I made a circuit board for the controller, but when the components showed up, the footprint for the h-bridge was different than the one in the Protel library (grr..) And I also noticed a few other things that I didn`t like. I should have used bigger diameter pads to make it easier to drill the holes. So I have made a new layout with bigger holes. I also did a few changes to the circuit. Added current limiting for the motors. Nice to have, especially if something bad happens, or I want to stop the motors quicker by shorting the inputs. I also added a 12v trigger input to make the masking system turn on when I turn on my preamp. Hopefully I will etch and drill the new board in a few days.
I also did some mechanical changes to my system. Like many others I used a system where the lower mask was pulled up by the upper roller, and the upper mask moved down by gravity. This give a slight error, since the diameter of the upper roller increases when the fabric rolls on. So, the upper mask will move faster and faster as the fabric rolls on. The opposite happens with the lower mask. So the masks will not open/close symmetrically about the screen center. I also got problems with binding. I tried to use gorilla tape as Scott did on his system, but It didn`t work for me.
So I made a continuous loop of cable with pullys as has been suggested and also implemented of some on this forum. The upper pullys have two wheels.
I use the old rollers, except that I put in a spring from an old roller blind in each roller. The spring keeps the fabric tight when the mask rolls in or out. I used counter weights before.
The masks are attached to the cable, see pictures. This works very well as a manual system as well. Just lift/lower one of the masks to the desired posistion and it stays where you put it. The other mask will of course move the same distance in the opposite direction..
I plan to use the same pully system for the side masks, but there is probably room to use panels instead of rollers. It`s alot of cable and pullys, but it is easy to adjust since each end of each mask can be adjusted. Also, I don`t think it will change much after initial adjustment.
The pullys come from blinds on a demolished building at my work.. The double pullys are modified single pullys. The system would move easier if I could find pullys with bearings, but it is still ok. I will need a little stronger motors than before.