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latching reed switches

Original page created on 01/04/2019; updated on 26/10/2022.

What’s the problem?

To control the lighting of coaches, several solutions are possible. First, no control at all! The coach stays lit in daylight. Why not, if the light intensity is moderate. Then, control by DIP switch placed under the chassis. This is not always very discreet, and this requires passing wires between the coach’s floor and ceiling. In addition, it is obviously necessary to remove the coach from the track to actuate the switch.

After, you have the solution using a reed switch, operated by a magnet. Placed under the roof, it is easily actuated. The problem is that for most reeds, the closing is momentary: when you move the magnet away, the switch reopens. So some have imagined more or less complex electronic circuits, based on flip-flops or bistable relays, or even microcontroller, to memorize the state of the reed. Forgetting that at the slightest power cut, this memorization is lost! Not to mention the whole bulk.

And then there is THE solution: the latching reed switch. It works as follows: a tiny magnet, unable to close the contact by itself, is stuck on the reed switch. When approaching the control magnet, with its poles oriented in the same direction as the tiny one, the magnetic fluxes add up, and the contact closes. When the control magnet is moved away, the tiny magnet is able to hold the contact closed by itself.

Et voilà! We memorized the order! Yes, but how to reopen the contact? Well, nothing easier: only approach again the control magnet, but with its poles reversed. This time, the magnetic fluxes will be subtracted, and the tiny magnet will release the contact that will open again.

So, I’ve been using such latching reed switches for a long time, see for example Turning the red front lights off in the Picasso Mistral, the lamp control in the Dd4s Roco luggage coach, that of the lamps of a Jouef cereal hopper, or that of many coaches lighting.

This is definitely good, you say, but why want to make it yourself? Well, it started with the fact that the model I was using, the PMC-1424THX I bought at Conrad, was definitely sold out. So I looked for a size equivalent, to avoid redesigning the circuit board on which it was placed. For example, I found the Meder KSK-1E66, but it is available nowhere, and it is expensive, almost €10 each!

PMC-1424THX model

KSK-1E66 model

So I wondered whether I could make it myself, without believing it was possible because I thought that the setting of the magnet had to be particularly fine and precise. But I tried, and the first result was encouraging. So I’ll explain how I make latching reed switches very easily, for a price of just one euro each, which is not its least interest.

Necessary equipment

The main problem is to find magnets small enough, still for a question of size, knowing that the diameter of the reed used is about 2 mm, and sufficiently weak to not be able to operate the reed by themselves. I have found some, I have to say, by chance. These are small parallelepiped magnets measuring 1.6 × 1.6 × 12 mm, probably ferrite.

The other indispensable element is obviously the reed switch. I chose a small model (length 14 mm for a diameter of 2.2), quite sensitive and cheap. This is the Meder KSK-1A66, the sibling of the unobtainable KSK-1E66.

Here are the components involved.

Heat-shrink tubing is still required to hold the magnet on the reed, with a diameter of 3.5 mm before shrinking. Larger, the magnet would not hold in place for adjustment; smaller, it would be very difficult to slip the magnet on the reed’s body.

Making the switch

Adjusting the magnet’s position

Next, approach the control magnet. If we direct its poles in the same direction as the tiny magnet (same colour on the same side), the fluxes add up and the contact must close again: the LED lights up. When moving the magnet away, the LED should stay on. Otherwise, it is necessary to go up the small magnet slightly towards the middle.

Obviously, we must also do the opposite test, that is to say, approach the control magnet with its polarity inverted, to turn off the LED. The setting is correct when switching on and off occurs at about the same distance from the switch. Do not try too much refinement, because it may be necessary to alter the setting once the switch implanted in its circuit.

If the magnet piece is really very small (less than 3 mm), the LED may not light when the magnet is in the middle of the reed. But this may not be lost. Approach your control magnet: the LED will light up. Move away: it may stay on. If so, we won! If not, discard the small magnet piece.

Example of placement on a REE UIC coach

Does it work well?

I have already mounted these “DIY” latching reed switches into about fifteen coaches. Actuation is certainly less accurate than with commercial devices. Some are too sensitive: it happens that two coaches are controlled at the same time if their switch, placed at the ends, are face to face. But this also happens with trade switches. Overall, the operation is very satisfactory.

