Help choosing a permanent magnet for a reed switch

[cdevidal]

Sanity check please, help me to know that I'm thinking correctly. I need a reed switch for a chicken coop door opener. (Edit: And yes, I've already tried the purpose-built security system outdoor reed switches.) The reed pulls in when a magnet attached to the bottom of the door is parallel with the reeds, indicating that the door has reached its end of travel. The more horizontal distance allowed between magnet and reed, the better*, since it may not be possible for the end user to get the magnet very close to the reeds, depending on their door design. If I have this reed switch (datasheet) which requires at most 30AT, according to Figure 10 on page 7 of this document (screenshot below) I want at least a 75 gauss magnet. It needs the correct orientation so that the strongest portion of the field radiates in the required direction. And of course, I need a weather-resistant coating such as nickel or plastic. Using this calculator, I should be able to assess magnets of various sizes and materials to find one that gives at least 75 gauss at the distance required.

Did I get this right, or am I missing something?

* But not too far. If the magnet is too strong, it might pull in the switch before the door has reached the end of travel.

[Xena E]

This is not what you are asking, apologies.

However there are ready weatherproof encapsulated reed switches with matching magnets available that are used in industry, they even have built in mounting systems. (Screw holes!)

Just as an example:

Actuator magnet:
https://uk.rs-online.com/web/p/sensor-accessories/2897812?gb=s

Reed switch:
https://uk.rs-online.com/web/p/magnetic-proximity-switches/2897799?gb=s

Normally closed switch versions also available. Sure there will be similar available local to you.

X

[janoc]

Don't reinvent the wheel, use a common window or door magnetic sensor as are used for heating (detecting open windows and closing the radiator valve) or burglar alarms. Those are cheap, plentiful and some of them are designed also for outdoors use. The ones for indoors will likely not last long due to UV radiation from Sun destroying the plastic.

[cdevidal]

Appreciate the responses, both of you, but I've already been down those roads. Got some of those sitting eight feet away from me right now.

[floobydust]

You need the magnet to have right shape for the lines of flux to pass through the reed switch so that the leafs are pulled together. Axially like in pic.
I'm not sure of what your configuration looks like. Standex magnets configs
If it's not ideal, a steel screw or washer can redirect/bend the flux.
A rod magnet parallel to the reed switch is the usual configuration. Use a strong magnet (rare earth) if you need range.
Not a thin disc/coin type that will not work well because the flux lines keep too close to include the reed switch, but the strong rare-earth ones can overcome this.
Why not just use a home alarm door switch/magnet? Take it apart if you have to.

Side opinion - I'm not a fan of reed switches, they are really fragile devices the glass breaks or the pin rotates etc. and I never had good luck with them.
There are low power Hall switches that are more sensitive. I made a few for an alarm (door) of mine and zero troubles for years.
Last ones I used are A1120 are they are great I get operation at least an inch away, good sensitivity with strong magnet. But need steady power 4mA.
A3211 is micropower for cell phones etc.
You can also get a bipolar Hall sensor and use two magnets for the door up/down detect.
ESP32 has a Hall sensor built in.

[cdevidal]

A rod magnet parallel to the reed switch is the usual configuration.

That's the configuration I'm going for.

Why not just use a home alarm door switch/magnet?

Long story, but I've already been down that road. Some of them are sitting right next to me.

Use a strong magnet (rare earth) if you need range.

That's the idea, just need help choosing the right strength; Am I correctly converting AT to gauss and calculating the strength at the distance required, or am I missing something in the way I calculate it?

[floobydust]

TBH I don't think math works well here, it's complicated. Field strength falls off fast with distance 1/r² anyhow.
K&J Magnetics has calculators and good info too, although they only sell super strong ones.
That R&S PSM at 200 gauss is pretty weak, it's just a vanilla iron rod magnet inside.
Realistically you want the strongest magnet you can afford, and if it's too strong move it away 

[inse]

An industrial type position switch/end switch is not a viable solution?
Those are pretty rugged and can be found at a reasonable price.

