I think that your best bet are wireless doorbells. You can remove the
speaker and do some tricks involving a solenoid or a small motor instead.

The approach of winding the string to close the trap door befits Rube
Goldberg 150%. It would probably work to catch newts or tortoises.

A normal trap has a spring or some other mechanical energy storage device
and the trigger releases this. In practice you could have an elastic
trying to close the door and a 3V motor with a half-cup shaped paddle
soldered onto the axle hold the door back (by a protruding piano wire or
such). When the motor rotates the wire escapes the escapement and the door
slams shut and stays shut. You do not need to limit the rotation of the
motor in any way. The escapement must be adjusted with pliers so the
required force is low and the motor can spin that first turn. When I
played with such things I used a strip of 0.5mm tinned mild steel about 5
x 3 cm, folded it down the middle and solder-tacked it to the axle of a
motor from a toy, with the axle inline with the fold (inside it). The
spring it released was a piano wire hook. This was to throw tennis balls
;-). The fold made an angle of about 60 degrees (not critical).

hope this helps,

Peter

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