A new Casio movement for the GS-1200 Tough line – a radio controlled watch model built for shock resistance – has internal LEDs that calibrate the analog hands of the watch.
The movement has a structure where the gears for second, minute and hour are sandwiched between the LED and the phototransistor. With this setup, it is possible to detect the positions of three gears by one sensor. The LED emits light 16 times a second to check the positions of the 300mum diameter detection holes that are formed on the gears.
Most mechanical analog watches eventually slip out of alignment due to the internal motor. Buy adding the tiny LED and photosensor, the watch processor can tell exactly where the hands are on the face and ensure that the user won’t have to recalibrate the watch later.
This is similar to the methods used to sense positions in the old-time mechanical mice.