[Xena E]

Use a strong magnet (rare earth) if you need range.

That's the idea, just need help choosing the right strength; Am I correctly converting AT to gauss and calculating the strength at the distance required, or am I missing something in the way I calculate it?

That graph is not helpful. AT on a standard coil equivalence to permanent magnet is the minimum strength needed to operate the reed reliably.

Utilising a more powerful magnet will extend the operating range with a reed switch. So the increase in the strength of the magnet will increase the distance of the unclosed gap as you require, In close proximity the relationship between magnet strength and gap is almost "square law" ie., 4 times strength, roughly doubles the gap. (Thats less the magnetic leakage, and the pull in hysterisis of the reed.) Increasing the range will proportionally increase the unclosed gap.

What can then be done is the reed can have a U shaped feromagnetic shunt around it to control the range in the presence of a very strong magnet, or alternatively a seperate bias magnet to increase the range. Unfortunately if this device is to be fitted by non technical folk then theres a lot of scope for failure, if it is mounted near or on ferrous material for instance.

X

[cdevidal]

An industrial type position switch/end switch is not a viable solution?
Those are pretty rugged and can be found at a reasonable price.

Thank you for the suggestion. Long story: Also no on these.

The design decisions are all complete except for the size and composition of the magnet. Just need that and it's all done.

[cdevidal]

That graph is not helpful. AT on a standard coil equivalence to permanent magnet is the minimum strength needed to operate the reed reliably.

That's how I understood the graph. Since it is showing the minimum strength needed, I need to calculate the strength provided by certain magnets to know if it is the minimum strength needed to operate the reed reliably.

Utilising a more powerful magnet will extend the operating range with a reed switch. So the increase in the strength of the magnet will increase the distance of the unclosed gap as you require, In close proximity the relationship between magnet strength and gap is almost "square law" ie., 4 times strength, roughly doubles the gap. (Thats less the magnetic leakage, and the pull in hysterisis of the reed.) Increasing the range will proportionally increase the unclosed gap.

Yeah, that's what the calculator shows.

What can then be done is the reed can have a U shaped feromagnetic shunt around it to control the range in the presence of a very strong magnet, or alternatively a seperate bias magnet to increase the range. Unfortunately if this device is to be fitted by non technical folk then theres a lot of scope for failure, if it is mounted near or on ferrous material for instance.

Yeah, I'm not interested in solutions that have a lot of scope for failure by non technical folk.

I just want a confirmation on how I'm thinking: Since as you said the chart shows the minimum strength needed to operate the reed reliably, if I use the calculator to find a magnet with the correct strength at that distance, it should work reliably, yes?

I will of course buy and test before production but one can spend a lot of money buying and guessing wrongly on different sized magnets. At this point I am only looking for someone to confirm how I am thinking.

[Xena E]

That's how I understood the graph. Since it is showing the minimum strength needed, I need to calculate the strength provided by certain magnets to know if it is the minimum strength needed to operate the reed reliably.

As long as youre aware that theres a lot of vaguery involved in the application, then what youre doing is perfectly valid, but, it may be a case of calculate a figure and double it.

If you can convince the supplier of further bulk orders they may be able to offer free samples or sale or return on items you need for development.

Also knowing what you have already can give you informed choices of what you need. Have you that information on the magnets you already have?

Regards
X

[cdevidal]

As long as youre aware that theres a lot of vaguery involved in the application, then what youre doing is perfectly valid, but, it may be a case of calculate a figure and double it.

Yes definitely.

If you can convince the supplier of further bulk orders they may be able to offer free samples or sale or return on items you need for development.

Also knowing what you have already can give you informed choices of what you need. Have you that information on the magnets you already have?

Yes, the calculator only needs the shape, dimensions, and material of the magnet and all that is widely available.