Alternatively, you could do some clever metalwork and make something like this:- YouTube - The Big Countdown for the Chronophage where the lights that go round the clock are just slots in the metal aligning or not.
Ok.. I guess I just jumped right in without looking.
I have no idea how that works, I was thinking it would be something simple but looking at the 'datasheet' for that, I'm way out of my depth.
And I don't understand how the second one works, though it sounds pretty simple.
BTW in case it wasn't blatantly obvious, I have never done electronics before.
The idea is not worthy on making on existing mechanical clock. If you're making a digital clock using a pic module then it would be easy to program. I think reed switch or any other mechanical arrangement will interrupt the free movement of the clock. Other things like a motion detector or IR interrupt based switching we can't do here . If you're a clock manufacturer then the below setup will work out.
*)Imagine you've placed holes corresponding to digits 3,6,9,12
*)There's a one more round plate below the dial attached to the hour wheel and a single hole corresponding to hour hand direction.
*)LED backlight.
*)Obviously the corresponding holes of digits will be lit when the plate moves below that