Which sensor should I use?

Here is an image of the type of product

The puck will travel underneath rotating connecting arms. The sensor needs to be able to pick up the puck breaking the beam from leg to leg, underneath the connecting arm.

The sensor needs to be able to rotate with the connecting arm.

Was looking for the smallest solution, yet also the least expensive.

Hoping for positive advice.

What's the puck made from?

if it is not steel would it be possible to stick a thing sheet of steel to the top of it. That should allow you to use hall-effect sensors to detect it. They could be mounted on the bar rather than on the legs.

...R

Are the legs fixed to the arms such that the same spot on a leg always points along the length of the arm? If not, your beam break idea is going to be very difficult!

Will the puck be a strongly contrasting colour to the floor? If so, a series of sensors along each arm, pointing down at the floor, might work. The sensors will need to be close enough together that the puck is always detected by one of them, no matter where under each arm it passes.