Who's Right ? Hope it's not them. Normally OPEN/CLOSED? Reed switch - Page 1
Forum section: electronics / beginners

author: Voxman
author_rank: Contributor
timestamp: June 16, 2024, 03:43:36 pm
content: For more years than I can remember, I've always thought NC reed switch means whenever the activation magnet is far away, the switch reverts to a closed position in order to complete circuit for something to light up or energize.However I was needing a new switch to operate a LED to indicate when the freezer door is open or closed. Amazon sellers apparently are on a different planet.Normally one would use a NC (normally closed) magnetic reed switch to turn the LED on when door is open (magnet no longer keeping switch in NO position).Well searching Amazon, I found several items from sellers that say they are NC switches but the circuits they show contradicts that. Wondering if they are misinformed or if my understanding of NC and NO is backwards.Here's one of the circuit in question? Who's got it wrong?!

author: pcprogrammer
author_rank: Super Contributor
timestamp: June 16, 2024, 03:59:03 pm
content: Well I hope with you that it is them.From one point of perspective you could argue that the normal state for a window open detector is with the magnet near by giving a closed circuit, but from the plain reed switch perspective this one would be in an open state without the magnet near, and thus normally open.A non latching push button that has both states will normally be in the up state meaning that the normally open contact is indeed open and the normally closed contact is indeed closed, because that is the normal state.So I guess it is all about what you think is normal.

author: Voxman
author_rank: Contributor
timestamp: June 16, 2024, 04:43:16 pm
content: Thanks,I'm just wanting to order the correct switch and apparently can't trust sellers to know what the correct label is. I'll just find a switch that has both NO and NC options on it.Geez you would think this would be a no-brainer!

author: Benta
author_rank: Super Contributor
timestamp: June 16, 2024, 04:58:10 pm
content: NC/NO refer to the state of a switch/relay in the non-energized/not-activated/non-magnetized state.That in your application the magnet is normally close to the switch is irrelevant.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 16, 2024, 07:03:42 pm
content: You are in the US. If the shipper is Amazon, don't worry. If the seller is also Amazon, don't worry at all. If it arrives and is not NC, return it. I suspect the seller and shipper are marketplace, then you may be screwed. Just don't buy marketplace unless you know the seller.

author: tooki
author_rank: Super Contributor
timestamp: June 16, 2024, 08:24:47 pm
content: Having recently purchased and installed multiple reed switches to use as door interlocks: from any reputable vendor, NC means NC without the magnet nearby, just like Benta said. Those vendors are wrong in their terminology, even if there is some logic behind it.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 16, 2024, 08:33:31 pm
content: Nearly all reed switches are NO, because of the inherent nature of those things. The contacts are made out of steel (or similar magnetizable Fe material) and when the magnet gets close, the parts become magnetized and attract each other. When you buy a reed switch with only two contacts, then you can be pretty sure it is a NO contact because of this.There are some reed switches with three contacts. In those the normally closed contact is made of a non magnetizable material. Those switches are quite rare though.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 16, 2024, 10:33:54 pm
content: Quote from: Doctorandus_P on June 16, 2024, 08:33:31 pmMany images of products on Amazon are stolen from other products. Marketplace (and other) vendors don't care. If Amazon is the shipper, buy and return if not NC, or buy from a reputable dealer for more money. I have had no problem returning items for full refund that were not as advertised.

author: mikerj
author_rank: Super Contributor
timestamp: June 17, 2024, 10:56:54 am
content: Quote from: Doctorandus_P on June 16, 2024, 08:33:31 pmNormally closed two wire reed switches are commonly available, they include a small biasing magnet that holds the contacts together in the absence of an external magnetic field.

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2024, 11:56:17 am
content: They also are rather critical of magnet strength and operating distance, along with polarity, as too strong a magnet, or it coming too close, will result in it opening then closing again.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 17, 2024, 12:11:22 pm
content: Quote from: mikerj on June 17, 2024, 10:56:54 amThis was new to me. Also confirmed by:https://uk.rs-online.com/web/content/discovery/ideas-and-advice/reed-switches-guideAnd that site also states:Note that you can create an NC switch from an NO switch by simply leaving a magnet permanently affixed to the switch casing. In this example, a second magnet with (stronger) opposing polarity would need to be introduced to override the effect of the first.Edit, and more:Via this link:https://electronics.stackexchange.com/questions/698849/converting-reed-switch-from-no-to-ncI stumbled upon:https://standexelectronics.com/wp-content/uploads/ReedSwitchNoOrLowPowerSolution-0209.pdfThey even mention bistable reed relays, by using a magnet in such a way that it's strong enough to hold the contact closed, but not strong enough to close the contact by itself.

author: Gregg
author_rank: Super Contributor
timestamp: June 17, 2024, 08:25:26 pm
content: The NO vs. NC description might be a throwback to the mostly backward thinking alarm industry; where the normally closed state is when the alarm is ready and not activated. Many early wired alarm systems were in a normal state when a loop had continuity.I've installed a few alarm systems and the alarm system nomenclature tripped me up a few times.edit: In the alarm industry the magnet is normally mounted to a window or door and the reed switch mounted to the structure, When the door/window is closed, it closes the reed switch resulting in a "normal" alarm loop.

author: BillyO
author_rank: Super Contributor
timestamp: June 19, 2024, 12:20:39 am
content: Quote from: Gregg on June 17, 2024, 08:25:26 pmYes, I was going to mention this. To be fair, alarm systems are (were) the biggest application for reed switches so maybe their way of thinking is what the vendors are following.

author: kevin.gibbs
author_rank: Regular Contributor
timestamp: November 20, 2024, 03:02:33 pm
content: Reed switches are available with both NC and NO contacts. You can order these without worrying about making a mistake, but they are a bit more expensive.You can also modify the circuit by turning on the LED permanently and closing it with a reed switch. However, when the LED is switched off in this way, there will still be current flowing, but it is only 3-5 mA.