A couple of months ago I received two very small pager motors, much smaller than the usual size. They seem ideal for an SMT version of SunEater_III. But the snag was obvious: SMT reduces the size of everything except the storage cap.
As you know, photovores are at their best when they take small steps toward the light. With a good SE, that makes them `pop' every few seconds, even in ordinary daylight (as opposed to a direct noon sun). But this requires a cap of a couple of thousand uF. Make it larger, and you get boringly long charge times. Make it smaller, and the `pop' causes insufficient motion. That saddles you with a rather large ordinary electrolytic cap - you can't use a physically smaller SuperCap, because it has too much capacitance.
Obviously, the problem is caused by having the SE switch on and off at voltage levels. So I modified SunEater_II to have a switch-on level and a switch-off *time*. It turns out to be a significant improvement. Not only can I use the smallest SuperCap (47000uF, 13mm x 7mm), but the motor bursts have more torque at a lower voltage. This is caused by the voltage not tapering off during the burst. The drop is just 0.1V or so, giving the accelerating motor much more follow-through.
The result is a considerably more efficient SE, which can be built quite small and light. Expect a very tiny SunEater_III type `bot in the not so far future.
Btw: The schematic mentions a burst time of .15s or less. That may look short, but even the large and heavy SunEater_III. gets a 1cm `step' from a burst of only 0.1s. And longer bursts are no problem, just increase the value of the 220N cap. The high internal resistance of the SuperCap is not as much of a problem as it used to be, because the trigger now allows the downward voltage peak at switch-on to be more than 1V. Of course it still helps to use good motors.