However, I noticed incidentally a curious phenomenon, which occurs as well with trade latching reed switches: some locomotives, Roco in particular, operate the switches of a train parked on a parallel track when they pass along. It must be believed that their motor radiates a large magnetic field. It’s not very embarrassing, but it’s pretty surprising at first…

After investigation with twenty locos of different brands (Roco, Jouef, Electrotren, Mabar, REE, LS Models, Van Biervliet), I strongly suspect open motors, that is to say, those whose rotor can be seen, to be responsible for this malfunction. And, indeed, most Roco models are so equipped, while other brands mostly use closed motors (can motor) instead.

Useful addresses

• For reed switches (and many more devices), TME, in Poland, unbeatable prices except perhaps by the Chinese, but with much faster delivery times
• For magnets, CDF, a seller well-known by railway model electronics enthusiasts.
• For the heat-shrinkable tube, TME again… With 15 meters, you can mount thousands of switches! The blue sheath is cheaper than the black, I don’t know why…

Let’s talk about prices…

With the quoted suppliers, on the basis of 50 switches manufactured, with a 12 mm magnet for 3 switches, here is the bill:

Designation	Unit price (€)	Quantity	Price (€)
KSK1-A66-1015 reed	0.42	50	21.00
CB-HFT2X-32-BOX tube	2.80	1	2.80
CDF 8041946 magnet	6.50 / 5	20	26.00
		Total	49.80

As promised, a unit costs less than €1.00 (excluding shipping). And I counted all the tube, while it takes only a small piece.

Could we still do better? We see that the price of magnets raises the cost price. We can find, on the Internet, more and more shops that offer an infinity of magnets of all kinds, some at prices much lower than at CDF. So I looked for magnets that could be suitable.

To make a choice, we must determine some characteristics, including the bulk (about 2 × 5 mm max.) and the pull force. What is the pull force of magnets used above? To get an idea, I tried to lift a 25 g ballast steel plate with an entire 12 mm magnet: we’re just getting there. I deduce that the proper magnet should have a force of about 8 g — one third.

In the neodymium magnet category, we can find some very tiny, €0.03 each, against €0.43 at CDF. But their force is 25 g! It’s way too much! So, for the moment, I haven’t found anything better than CDF’s magnets.

A few months later, continuing reflections

If we look at the characteristics of the Meder KSK1A66 reed switch as they appear in the TME online catalog, we see one named range, with AT as unit.

Explanation: this is a range of possible values of the magnetomotive force (MMF), which is expressed (or rather was expressed, see Wikipedia) in ampere-turns (At).

Expressed in a simpler way, the question is: if we want to operate the reed with a coil (a solenoid), we need to drive a current of how many amps into how many turns? For example, for the KSK1A66-1015, we can wind 10 turns and drive 1.5 A (that’s a lot!) or 100 turns with 150 mA (this is better), etc. The important thing is that the product gives 15 each time: I amps × n turns. It can therefore be considered that the value of the magnetomotive force gives an idea of ​​the actuating force required for the control magnet.

What am I getting at? Well, with the model chosen above, the operation was disrupted by some locomotives passing by the reed. By choosing a model requiring a greater magnetomotive force, this disadvantage may be avoided. However, it will probably require a somewhat stronger holding magnet — perhaps a half bar instead of a third — and to get the control magnet a little closer to operate the reed.

Given the price of these reed switches, it is worth trying. In a future order at TME, I will not fail to buy the model 2030 to test it.

New, less sensitive, reed switches

Well here we are, the new KSK1A66-2030 reed switches have arrived; it’s time to do some tests.

First observation: the holding magnet can be the same as before, i.e. one third of a 12 mm magnet. Simply, you have to position it more to the middle, which is not surprising, since the switch needs a larger MMF.

So I installed on a breadboard two latching reed switches, a very sensitive “old” and a “new” one, less sensitive, each with an LED and a series resistor. I start by doing some tests with different magnets, to find that indeed, we must get the magnet closer to operate the new switch, about 25 to 30 %.

Then I start the tests with a loco that disrupted the operation of the older switches. This is the ROCO ref. 62995, NMBS 6003. I will pass it along my breadboard, at shorter and shorter distances, to see whether it actuates the switches. I recall that the goal is to prevent the loco from changing the lighting or the tail lights of a train that would be on an adjacent track, which is simulated by my breadboard.

The actuation distances are in millimetres, measured from the axis of the switch to the loco’s. The values are average. The switch-off action means when the switches were previously closed (LED on). This can only happen when the loco presents its motor’s north pole to the switch. For the switch-on action, we have to turn the loco around so that it presents the south pole. The following photos show only switch-off actions.

