Magnetic reed switches

[RickyRails]

I'm looking for an article that was written many years ago about the use of magnetic reed switches and how to use attatched magnets to make them act like a SPST switch. Does anyone know where I might find tis information?

Thanks for the help.... Ricky B

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[fwright]

RickyRails

I'm looking for an article that was written many years ago about the use of magnetic reed switches and how to use attatched magnets to make them act like a SPST switch. Does anyone know where I might find tis information?

Thanks for the help.... Ricky B

I'm going strictly on memory - I 'll have to look it up at home - I think Linn Westcott's original article on magnetic reed switches appeared in Jan 1963 Model Railroader.  He described using "bias magnets" to turn momentary reed switches into latching SPST switches.

I believe there are many latching magnetic reed switches on the market today (there weren't in 1963) so that in most cases the bias magnets are not required.  But so far I haven't used the switches, so I can't say for sure.

Fred W

[MisterBeasley]

Not sure about the article, but the principle is pretty simple.  I have some of the Miniatronics ones.  They make them for HO and N guage.

The reed switch itself is a little thing about a half-inch long encased in glass.  It's got two leads, and looks like many other electronc components.  For trains, I mounted mine between the rails, lengthwise.  (They wouldn't fit crosswise anyway.)  I'm modeling in HO.  Before I put them in, of course, I attached longer wires to the leads to thread through the subroadbed.  I looked carefully at the switch, and figured out which way the reed inside moves when a magnet gets close.  I oriented the switch so that the read moved up and down, not side to side.

The magnet attaches to the underside of the engine or car you want to use to trigger the switch.  In my case, I've got subway trains, so I just glued the magnet to the bottom of the car.  Simple.  When the car passes over the reed switch, it closes.  When the car moves on, it opens again.  One thing to note is that this is a short-range device.  An inch or two away, and the switch will stay open.

I suspect that the distance is the critical thing about N-scale vs. HO-scale.  The car floors are closer to the ties in N-scale, so you can get away with smaller magnets.  I have one of these installed on a curve.  I was so concerned about the distance issue that I located it closer to the inside rail of the curve, so that the magnet, mounted in the center of the car, would pass directly over it.  I put these on top of the ties, by the way.  Nobody is going to see them in my subway tunnel, anyway.

I use mine to light LED occupancy detectors on stub-end staging tracks.  That's a pretty low current draw.  I'm sure they can handle more, but they are small and I wouldn't put too much of a load on them.

[Blind Bruce]

Those new rare earth magnets will work at a greater distance than the old ones. Also, a magnet on the underside of a car will pick up any stray spikes or the like as it traverses the layout.

I like the idea of using them reed switches for car occupancy too. What other uses can we think of for a momentary reed switch?

[Mark R.]

I built my entire signalling system using reed switches ! This was back in my DC days when track occupancy was more difficult to attain. However, since the entire system is independant of track power, when I switched to DCC, nothing needed to be altered and the signalling system continued to work as it did before.

I found some miniature DPDT latching relays at Digikey which were the key element. The momentary reed switches throw the latching relay either way and the relay is held in that position until a pulse from an opposing reed throws it back.

Each engine has a 3/16" rare earth magnet mounted to the truck. I originally starting with mounting them under the fuel tank or pilot, but found those locations to not be 100% reliable in a curve should the tank or pilot not be over track center where the reed was. Mounting the magnet on the truck itself garanteed it would be centered at all times.

Sure, it's rather archaic by todays standards, but it worked flawlessly for the time I built it and continues to do so to this day.

Mark.