Problem, as I see it, is the inertia of that weight.

I personally would have chosen for a simpler solution. Stop with a bolt sprung in a hole like a catch pin in a large round plate (like flywheel). Every time the thing has to rotate the motor kicks in but it is mounted on a tilt plate that engages the teeth as it starts the rotation. As it slowly rotates the weight over the top again the force should come off the teeth so that as it gets to 0.1 degree within its top position, the force on the teeth is almost zero. At this point the spring catch flips into its hole again but at the same time it actuates the motor stop which also retrieves the gears.

I am not going to do any calcs here but it feels to me you might get away with a relatively small motor like 1/8Hp on a 1:200 gear ratio which gets your power up but speed down.

Little Siemens logo required for the solenoid actuation of the catch pin and bob is your uncle I think.

Stopping the motor and the gears with the weight attached will require the brakes of a jumbo jet I think. Not wise if there are other ways and means.

I am sorry Sir to make the remark that a gear box will NOT increase the power but only the TORQUE, in fact the power output will be less due to internal losses as friction in bearings and between the gears due to sliding.

"small motor like 1/8Hp on a 1:200 gear ratio which gets your power up but speed down"

I think that such errors are more important then spelling. What do you think? Words have their meaning and we are technicians.

From the principle it self it has a flaw. When a gear motor is stopped it offers only its own friction resistance to a torque present at the output. The discussed case is a situation with an active torque at output and with an instability since if the angle grows the torque will increase due to a bigger lever arm of the sand weight.

The best solution is to have a self-locking drive as worm or globoidal gears which have only one active direction and cannot be driven in the opposite direction i.e. from output to input. Now since the inertia is important it would be good to reduce even more the speed before stop in order to avoid a dynamic too important load of the whole chain. This can be done with a series of 2 contacts the 1st reducing the speed and the 2nd giving the stop signal. The speed should be small enough to avoid sliding on the drive rollers. In the chain it is also possible to introduce an elastic element with same goal of reducing dynamic overloads.

If I read your setup correctly, the disk is friction driven by the rollers.

It's true that you can't depend on the motor stopping at an exact point unless you use either a control loop, or a stepping motor that makes a definite number of steps.

How about this. Make a stop for the wheel that can be retracted with a solenoid. When you want to start the rotation, retract the stop with the solenoid long enough for it to pass the stop block and then let it reposition. When the disk hits the stop, it will slide on the rollers.

Look at the set up of a garage door opener. It spins an actuator on a threaded shaft back and forth to engage switches. Will not rotate the hour glass continuously around in a circle. Will be able to turn the hour glass back and forth 180°. So you will need to set up circuitry to reverse the motor. Brake will help stop the motor from run on.

The ideal solution is a reduction drive that does not need a brake, but will stop "dead on" when the motor is stopped. Stop the motor,and the load CANNOT result in drift. I cannot at this moment say more, as I do not wish to jeopardize my patent status, but that consideration will soon be moot.

try a chain hoist the electric kind ,if it has a snatch block then you will have you sprocket for the "clock" shaft you will also have both dir. of travel

You can purchase your gear motor with a braking option.

Ozzb has it right, a garage door opener would be perfect for this...

At least some old garage door openers used a sprocket chain for the drive. Mount your drive at the nine o'clock position [US 12-hour clockface!], and wrap the chain around to a fixed point on the rim at twelve o'clock (in both the 90 degree and 270 degree directions). Run the motor one way to move that fixed position down to six o'clock, and the other to go back to twelve; the fixed attachement goes around the 1-2-3-4-5- side, never passing the drive. The motor drive is already designed to have two stop positions, so switches suitably actuated should complete the job.

One way to stop an AC motor is to use a DC voltage.

One other way is to use an electric clutch and an electric brake

I would use an electric clutch and a Geneva drive for timing coupled with a timer (microprocessor).