it on and off
- The original circuit was mysterious…if you kept taking pictures, it would keep recharging the flash. However, if you let it sit for a while, it would stop charging the flash, and the capacitor would gradually discharge. I wanted to control specifically when it is and is not charging.
- be able to run off batteries or an external power source
- It seems wasteful to keep replacing batteries on the thing…
- be able to trigger a flash from a microcontroller pin
- this also implies that I’d connect the flash’s ground to the same ground as a microcontroller, and I’d expect the flash to trigger from a +5VDC pulse with respect to that ground.
- be able to trigger a flash manually
- for testing & to discharge the storage capacitor most of the way after turning it off to put it away or hack on the circuit some more
Here’s what I came up with:
FIXME: insert schematic here
As you can see, there are 4 wires that I patched into from the original flash cirtuit:
- battery +
- power input
- these two were formerly connected, but I removed the jumper wire so I could switch the power on & off
- ground (i.e. battery – )
- flash trigger
- will activate when connected to ground
Aside from removing the jumper between “battery +” and “power in”, I made one other change to the camera’s curcuit board: to make sure that the flash is always charging, I soldiered a jumper across the “flash charge” button contacts.
Next, I wired it up, I put in a battery, and turned it on…
Results
After switching it on, I could hear the flash charge (the boost converter to charge the storage capacitor creates a lot of audible noise), so I knew I hadn’t screwed up too badly. However, when I pushed the button, nothing happened. After some frustrating debugging, I found the problem: I had the polarity wrong on the “trigger” wire. I had thought that it was +300VDC with respect to the “battery -” wire, but it was in fact at -300VDC. My SCR needed to go the other way around.
Additionally, I found that when I hooked up an external power supply, the flash wouldn’t charge completely. I did some more measurements with a battery, and found that the flash drew about 2 Amps from the battery! My Linear regulator was only rated for 1A.
Unfortunately, at this point, I gave up for a while. Switching the SCR around was non-trivial, and Halloween was arriving soon…too soon. I did manage to get it working when I tried again the next year.
