Raghu:
I would suggest that the grandfather clock pendulum may swing a bit too slow for the LED image to look like a non flashing display. Most of these things would only swing back and forth once every second or two. The mechanism I sketched and which I saw in an actual real commercial product some years ago had a pendulum frequency of something like 120 Hz or more. The wand was a total blur !!

And also I recall that the mechanical spring thing was simply a small flexible strip of spring steel maybe 4 or 5 mm wide and rather thin. It was mechanically afixed to the wand arm and then clamped into a slot in the moulded plastic base of the contraption. The arm of the wand was all but 5 mm wide too. At the top it was wider but surface mounted LEDs were used to reduce the mass of the wand. Total length of the wand was probably about 180 mm. What I do not recall is if there was any kind of countersweighting done to balance the want mass across the hinge point. Actually now that I think about it the metal flag assembly was mounted further down the arm so that a good part of its mass was down past the center point of the spring hinge.

Another thing to note is that when this device lost power the wand would spring straight up to a center rest point. When one applied power it was necessary to swing the arm manually all the way to one side and then let go. The string tension would then let the arm spring on its own all the way back across to the other side. As the photo detectors would see the end of range detection the microprocessor apparently energized the coils in the manner to either kick the arm back the other way or to magnetically hold the arm for a short instant and then release it to let the spring return take over. I never had one of the devices apart to the level of being able to "scope" it out inside.

If I had built one I think I would have experimented with mounting two small round bar magnets on the end of a brass rod that was bent into a circular arc and permitted the magnets to enter and leave a hole in the middle of a coil on each side. Something like this....



Michael Karas