Amazon.com.au:Customer reviews: SpzcdZa 2Sets Magnetic Reed Switch Normally Open Closed NC NO Door Alarm Window Security/Magnetic Door Switch/Magnetic Contact Switch/Reed Switch for GPS,Alarm or Other Device,DC 5V 12V 24V Light
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I have several sets on 50-100' wire lengths back to an MCP23017 I/O expander chip at 3.3V, set to generate an IRQ on edge events. As with most switches, they were quite "bouncy" at first, but a 1uF cap across each (at the controller end) solved that
Contact resistance was generally 3-6 ohm, which was fine, but one side of one of the switches (I can't remember if it was NO or NC side) was up over 600-800 ohms at first, varying slightly on every cycle, but never as bad as the one in Nancy's review. However, this "fixed" itself after I accidentally dropped it about 5' on to a concrete floor, and has operated reliably ever since! I used large enough pull-up resistors (22k?) on these sense lines to hopefully tolerate that level of contact resistance if it returns, but after 4 months, it's still under 10 ohms.
The double-throw design of reed relays inside these could make intermittent marginal contact more likely. I'd have chosen NC-only (closed when magnet is present) if those were available for the same price.
You really don't want to run any significant current or voltage across these. It may work at first, but contacts are unlikely to last very long in this role. Use a FET, etc. if needing to directly switch a load.
Not reliable. On one of the two the normally closed pin fails to make contact fairly often. For whatever reason you have to get the torque on the screws just right for it to work.
The other one breaks contact on the normally closed pin well before it makes contact on the normally open pin. In my case using it as a limit switch it means stops movement and never engages the light that is supposed to tell me the action is complete.
I installed this in a gun safe to activate LED light strips. Almost immediately the switch didn't tun off when the magnet was placed in close proximity to it. The switch also became hot to the touch. The set came with two switches and had problems with both switches. I will not be buying this item again. Don't waste your money!