A trigger release, such as found on crossbows – suggest a captive bolt wound or drawn back by hand. Similarly, could use the basis of an airgun cocking and trigger mechanism or for simplicity a simple hanging rod & weight that snaps into a release mechanism with a pull-string to reload. The problem with the above options is that the clapper will likely rest against the tuning fork and dampen it although with the pendulum version it could be arranged not to swing back far enough for a second strike.
Someone cleverer than myself could likely knock up a simple circuit so a single strike via a solenoid then reverses and resets – wouldn't even need a button push if using a break in a light path as the trigger – just a finger in a gap.

pgk

Two tone door bells do this.

Martin C

A simple hammer on a semi flexible rod mounted vertically from a horizontal rod pivoted t both ends. The horizontal rod carries a counterbalance on 2 right angled levers to bias the hammer against a stop just shy of the tuning fork. A short tail on the pivioted rod to act as the activating lever/key. The action is such that pressing the activating lever lifts the hammer away from the stop and on release allows it to fall back. The flexibility of the hammer rod allows a small over run such that the hammer hits the tuning fork. Adjustment is by counter weight choice and adjustment of the stop.

regards Martin

I'd suggest the same idea/mechanism as used in a Sten gun, adapted to "fire" vertically. Basically it is a fairly heavy steel rod running vertically inside a loose fit tube. There is a spring below the rod causing it to jump up and hit the fork when triggered. After impact the rod falls back down by gravity, and if the spring is adjusted just so, it resets the rod behind the trigger catch / sear, and re-compresses the spring for the next actuation. Again, weight of rod and strength of spring have to be adjusted just so to get the rod mass vs the spring force to work as described. A nut soldered or welded on the bottom end of the vertical tube, with an adjuster screw installed inside, would provide a simple length/force adjustment for the spring, to " dial it in ". The trigger could be made with a button on a release lever, or left as a plain lever. It also needs to have a light spring and a stop to allow automatic resetting with minimal force. Just food for thought.

As with all simple mechanisms gravity is your friend.

Be sure to post some photos of your progress An Mich, would love to see how this works out. Cheers!

A variant on the 'detaching hook' used to prevent overwinding of mine shaft cages would be neat way of doing it