3 Dec 2009

Electrical Pick-Ups, Occupancy Detection and LEDs

As part of my grand plan, I need to get good occupancy detection of trains on the layout. Achieving this is easy for locos - the Digitrax BDL168s referred to in earlier posts see the loco current draw and know it is in the relevant section - however the occupancy detection really needs to see the rear of the train as well as the loco at the front. Therefore I decided as a test to try and convert a brake van to trigger the occupancy detection.

While I am at it, I will also try and add some LEDs to the brake van to provide some lighting effects, in particular I want to try some of these fantastic looking oil lamps from DCCConcepts.

I started with a Bachmann 33-306A toad brake van.

Now the two aims - lighting and occupancy detection - both require electrical pick up from the track - this is not that easy at least for me it isn't. By default Bachmann insulate both wheels where they join with the axles - obviously if there was no insulation then the wheel / axle combinations would short the two rails out.

I intended to use the DCCConcepts pickup srings which fit over the axles but these need one of the wheels to be electrically connected to the axle. My initial attempt was using silver paint

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but i just couldn't make this work - the paint is a suspension of silver and even after repeated attempts i couldn't get any electrical connection from the wheel to the axle. So on to plan b - more expensive - but more reliable perhaps. Replace the wheel sets for ones that have an metal bush on one wheel thereby making the axle 'live'.

One company that advertise live axles are Markits

I ordered two Markits wheel sets the 12mm for the brake van, and the 14 for a Bachmann Mark 1 coach - that I wanted the same treatment for.

2 X MRJ12TSNSL1 (12mm 10 spoke n/s live 26mm Axle)
2 X MRJ14BR1-1L (14mm 3' 7" contoured n/s coach disc live 26mm Axle)

These were not cheap, by the time postage was included these four axles cost 15 pounds. Worse the van really needed 13mm wheels and putting the smaller ones on it, meant that now the van was too low for the kadee height test...

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Still I carried on anyway, thinking that if the project worked I could always go for some larger wheels later - the model came with 13mm wheels but Markits only do 12 or 14mm - and fitted the spring takeoffs to the axles ... you need some calipers to reset the wheel axle widths after you have done this.

The I soldered some insulated wire to the springs and covered the join with some heatshrink. So the underside looked like this -

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To get the wires through the chassis I needed to drill a hole in the floor of the brake van, and to do this I had to prise the body off - the body is glued to the (black) chassis, and the body has a (grey) floor that is also glued in place and can also be (carefully) prised out. We also need to get into the body later to place various bits of circuitry and the interior LED. Here is the body separated from the floor prior to drilling...

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Next came a very simple occupancy test - if you place a 10K resistor across two rails then that is sufficient to fire the BDL168 to show occupancy, but far too small to short the track or cause any significant current to flow. So next I tookthe two wires exiting from the top of the chassis and connect to a 10K resistor with some croc clips and voila my Heath-Robinson looking device is detected by the BDL168s.

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Now having got power to above the chassis in the brake van - i need to work out what circuitry i need to both power the two very different LEDs and to make the occupancy work.

Now DCC is an odd sort of supply as it is squarewave AC - and so i asked Pearson(elder) for advice - you can see his response here (broken link).

So from that I believe that I need to adopt the following circuit. The choice of 10K for the locolamp is not from any theory but because the resistors supplied in the pack from DCC concepts seemed to range from 5kohm to 20kohm - so obviously they expect a very low forward voltage.

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I lashed up a crude test of this with some components and sure enough it triggered the occupancy detector and lit LED 1 but not LED 2 ( the locolamp).

A bit more messing around led me to the following answer...

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If like me you are a bit unused to identifying diodes then really anything that isn't a zener is likely to work - ideally look for a minimum breakdown voltage of > 50V other than that go for a nice small size that will fit in your model!

I built this circuit on a little bit of veroboard cut from a larger piece bought at maplin and using a yellow LED from Rapid that I had in stock - yellow to simulate an oil lamp lighting the inside of the vehicle.

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Here is the circuitboard mounted on the van base - with the led being lit...

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And even more satisfying here it is both stopping an oncoming loco and turning the associated signal to red - via the CML SIGM20 I mentioned earlier in the blog.

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Disappointingly though the connection via the rails up to the circuit board is too poor to work well for occupancy detection. The led flickers all of the time as the van circulates on the track - i could fix this using a capacitor circuit like flickerfree (also from DCCConcepts) but this won't help with the occupancy issues. If the computer and/or signals see the occupancy going on and off they will get confused. A bit of messing about shows the problem is not the spring pickups on the axles but the fact that the two live wheels aren't always in contact with the rail.

Accordingly I am going to have to try and get eletrical pick up from the other two wheels on the van and probably also weight the van down more. This will mean fiddling about with nickel silver springy wire - not something i am looking forward to.

So in the meantime I have been messing with the locolamp. These are really incredibly small.

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They come with enamel coating over the wires for insulation. This is not easy to remove - I finally did it with a combination of a match to burn the enamel off and a piece of emery cloth.

Here is the hole drilled on the van for the red oil liamp light...

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More when I try and connect a pickup to the other two wheels.

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