With a couple little kids going in & out of the house, they often forget to lock the doors when they come back in. So, I've been looking into ways to monitor the status of the deadbolts (and the sliding door lock). I have some limited options as I'm not putting in any new door frames, so the walls won't be open.
I looked at using plunger switches drilled into the door frames that would get pressed when the deadbolt was locked. I didn't really want to drill into the door frame to install it and have to figure out what to do with the wire on the other side of the door frame.
I opted for the typical magnetic reed switch solution. Fortunately, a couple doors had deadbolt holes deep enough to fit mini reed switches without interfering with the deadbolt. I was easily able to glue them into the deadbolt hole and route the wire to the bottom of the door frame and out to where I needed it to go. For the magnet end of things, I got some 10mm x 1mm rare earth magnet discs from eBay. I superglued these to the end of the deadbolt, so that when the deadbolt closed, the magnet would be close enough to cause the reed switch to close.
One deadbolt hole wasn't deep enough and I didn't feel like drilling deeper into the doorframe. Instead, I found some subminiature reed switches at a local electronics surplus store. They're very tiny with a glass casing, so you need to be careful installing them as they're easy to break. I etched out a place in the deadbolt hole so I could glue the reed switch in, making sure it was far enough away to not get crushed when the deadbolt closed.
The sliding door has a different locking mechanism so I had to be a little creative monitoring that. Again, I used a subminiature reed switch and mounted it on the door frame. I used half of a rare earth magnet and glued that to the lock latch. It took a few iterations to get the right position of the reed switch and some careful wire placement to run the wire, but I was able to get it monitored.
I have everything run to the gameport adapters that I used for contact closure monitoring. I'll post some pictures when I get the chance.