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I am a model railroad hobbyist and am using Miniatronics brand reed switches (item #32-058-05) to sense the passage of a train car (caboose) with a magnet attached to the bottom of its frame. The problem I am encountering is that these Miniatronics reed switches are also sensing the passage of the permanent magnets in the motors of the locomotives, which I don’t want to have happen. In other words, these Miniatronics reed switches are too sensitive.
I understand that the sensitivity of a reed switch is described by the number of ampere-turns (ATs) that it has. I don’t know the AT rating of my Miniatronics reed switches. I am wondering if DigiKey carries a low-sensitivity reed switch (high ATs) that I could try out in lieu of my Miniatronics reed switches.
These reed switches need to be small so that they can sit flush with HO scale track. Also, they only need to handle minimal current (about 0.02 to 0.05 amp).
I’m afraid that without some more specs, it will just be guesswork to try to find something that might work for you. You are correct that having a higher ampere-turns sensitivity spec means lower sensitivity. Regarding physical size, I have no idea what the clearance requirements are for an HO-scale track, but we list dimensions for our reed switches, so you can filter out the ones that are too thick or long.
Here’s a link to our currently available reed switches listed roughly in order from least to most sensitive. You’ll note that some have a much narrower range of min and max sensitivity than others. For those with a wide range, two of the same part number may behave somewhat differently than each other in a given set of magnetic field conditions, whereas those with a smaller range will tend to behave more consistently with each other.
I would suggest that you first filter out all of the reed switches that are too large by selecting only those in the filter which are at or below your max dimensions.
Then, you can either guess as to what AT specs you need and pick a part based on that, or pick a few different ones with different sensitivities and see which works best.
Also, you didn’t mention your switching voltage, but make sure it is below the maximum rating of the switch, as highlighted above.
If, after testing a few, you can’t find one that consistently picks up the magnet but not the motors, you might need to try using stronger magnets to increase the differential field strength between the motors and the magnets, and then use a lower sensitivity reed switch. This would make it more likely to still pick up the magnet’s field but not the motor’s.
If possible, you could also try rotating the reed switch (axially, laterally), as the sensitivity may depend on the direction of the magnetic flux (for unknown reason makers do not publish that data).
Thank you for the detailed response. I appreciate it. The switching voltage is only 5V, from an Arduino, so it’s a pretty light application for a reed switch. I will use my digital caliper to take measurements of my Miniatronics reed switches (length and diameter) and use that as a go-by to select something that has a higher ampere-turn factor from the DigiKey menu. I’ve already got some mu metal on order from a model aircraft supplier and I’m going to try that as well: adhering the mu metal to the bottom of the locomotive fuel tank to intercept the magnetic field from the motor. But the mu metal might not work, or it might look unsightly on the model locomotive (even though it will be in an almost invisible location).
Thanks, heke, I will give that a try. I have some extra Miniatronics reed switches here. I’ll place one perpendicular to and under the rails and hook up my continuity tester to it, then run a locomotive over the reed switch and see if the reed switches senses the magnetic field of the motor. If it does, the switch will close and the continuity tester will sound.
