How to get the angle from vertical window blinds How can i know the angle of the vertical window blind by placing a small sensor on one of the blind slat

i am trying to make a arduino/esp8266 controlled window blinds, the controlling can be easily achieved in horizontal blinds, but my apartment has a vertical blind which are not interconnected at the bottom  

Tried so far:


open and close based on time which it takes to open and close, but it keeps drifting over time
Cannot use light sensor to sense the ambient light, does not give correct information (day and night) 
Tried to put a potentiometer on top of one of the blind slat ( no enough room to place things there, potentiometers have more friction)  


any suggestions for angle sensor ?
A hall-effect sensor, plus a small magnet attached (glue?) to the outside edge of one slat ought to be able to tell you the strength of the magnetic field, and thus the distance of the magnet from the sensor. From that, given that the rotation is with a fixed radius, you can deduce the angle while introducing zero additional friction.

Edit:
I had imagined something like:



However, looking again, this introduces difficulty in detecting the 90deg position, where the magnetic flux lines are exactly parallel to the hall-effect sensor plane.

If you experiment with this enough, you may be able to get repeatable values from the sensor to know the angle of the blind slats.

Edit 2:

The timing measure you noted in your question could also be a possibility. If you could use a microswitch or photo-interrupt sensor on the extreme ends of travel (0deg and 180deg), so that any time those extreme ends are detected, you can eliminate drift.