Misc. I want to mechanically detect when an egg-timer runs out - ideas?

[some bloke]

TL;DR Summary

I'm trying to design an egg timer which will actually sound an alarm when it goes off.

Hi everyone,

I'm trying to come up with a method for designing a classic egg-timer (falling sand) which will sound an alarm when it goes off. The alarm itself is going to be some contraption akin to a Rube-Goldberg machine, which I can design after I work out how to mechanically detect when an egg timer has run out. Making noise isn't the problem here -detecting when an egg timer runs out is!

My first thought on this was to have the egg timer suspended in a tube of water, and I initially thought that it would float higher in the water as the sand poured down, as its center of gravity would change. I have since worked out that this wouldn't happen - the mass of the egg timer would not change, so it wouldn't change how high it floats in the water - thus my first thought is foiled from the off!

I am after ideas on how to mechanically detect an egg-timer running out. One option would be to put something in the path of the falling sand, so that when the sand stops flowing it would swing back to a position. Another option I am looking at is making an egg timer in two pieces, so that as one side fills up its weight can trigger a mechanism.

I am also considering how to make this in a different manner entirely - my goal is a mechanical timer which sounds an alarm when it is finished. I could use a viscous fluid and have something sink through it, or any other means of giving a reproducible time effect. If I can, I want it to be adjustable on time, but I'm not getting my hopes up on that one! Just a mechanically detectable egg timer type thing would be ideal! And no electricity!

Do you have any ideas that would fit the bill here? Any suggestions I can look into?

 

[Rive]

Quite interesting challenge

 

[Tom.G]

A standard sand (often salt) egg timer
suspend from a string attached to the periphery of the top edge

1) As the sand flows to the lower chamber the center of gravity will change.
2) The angle of the egg timer will then vary to keep the center of gravity directly below the support point.
3) Use the sensing of your choice to detect the horizontal movement of the bottom edge of the timer.
3a) Following edit somehow didn't get in this post when created: (Perhaps by pushing a marble off a platform down a ramp to increase available energy.)

Have Fun! ...and be sure to show us the result!

Cheers,
Tom