Magnetic Reed Switch NC NO COM Surface Mount Wired Door Window Contact Sensor with Adhesive Paper for Home Security and Burglar Alarm Systems MC-38 5pcs

These work great. They're about 1" long, which is smaller than I was expecting, but they work great. I've got them hooked up to a D1 Mini with about 1.5 m of 22/2 gauge cable and they work without any interference. I tested them all with a multimeter and they all worked.

I put this on my service truck tool boxes, then I added an alarm.

Is smaller than I thought it was going to be.