This is an analogue computer with a digital option. It is an elegant, unique idea for MPPT. I have some documentation videos and circuit inside the link. The circuit works but I never used it much because my wind turbine project stalled. I am back: April 2016.
This is using two LEDs to sense the sun and send +- or -+ logic signals to a full bridge motor driver. You could also use this circuit to turn on an irrigation motor for a very short time every several hours.
My solar power has to be manually switched to city power at sunset in my basement. I made this wireless upstairs button, instead of running a long 120vac wire to an upstairs switch.
The project video shows a fridge timer which can be set to trigger during high battery voltage sunny days so the defrost cycle never uses valuable battery power. It is switchable to act like a regular defrost timer for long cloudy stretches of time, like winter.
The project video shows a way to power solar panel rotator motors from a solar panel, supplemented by a walladapter.