A further experiment in coach lighting.

Discussion in 'Other Electronic Interfaces' started by Keith M, Aug 25, 2018.

  1. Keith M

    Keith M Staff Member Moderator

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    I liked the idea of magnetically switched coach lighting as fitted in my recently bought LNER Dynamometer car so much that I've decided to experiment myself. I could always buy 'Function only' DCC decoders, but with a stock of about 35 coaches, that could work out quite a pricey option, so instead, I'm going for the magnetically operated reed switch method. There are a number of sellers of 3 pin NC/NO (Normally Closed/Normally Open) small glass reed switches quite cheaply (£3-£4 for 10) on that certain auction site, so with that in mind, I've ordered 15 to start with and we'll see how it goes. I'll hopefully be able to use the 'Magic wand' that came with the Dynamometer coach to do the switching, although I could easily make more up if needed as I have a stock of 3x3x3mm Neodymium magnets, and the reed switches themselves are easily capable of switching the low currents of the coach lighting, and it'll be useful to be able to switch the lighting off on coaches 'parked' in sidings etc and it will also reduce the total current load drawn from the DCC rail supply when coaches are not in use. I anticipate using small sections of Veroboard (stripboard) to mount the reed switches onto, gluing them into each coach roof so that I can just wave the "Magic Wand" over each coach roof to operate as needed. I'll likely do a "How-To" as and when I receive the switches, so 'Watch this space'.
    Keith.
     
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  2. Toto

    Toto Staff Member Founder Administrator

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    Great Keith. should be interesting.

    toto
     
  3. jakesdad13

    jakesdad13 Staff Member Moderator

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    I will look forward to this thread, I've got a few reed switches from an abandoned project and half a dozen coaches to build so it would be nice to install lighting as they are assembled.

    Pete.
     
  4. Keith M

    Keith M Staff Member Moderator

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    Standard 2 pin reed switches won't do the job Pete, as they change when the magnet is near, then revert back to the previous state after the magnet is removed. You'll need the 'changeover' type so that for example, in the 'Normally Open' state the circuit is not connected, wave the magnet over the switch and it will change to 'normally Closed', close the circuit and stay in that state until the magnet is waved over it again. This type of reed switch will usually have 3 pins, common, normally open and normally closed so it works in a similar way to an electro-mechanical latching relay. You could use the Normally Open type in conjunction with a latching relay I guess, but that's just adding to cost, space (in 00 gauge) and complication, so probably not worth it, easier to stick with the 3 pin reed switch I reckon.
    Keith.
     
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  5. jakesdad13

    jakesdad13 Staff Member Moderator

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    Cheer's Keith, to be fair it had crossed my mind but being a tech luddite I will go with the three pin type once I've clocked your method!

    Pete.
     
  6. TimberSurf

    TimberSurf Full Member

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    I will follow with interest, but I have a mind to use LDR's myself.
     
  7. Keith M

    Keith M Staff Member Moderator

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    It's been a while since I posted on further progress regarding using reed switches to switch coach lighting, basically because I've been unable to source suitable switches at a sensible price. I'm aware that layouts4u list suitable switches, but at £3 a pop, and with about 40 coaches to do, that's not gonna happen, so I've spent some time on the internet researching and found that the way these switches work is with a weak magnet attached to the side of a standard switch (layouts4u switch appears to have a 'wrap-around' weak magnet), the idea being that the magnet just holds the reed in place in the 'Off' position, but moving a stronger magnet alongside causes the reed to change over to the 'On' position, and the weak magnet is not strong enough to pull the reed back to it's original position. To change the reed back to it's original position, all that's needed is to apply the opposite pole of the strong magnet to the reed, thus changing it over to be held by the weak magnet again until the stronger magnet is again applied. I'm thinking that using maybe a small section of flexible magnet, such as is often used in refrigerator door seals, might do the job (these door seals are usually encased in the plastic sleeve that seals the door to the casing when it's closed) so I need to look at this further. I also had a look at maybe using electronics to switch the lighting, using the reed switch purely to switch the electronics, but cost and physical size make this unappealing, so it's back to experimenting with the reed switches again. More 'dabbling' required, unless other members have had success with other methods, if so, do tell!:giggle:

    Keith.
     
  8. TimberSurf

    TimberSurf Full Member

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    Keith
    I did a little research a while back and found these, but never got any further. We need to find an accessible supplier.
    The best I found was £2.04 each if 100 bought (but they are also very big)

    From my research I found out that they are a reed switches with a weak magnet attached to them. It is not strong enough to close the contacts until the correct polarity external magnet is placed close by. Then the contacts close and the built-in magnet is just strong enough to keep the contact closed. To open the contact, the external magnet has to be applied with the poles reversed. That weakens the magnetic flux of the weak magnet allowing the contacts to open.

    https://www.reed-sensor.com/reed-sensors/biased-smd/standard-latching-reed-sensors/
    2000 at 85p ea from https://www.digikey.co.uk/products/en/switches/magnetic-reed-switches/193?k=latching reed switc

    I can find expensive ones, or cheap by the thousand! Hence I gave up!
     
    Last edited: Nov 27, 2018
  9. Keith M

    Keith M Staff Member Moderator

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    As luck would have it, layouts4u had a stall at the Peterborough show last weekend, so I bit the bullet and bought 3 of the latching reed switches from them. I've had a chance to try one and it seems to work ok, so am building it into one of my Birdcage coaches, currently waiting for glue to dry, but the pic below shows the difference between a standard reed switch (ignoring the fact that this is a changeover 3 pin type) and the 2 pin 'latching' type. As can be seen, it has a tiny red weak bar magnet heat shrink sleeved to the side of the switch, which holds the reed until a stronger magnet is moved near to it. All I need now is a source of tiny weak magnets!
    Keith.

    IMG_1351.JPG
     
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  10. paul_l

    paul_l Staff Member Administrator

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    How about a logic OR gate latch - a quad 2 input orgate can be had for less than a pound, I've seen on ebay 35 for a tenner.

    Circuit

    upload_2018-12-11_23-53-21.png

    The reed feeds gate 1, once the output goes live the the latch back to the second output keeps the led on.

    Paul
     

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