Cab light

[AdeRail]

Hi,

 

i should know this with with all the dcc stuff I’ve been doing but it eludes me!

 

 

Ive fitted a Zimo MX644D Sound decoder to my Class 57, 8 pin loco, and while it’s all apart I thought I would fit a cab light... seeing as it’s an 8 Function decoder and most functions wont be used..

 

 

Whats the best way of fitting a cab light? Do I solder directly to the decoder, it’s a 21 pin decoder fitted with an 8 pin harness from a Hornby sapphire to convert it to 8 pin... Also can I just splice the positive into the common blue positive wire on the harness?

 

 

And what is the best led to use? Smd or 3mm.

 

 

Hmm the cab is going to be a bit of a stink to remove too, maybe I shouldn’t bother  😮

[96RAF]

The Sapphire harness supports 8 wires only (2 x motor, 2 x pickups, blue (common positive), green (function output 1), and yellow and white (F0 directional lights). so look for the blue and green for your cab lights switched by whatever F button your Zimo allocates. If you wire directly to the 21 pin board then look at your decoder manual for the pin-outs.

 

Rob

[37lover]

You will also probably need to install a resistor in the cab light circuit; I don't have enough confidence to advise what value but perhaps start with 2k. If shaded you might get away with a 3mm but I use 1.8mm.

[Fishmanoz]

1k resistor is more usual in either leg of the LED but depends how bright you want it.

[AdeRail]

I’ve used a small smd I had in my box o bits with a 4k resistor to get the correct dimness for a cab light..

 

 

I am wondering though, my particular cab has things that cast shadows and the light will only light up half of the cab, so i want to use two smd leds, one each side of the cab.. so do I wire them in series and will they dull down a bit if I do? Also would 2 drawing power from the same function tab on the decoder be ok?

[Fishmanoz]

Wired in series, the resistances add decreasing the current.  With a 4K resistor, the maximum current you could be drawing is 12V divided by 4k = 30mA, well within the maximum 100mA of the function output.  In series, the LEDs will be dimmer, if you put them in parallel but still in series with the resistor, you’ll get about the same brightness as with the one you have now. 

[AdeRail]

if you put them in parallel but still in series with the resistor, you’ll get about the same brightness as with the one you have now. 

How do I do that?  😳

[Chrissaf]

Would 2 drawing power from the same function tab on the decoder be ok?

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With the 4K resistor you are using, the current flowing through the SMD LED will be in the region of 2.5mA. Negligible compared to a 100mA function output. So yes, adding a second LED is not an issue. See EDIT at bottom of page.

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So if I do I wire them in series will they dull down a bit if I do?

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Yes they will dull down a bit if you do not modify the resistor value to compensate, see suggested revised values below:

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You can put two SMD LED in series or in parallel, either way the resistor value will (theoretically) need to be amended to obtain the same light level as before.

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If in parallel, I would suggest halving the resistor value to 2K so that the current is doubled from 2.5mA to 5mA (2.5mA per LED) with both LEDs sharing the same common resistor**. This is not an issue if both LEDs are the same specification. Just for info...there are issues sharing a resistor [Edit: see schematic at bottom of post] if using LEDs with different 'forward bias voltage' values (this would typically be the case if using two different colour LEDs.....but not an issue for you as both LEDs will be white I assume).

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Note** if you leave the resistor at 4K and put the LEDs in parallel, then the two LEDs will have to share the available 2.5mA and only get 1.25mA each. At these low current values, you may or may not see any noticeable difference in brightness. Just based upon the pure mathematics of Ohms Law then the theory is that the resistor should be halved as suggested to maintain the same 2.5mA current flow per LED. [Edit: see schematic at bottom of post for two LEDs in parallel sharing the same resistor].

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If in series I would suggest a resistor value of 2.4K rather than 4K to maintain a 2.5mA current through them.

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My resistor calculations (using Ohms law) are based upon maintaining a LED current of 2.5mA or thereabouts in each LED.

