Okay, after running some prototypes, I finally got it installed.
Here's my "Primer Out" warning device.
Couple of goals.
Not battery operated, but low voltage
LED light for reliability
No modification to the press
Here's what it looks like as long as there are primers in the tube.
And here's what happens when you go to prime and there's no primer.
I'm going to put a red lens (gel) on it, but for now it's white.
This was a small battery powered reading/work light for $3 at the local grocery store. I removed the batteries and the switch on the rear and soldered the wires to the back of the light assembly. It has a little clamp and attaches neatly to the press.
Behind the press is a micro switch. It is mounted on an angle bracket. When the primer tube is empty the primer follower rod locks the slider in the rear position which engages the micro switch as the ram comes down.
The bracket is held in place by it's lip which is udner the press. And the power supply wire is run through a hole right behind the press. I have a bunch of "wall wart" power supplies, so it was a simple matter to find a 3volt one that would power this lamp assembly. I wanted something cute'r like a railroad warning signal or a railroad block signal or a traffic light. I haven't found what I want yet. The problem with scale model stuff is that the brightness is to "scale", so they're not very bright.
One last picture where you can see the little "wheel" on the primer slider tray thingy. It's stuck the rear position because it has trapped the rod where there should be a primer.