MC-38 Surface Mount Wired NC Door Sensor Alarm Magnetic Reed Switch Brown 10 Pair

I got these to wire an alarm in my shed. It's a simple project, and these have many applications. They work well for my needs.

With magnet in place .2 ohmsNo magnet .5 ohms......

These seem to work fairly well. Their size is very small, and can be hidden away easily. However, the sensitivity of them was more than I expected. In some cases that's good, as it doesn't take much movement to cause them to alarm. But in other cases, especially on steel door frames, the slightest movement causes them to alarm. Buy them knowing this fact, and you'll be very happy!

I measured the resistance of the reed switch when opened (no magnet) vs closed (magnet near switch). 50% had greater than 3 ohms of resistance with one being completely open no matter the position of the magnet.Your application may not be critical enough to worry about the higher contact resistance but I would be concerned that the resistance would increase over time. I would not trust these in a house alarm since failure can cause significant hardships due to false alarms.High quality reed switches normally read less than 1 ohm and use Ruthenium Oxide over Palladium as the contact material. I doubt these use quality reed switches.

I need a normally closed switch. That is what is in the description. It is normally open