Note: the most annoying action is the switch-off, because more visible at night than the switch-on at daylight.

Test with horizontal switches, perpendicular to the track

This is the most common case, with the control circuit placed under the roof of the coach. The N label on the loco indicates its motor’s north pole.

At this first distance, neither of the two switches is actuated.

At this second distance, the most sensitive (left) is activated, its LED is off.

At this third distance, both are actuated, both LEDs are off.

Results:

Action	Older switch	Newer switch
Light on	60	45
Light off	90	85

Test with vertical switches

This is the case when the control circuit is placed in the toilet of the coach.

Results:

Action	Older switch	Newer switch
Light on	50	30
Light off	70	50

We see that the actuation occurs more closely: this arrangement is better than the previous one.

Actuation test with switches parallel to the track

The configuration with the switches parallel to the track is not my most usual, but I used it for example on my M2 NMBS coaches.

We can see that, while passing along, the loco turned off the most sensitive switch (the older one), here on the right.

Results:

Action	Older switch	Newer switch
Light on	35	20
Light off	50	40

It’s even better here.

Conclusion

As I said, the most annoying action is light-off, and this is unfortunately the one that occurs from further afield.

That being so, we can conclude that:

• the new switch is actually less sensitive and will therefore be less affected by untimely actuations;
• the first case studied is the most unfavourable, and yet it is the one I use the most. It will therefore be necessary if possible to put the reed switch vertically (in the toilet) or, even better, parallel to the axis of the coach.

What would ultimately be the optimal arrangement of a reed switch for lighting control? Well it would be, we just saw, parallel to the axis of the coach, and in the centre of it. Indeed, on the one hand, this point is easier to locate to position the control magnet, and, on the other hand, it is further away from contiguous coaches’ switches. There is less chance of interference between them.

Before these tests, I almost always placed the reed switches at an end of the coach, simply because the anti-flashing capacitor is placed more easily in the toilet, without the need of long connection wires. I’ll have to review this arrangement in the future.

Since then, for the new strips I got done, I have systematically used the axial positionning. Last realisation: the control of the lighting (central reed switch) and the lanterns (left reed switch) of the REE B⁴Dd Sud-Est coach.

Click to enlarge.

Using neodymium magnets

Problem of magnet availability

As time went by (it is now 2022), it became increasingly difficult to find suitable magnets (Littelfuse or Hamlin H31). I had ordered some from Mouser, but they were not available. Month after month, the delivery date was postponed. I decided to try neodymium magnets, which I had eliminated because I thought they were too strong.

Test with neodymium magnets

So, I ordered “very powerful” (only “technical” data specified!) neodymium magnets, on ebay France. A bit expensive compared to other sources, but quickly available. Beware: they may not be available at the time you read this. Otherwise, there are many suppliers, the most interesting of which is at Aliexpress. But beware: there is no guarantee that these magnets will have the same strength as the ones tested here.

Handling such small magnets is not easy. I recommend the use of non-magnetic tweezers. I take advantage of the fact that the reed switch wires are magnetic: the magnet sticks to them.

The process is similar to the previous one: I start by colouring the ends of the magnet in relation to a known magnet, the south pole in red and the north in blue.

I put a magnet on the reed switch: it doesn’t work. If I approach the control magnet, the reed switch closes; but it reopens as soon as I move away. With two magnets, the same happens. But with three magnets, it works! Being under way, I put four, but then, sometimes, no more way to open the reed switch.

So, and I insist, for the magnets used, you need three for a correct operation.

Manufacturing process

Here are the elements: reed switch, tubing, magnets. You can only see clearly these tiny magnets by zooming in! I can see on the enlargement that they have already managed to capture tiny bits of filings! You can see the red coloured end compared to a known magnet (here, a Jouef tournebroche magnet…).

Click on the picture to zoom in.

The magnets are placed on one of the reed switch wires, then a piece of ⌀2 heat-shrink tubing is slipped over it.

The triple magnet is advanced onto the reed switch body and the tubing covering is initiated.

While holding the magnet, the tubing is pushed over it.

Gradually, the tubing covers the magnet and the whole assembly is slid towards the middle of the reed switch. Note that there is not even a need to shrink the tubing.

After adjustment as in the method described at the beginning of this article, the ends of the reed switch are marked with a felt-tip pen like the magnet, which is still more visible.

That’s it. It’s easier than before: no magnet to cut. It’s even less cumbersome, and even less expensive!