help! mechanical shutter

rotating disk with hole(s) in
rotating disk with segment(s) removed
rotating thing with mirror stuck on it

(Unwanted CD, lid of tin can, etc)

Toy type DC motor
variable resistance or variable voltage power supply

thats a good idea contrex, thankyou.
but unfortunately,the rotation must be quick and yet slow enough so that the hole reappears only after 1sec : the output should be only 2pulses per sec.this, consequently calls for a large disk and that won't fit snugly in the little space in front of the yag laser.dats why my query was specific to a mechanical 'shutter'.

Use a solenoid controlled by a 555 timer.
Attach a shutter of opaque material to the throw of the solenoid to block the path of the beam when it is activated.
You can set the 555 timer circuit to perform as you desire so the shutter can be opened and closed in whatever duty cycle is required.

Of course a Kerr cell would be neat, but kind of hard to rig up quick, and not exactly mechanical. The power supplies are kind of hairy as well. Although I believe modern ones don't need 20 kV square waves to blank them.

thanks guys