Is that a challenge?
So... I watched some video of a real one in action.
https://www.youtube.com/watch?v=TZj5jg8Ujpw (about 38 seconds in).
I do see how it is
possible but extremely difficult. It would be an awesome project to work out.
Maybe a piece of memory wire to replace the little stem that connects the round target to the motor casting above it.
Although I don't know how hot that stuff gets when it activates. You'd have to hollow out the mast and main arm
or make new ones so you could run wiring up to the memory wire.
Crazy Idea 1:
1. make a new main arm and main mast that are hollow.
2. mount the target by drilling something like a #80 hole through the top of its arm with a pin through it so
it can swing freely up there.
3. Use a really fine steel wire wrapped around that pin as a "whisker" of a spring that
you hook to the target so it holds it swung over to one end of its travel.
The above 3 steps are probably the "easy" part. Ha ha.
4. Hook a fine piece of nylon cord, like really fine fishing line, to the target arm, run inside the
newly-made hollow support arm and down the inside of the newly-made hollow mast.
5. Below the benchwork, you attach the end of the cord to a shaft of a slow-speed gearmotor.
The gearmotor runs, wrapping the cord around its shaft, pulling the wig-wag target over to the other side (working against that whisker spring).
6. An arm, like a simple steel wire, also attached to the motor shaft, would move as the motor shaft turns,
and eventually would touch a microswitch, to act as a limit-switch. When that switch is closed, power to
the motor is reversed in polarity (you could do it with a DPDT microswitch requiring no other electronics, or a
small DIP relay, or a flip-flop circuit... or any of a number of ways).
The motor then turns back the other way, until the steel wire hits another limit switch to reverse the process.
You could even attach something to the fishing cord under the benchwork that move back and forth between
two limit switches.
I actually built something like this to operate a Z scale mine car on a little piece of Z scale track on
one of my old layouts. I sure wish I had photos of this now.
A fine piece of clear fishing cord ran along the center of the track, under a series of
track spikes. It had a fine whisker sticking up from the cord, glued to the cord, and that was strong enough
to push the little mine hopper along. The cord disappeared through holes at each end of the mine track
and below the benchwork it ran over pulleys to a gearmotor, which did this same back-and-forth turning
trick I described above, controlled by limit switches and a relay.
Crazy Idea 2:
Wind a tiny electromagnet coil (from magnet wire).
Rig up the target on a pivot and spring like I described above. But
now, mount the tiny electromagnet up there where the motor detail is. Hopefully you could hide it inside?
Wires run through new hollow arm and mast as before.
The power to the magnet coil would have to gently ramp up and down. If you just hit the coil with power, it will snap
the wig-wag over to the other side, which is not what you what.
So some kind of sawtooth generator (at a really low frequency) circuit has to be built to gently ramp the voltage
to the coil up and down. That would pull the target over gradually... but I'm still not sure how well it would work
because as the target moved, getting closer to the coil, it would still speed up as it neared the magnet.
The "sawtooth" might have to be a more like some kind of exponential curve generator, so that it ramps up
slower and slower as it goes (so the magnet increases in power at a lower rate after it gets going.
Like I said... a little bit crazy, and very difficult.
