Thanks, Digger. I've seen several systems that use coils, and they all seem to include a bunch of extra stuff, like a unique, dedicated panel, that I don't want. I suppose I need some associated electronics to excite the coil and monitor for disturbances in the magnetic field, and then send a binary status signal along to the main alarm/automation panel.
The optical solution would not be as secure since a wad of paper jammed into the bolt pocket would give a set indication. I'm not sure that is very significant, though. I can check for a "bolt set" signal when the normal door sensors indicate "door open" and trip a fault report. I'm mostly after a basic system that allows me to look at a panel and confirm the garage doors are down and the bolts are engaged on the various people doors around the house.
