I've seen a couple posts in other threads considering the idea of modifying a game to fire the knocker when an extra ball is awarded. It can be fairly easy to accidentally miss the 'extra ball' for casual players, especially when you get it for a points threshold instead of a playfield target. This can lead to people taking each others turns and causing some confusion.
Anyway I decided to give it a go, and have rigged up Solar City to fire the knocker for extra balls.
I purchased an on-interval relay that essentially "passes through" a voltage for an adjustable period of time before dropping out. In this way it is possible to tap into the circuit that powers the 'extra ball' relay (which stays on for the remainder of a turn) and create a pulse for the knocker that cuts out so that the coil isn't energized for too long.
I purchased this relay:
https://www.automationdirect.com/adc/shopping/catalog/relays_-z-_timers/timer_relays/t2s-tt-30-240a
I also added another higher-current relay, triggered by the timer relay, since the timer relay is only rated for 1 amp.
So:
The 'E' relay happens to be the the extra ball relay on Solar City. I tapped into the feed side of the relay for the trigger of the on-delay timer relay. When 'E' is energized, so is the timer relay. I set the output of the timer relay to drop out after about 1/2 second and it will not come back on unless 'E' is de-energized and re-energized again. When the on-delay relay is active, it triggers the higher current relay. The higher current relay runs the knocker, I have tapped into the fused side of the 25v line at the fuse. I also disconnected the original feed to the knocker to prevent power from back feeding into the game. The only purpose the knocker originally served was if the game was set to award replays, which is not how I have it at home.
I've tested everything a few times and it seems to work fine. Time will tell if this is a worthwhile mod. Thankfully it's pretty simple and didn't require any irreversible changes to the original game.
Photo taken during initial testing, I wired everything up better once I had established proper function.