To bring a little motion to the boxes, it’s worth having a few motorised components, like turntables. But they need to turn slowly, which is a challenge. Since I’ve never done the Exploratorium’s workshop personally I just assumed they’d made their own, but compiling this page I’ve realised they seem to be using jewellery display turntables, like these or this.
Since I didn’t know that, I ended up reverse-engineering something which didn’t exist in the first place. Hmm.
Well, I’m proud of our turntables. They turn slowly (about 1rpm) and despite looking pathetically fragile have proven quite robust in use. The old exhibit standby of ‘if it looks fragile, people treat it with care; if it looks solid, they’ll whack the heck out of it’ seems to apply. They do tend to fall apart in transit, but a minute’s ministrations with a glue gun solves that.
The core is a geared 3–6V motor, driven off a single 1.5V cell. There’s enough torque to work reasonably well, and undervolting it slows it down even before you get to the gearing. The drive pulley is a short length of plastic tubing with a cardboard end-cap. A slot in the cardboard cap fits tightly on the motor shaft, then hot glue poured into the tubing fixes it well enough. Gaffer tape wrapped around the plastic tube provides enough friction for the edge drive to the turntable itself.
Since we hand-cut the turntable discs they vary in diameter somewhat. The pivoted chopstick and elastic band tension spring keep the drive pulley and turntable in contact. The main turntable bearing is a short M10 bolt glued to the frame – ridiculous overkill, but it’s what we had lying around.