I should note that two of these videos are of Magnesium sulfate coming out of solution. And the video in Salt Crystal garden was made with this rig.
Step 1: What I'm Using
a pair of old servos
pair of old batteries
and other junk
gearhead motor (optional)
Step 2: Disassemble the Servos
I disassembled to old tower hobbies servos that dies after spending a week on the bottom of the Econ River. Nice thing about these servos is that they use the same pitch gear through out making interfacing easy.
Once you disassemble the servos remove any stops that prevent full rotation and one gear is mounted on a pot, instead of fashioning a new axle I just removed the guts from the pot to let it spin freely.
Step 3: Make a Mount
I contrived a holder out of scraps of plywood, and after discovering the two servos would link together, made a few careful measurements and mounted them. With this I discovered i could attain a rotation speed between 1 per 2 sec and 1 per 40 sec. I wanted the ability to go slower still, so i found a gearhead motor in my junk box along with the right sized gear and now can go as slow as 1 per 5 minutes.
Step 4: Actuator
The actuator took a bit of thought i wanted something that wouldn't mash my camera to death, i decided on a cam made from a film canister lid hot glued to the servo arm.
Additionally the camera did not accept a power adapter and a pair of batteries was modified to feed power from a wall wart to the camera, the tape prevents the batteries constantly charging.
Step 5: That's It
A mount was made from spring clamp, scrap wood and an old aluminum ruler, set it so the cam activates the shutter and walk away for a few hours. speed is varied by voltage.