Step 3: The Circuit
To trigger your CHDK enabled camera we'll be using the USB remote function. In this case we have to use it via the 'syncable' method, which is lightning fast compared to the normal USB remote.
The syncable remote also operates differently. It triggers the camera on the falling edge instead of the rising edge of the 5-volt signal. When the camera detects the 5 volt USB signal, it gets ready to take a shot, waiting for the voltage to fall to zero.
There are high-speed camera trigger circuits floating around the 'net but I couldn't find any for syncable USB. So I cobbled together the circuit below.
It uses a 556 timer IC, an inverter, a photoresistor and some caps and resistors.
The dollar store had a USB cable identical to the one my camera uses. I lobbed one end off of it, instead of wrecking the one that came with my camera.
A 5-volt power supply is needed to power the circuit. If you don't have one, pick up a cheap USB charger, or add a 7805 voltage regulator to the circuit.
The photoresistor is not on the circuit board; it's mounted on a small piece of perf board at the end of a short cable. Glue some magnets onto the back for easy alignment with the laser.
The circuit should be built first on a bread-board and tested. Once you're sure everything is working then either etch a circuit board or use a prototype board like I did. Or just continue using the circuit on the bread-board.
NOTE: OCT 2nd, 2009 There was a huge mistake in the schematic that instructable member toxoof pointed out. The PDF has been corrected.
OCT 19, 2009: another error has been found in schematic. Arrrggggg!
July 30, 2010: Schematic revised to use photoresistor
Download the pdf here: Schematic