Couldn’t help thinking about this a bit more, and have a few more suggestions…
First, some fairly straightforward mechanical ones that are still worth mentioning:
- A dedicated level changer, like the one in the simulator
- A push button, i.e. is on only while it is pressed (useful for any circuit with latch/reset behavior)
- I only just got my head around the three-way junction, so I don’t know what’s possible here, never mind practical, but a four- or five-way junction seems like it could declutter some circuits (presumably with some gears sharing levels)
- Something like a (resettable) analog odometer, which could measure total charge flowing through a part of the circuit (I don’t know if it would be practical to measure the total energy by also detecting the voltage); bonus points for making it look like a part of Babbage’s Difference Engine
- Mechanical counter that detects number of pulses (same bonus; more appropriate here, really)
Next, a few electrical/electronic components I haven’t seen mentioned:
- A Zener diode, i.e. current starts to flow past a certain voltage (may or may not be a diode in the other direction – perhaps the Spintronics version has the “breakdown” behavior in both directions?)
- A transformer that allows winding/gear ratios other than 2 or ½
- A large capacitor that can actually store a decent amount of the battery’s input energy (not entirely sure this is useful, but maybe it’s more practical in some circuits than “leaving the energy in the battery for later use”)
Third, sensors. Light and sound sensors are clearly impractical unless you involve electronics (um, see below). An accelerometer might be practical, but this would presumably require something like the aforementioned “steampunk chassis” in order to be useful. That leaves:
- A pressure sensor – could even be used to build a simple scale?
- Temperature sensor – probably much harder to build reliably, but it would be cool to see a bimetallic strip incorporated somewhere
Finally, since I “went there” with the plug-in battery (or “electrical/mechanical energy converter”) suggestion:
- A “smart block” with a tiny screen and simple programmable microcontroller. Would presumably draw power from the plug-in battery. Obviously extends Spintronics in a fairly radical direction, but I do think it’s interesting to demonstrate conversion between different kinds of logic. There are a lot of toys in this space, but maybe this smart block could be compatible with some of them, in which case the toys could then be used together (and alternately, maybe the “smart” block doesn’t actually need to be smart or have a screen, just interface to something that does).