NOTE: THIS PICURE IS OUTDATED. You can see a photo of the Mark II sound trigger here
My custom sound trigger
This is the sound trigger I designed and built because I wasn't satisfied with the sound trigger schematics that were available on the net. I usually like re-inventing the wheel :-)
The sound trigger is designed to be quite sensitive and support two independent output channels. A little confusing perhaps but output channel 1 drive three output FETs (tip and ring of the middle socket and the ring of the top socket); output channel 2 drives a FET connected to the tip of the top socket as well as the LED. With this setup there are 4 isolated outputs (only shares a common ground) that fires as a group of three (channel 1) and single output (channel 2)
The PIC basically waits for its interrupt (trigger) pin to go low and then starts to play back a sequence consisting of output value and delay pairs. The maximum duration for any step is 16.7 second with 1us resolution. There can be up to 25 steps. The circuit adds about 6-10us latency.
The Set (arm) button allows you to set the output channels to a specific state while waiting for a trigger.
My tipical sequence looks like this:
Idle: Camera off, Flash off
Set pressed: Camera on, Flash off (start bulb exposure)
Trigger detected: Camera on, Flash off, delay 1.2ms
Step 1: Camera on, Flash on, delay 10ms
Step 2: Camera off, Flash off
Schematic can be found here.