R.D.
I got a battery powered one from Walgreens (priced at around $20) that you can set to light up AND ding when it dispenses a pump of soap! That way, if I hear a second ding, I can remind my son: "that's enough soap."
Recently, I had a flash of brilliance. I was trying to show my son that he doesn't need one whole pump for his little hands because I don't even use a whole pump! Well, since he just wants to do it anyway, I made it too difficult. I wrapped a plain (newspaper) rubberband several times around the pump so it can only be depressed so far. It works like a charm!!