The Arduino can not make another computer do something, unless that computer is listening

Yeah.... good point. Pity, that.

I suspect I know the answer is "No"... but it would be nice if there was a way to switch on the monitor when a sketch that uses it is loaded to the Arduino.

There isn't such a thing is there?

Seeing as you have dogs, plural, can't you just teach them to throw the ball for each other?

Those buttons might need turning thru 90 degrees...Have a look at the pic below to see how they switch internally... Might not be switching the connections you expect.

I Googled "toothed belts" on your behalf  and got, for example, this

I think I'm not understanding a basic principle, maybe related to "pull down", but I would have thought that simply substituting a switch for the pushbutton and leaving the rest of the setup intact would be reliable

Would have thought so too... provided you have the pull-down resistor still in place?

Its purpose is to ensure that the pin is low when it's not high. That might sound crazy, surely if it's not high it must be low?- nope. Pins "float" and really need to be deliberately forced to either end of the scale. Just because it's not high, doesn't mean it is low, so the resistor as the name implies, pulls it down to 0.

But it has to be through a resistor, not just a piece of wire... if it was just wire, when the switch was closed, the 5v would short circuit straight to 0v with the unavoidable release of expensive smoke.

Best you post a schematic of what you have...

They were a bit more expensive in the 60s.

Yeah...

My Dad, 97 this year, was an electronics boffin in HM's Royal Air Force and at Redifon (later known as Reduffusion Simulation, now part of Thales afaik) in London and Crawley. Circa 1962, I remember him making a multi-meter in a cigar box.. it was hugely impressive, with all manner of knobs and buttons and stuff.

He also built me a power supply for my Scalextric...

Well - I wouldn't recommend doing that... basically a very uncomfortable tingling...

Indeed, but we've all survived loosening the positive with the ground still connected and one hand on the fender.

Hands up all those who stuck 9V PP3s in their mouths as kids?

Just because something is published on the web doesn't mean it's error free or even correct.

WHAT? Are you sure? DAMN!

just in case I get wrapped up in this for a while

.... Hell yeah, we understand that, I've been Arduining all day and it's just gone 1900 here.

One disadvantage with the waypoint scheme is, the servo will slew over to the new positions
at maximum speed

Yep, that is indeed true.

Also, and maybe even more important, the waypoint idea might miss crucial waypoints...

Let's say it goes from 90 to 180 and back to 175. We could miss the 180 point, and capture the 175. So on replay it would go from 150 to 175, but it might have been important that it go via 180... like that was the point where the gripper opened so we drop a virus sample on the floor instead of in the medical waste.... YMMV.

zp you might have missed my post #6 in the clutter....

Now why did I think Tip120 is okay without a base resistor?

That was simply the East Coast seeing if the West Coast was paying attention?

AWOL beat me to it, but still,  here's the link to the example....

Here's a tutorial showing exactly what you want to do, except it uses a 122 not a 120: only difference is the voltage afaik.

Don't forget that diode.....

Edit... And it shows a base resistor