Ah. That's totally my bad. Sorry. I think of camera triggers. You specifically said it's a
flash trigger, but I automatically converted it to mean a
camera trigger and kept moving. :b
The reason you're having trouble measuring it, flash triggers are switches, they momentarily short the hot shoe/PC pins together. So it's going from 0V to 0V and back to 0V. Without injecting your own voltage to measure, you'll never see your meter deflect. You might be able to catch it on Ohms setting, but it's a fraction of a second, anyway.
good news / bad news:
The good news is the circuit is about the same... we don't need an inverter so you'll move the output connection from the collector side to the emitter side, and we need the USB to stay at +V
(somewhere between 3 and 5V, 6 tops!) until the trigger fires, so you'll move the other end of the base resistor to +V to act as a pullup. You can eliminate the collector resistor.
The bad... it's not going to be that simple. Working in darkness and leaving the shutter open and firing the shortest burst you can, the timing part only involves two syncronizations - the object in motion and the flash burst. Controlling the camera instead, you'll add a 3rd time-sensitive calculation.
Another bad, CHDK can fire on the rising edge of USB voltage, but it's much less accurate timing-wise than the falling edge. The camera doesn't function with voltage on USB unless there's a break.
(by default SDM & CHDK in sync mode uses two breaks, one to focus/setup and the 2nd to fire.) But, since your trigger has no idea about what it's triggering, you'll need to wire up a momentary pushbutton where you can do the short-to-ground first "half press" and then the flash trigger will fire the shutter. I could see where after forgetting a few times and the camera not firing, this would become "bothersome".
(no cussing here.) Finally - what's that truism about "bad news comes in 3s"? - there is a delay between detecting the USB voltage drop and firing the camera. It's much more repeatable than the rising edge, but it is still a delay, and it is still (slightly) variable. For most PowerShot models, about 1ms variance, which isn't a lot...
except in doing 1/20,000+ flash bursts... then it's huge! So your firing delay will have to be adjusted for by making the flash trigger fire that much faster.
(we'll say 50ms, it's per-camera, so it will take some experimenting.) This last one isn't really
new bad news, more like an error of omission. It would still be an issue when I thought your flash trigger was a camera trigger.
Oh, and one more bit of
good news: I had never heard of this Quaketronics flash controller, but it seems it's an Open Source project, and the current revision is running off a PIC16F616, so there's probably a very easy way to do this in software if you have this model. I've looked at the schematic and the output control for both CONN1 (the hacked-up disposable) & CONN3 (the strobe) come from the same pin off the microcontroller. But there is hope! There's a spare output pin (RA5) that could be appropriated for CHDK USB triggering. I would ask on his forum, or on the MAKE: forum, or just email him personally and point him over here. But someone else may have already
done this exact modification... anyone into DIY photography will end up finding CHDK.
Happy hunting!
