EZ Solar Engine
From BEAM Robotics Wiki
 EZ Solar Engine V1
The secret is the way the solar cell is connected to charge the storage cap through the motor winding. Compared to many previous SE circuits, the EZSE is tolerant of, and adapts to, different operating conditions.
For some simple application like a flagwaver, charging the storage cap through the motor can have another benefit as the motor first turns one way while charging and then reverses when it triggers. I tried several different motors, capacitors and solar cells and seems to work flawlessly with everything.
The SE trigger level adapts to the light conditions or rather the solar cell output current, by charging up to a higher threshold in bright light and triggering earlier in dim light. This is also a function of the motor resistance as the voltage drop across the winding adds to the trigger level.
Well I did say "deceptively" simple. There is no free lunch and no perpetual motion here. I was reminded of this idea of charging through the load when I commented on the earlier UJT SE which like many other solar engines suffers from "latch up" when the solar cell current is too high. This occurs because the solar cell can deliver enough current to stop the storage cap voltage from dropping below the reset voltage or provide enough current to hold the SE triggered on.
In other words, if a battery was used in place of the solar cell the SE would not reset. On the other hand, some SE start to draw a larger supply current when the storage cap voltage approaches the trigger level and as a result "hang up" when a low current solar cell or low light level does not generate enough current to overcome that quiescent supply current bump. These are the most common problems that people experience when using simple SEs. The Miller SE avoids these problems by using a nearly ideal voltage trigger (1381) which is not always available.
The EZSE uses a ridiculously simple solution to avoids these problems by simply ensuring that the solar cell is shorted out while the SE is triggered. Looking at it slightly differently. The current though the load must go through zero before the storage cap can be recharged. The additional positive feedback causes the SE to quickly snap on and off and makes it nearly immune of the aforementioned problems.