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EDIT: Just for clarification, Fishy's calculation had the decimal point in the wrong place. !2 volts divided by 4K = 0.003 (3mA) not 0.03 (30mA). My calculations take into account the forward bias voltage drop of a white LED (typically 2.7 to 3.1volts), hence why my calculation was 2.5mA and not 3mA.

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To answer your "How do I do that" query asked of Fishy.

He means like this:

/media/tinymce_upload/a3331e58d7db09699a8d2a53eea6c066.jpg

[AdeRail]

Thanks for everyone’s input, especially Chris  😎

 

Ive changed the 4K resistor to a 2.4K one on your advice Chris and I think it’s a nice brightness.. with two smd leds in the cab.

So much so that I’ve done the other side cab light too. In the end I plumped to go with the wiring configuration shown in your diagram as this was easy to tape around the fans in the roof.

 

 

One thing I have learned from this is how delicate smd leds are! I had some on an old strip from Ebay and had to de-solder them from this strip which worked out ok, but I used super glue to stick them onto the roof of the cab and pressed them down with tweezers, then the lights wouldn’t work, I then found out that pressing them with tweezers had most likely crushed them somewhat!

 

 

Then after replacing them and testing with a quick flash on a 9v battery I found I had flashed them without the resistor and so I had two blown leds... tut

 

 

This brings me to asking what kind of both Normal leds and smd leds do you guys use? And where do you get them from?

For reference it would be good to know for future lighting installations on other locos.

Like what shapes are best for the main headlights, and which smd leds are easiest to use and so on...

 

Thanks

 

[Chrissaf]

Million and one sources of LEDs on the net. I have found eBay seller "Bright Components" have a very wide range of LED products at very competitive pricing. They also have a flat rate 99p P&P with rapid 1st Class delivery.

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This is their store front, but can also be found on eBay.

http://bright-components.co.uk/

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With regard to LED types, in terms of what I would call standard LEDs, I tend to go for the 3mm variety (5mm are too big except inside buildings). Also 'lighthouse' style (sometimes called 'top hat' or 'tower') can be useful as they have a built in 1.8mm light tube. This means that you can fit them behind a facia via a 2mm hole and the bulk of the LED is hidden out of sight. Thus only the 1.8mm end is visible. Makes good head and tail lights on a loco.

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A red example of a 'lighthouse / 'top hat' / 'tower' LED - the long leg is the positive (anode) polarity side.

/media/tinymce_upload/784d5d31def1c5af357141604643ef9f.jpg

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With regard to SMD, I tend to favour the PLCC-2 type, they are small, but large enough to have wires soldered to them DIY. If I want smaller SMDs then I tend to go for the pre-wired variety from alternative suppliers such as for example "Express Models". A little bit more expensive, but worth it for being ready terminated on wires.

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A PLCC-2 SMD example - the side with the corner cut out is the negative (cathode) polarity side. These are the type of LEDs that you normally find on the self adhesive lighting strips.

/media/tinymce_upload/684a72dfb065e60de74001de91de3b3e.jpg

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Then for really small LED projects. I use pre-wired micro LEDs from "Express Models".

http://www.expressmodels.co.uk/acatalog/Micro_LEDs_on_Leads.html

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These LEDs are so small you need a microscope to see them clearly. I used them on my 'Police Car' project previously documented on this forum in the 'Off Topic' section. How 'Express Models" solder wires to them is a mystery to me and would take either real skill or specialist equipment.

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The pencil is pointing at a SMD 0603 LED (far smaller than the PLCC-2 SMD). The LED to the right, that you can hardly see the detail off is the "Express Models" micro LED. The red wire to the left has a 0.1mm conductor. The ruler scale is 0.5mm.

/media/tinymce_upload/20030b1dc8126b36fc3a879fdb6042c6.jpg

    This photo was taken with a PC USB Microscope.

[96RAF]

The 0603 size leds are fairly easy to solder, its keeping them in place while so doing is my problem.

Rob

[AdeRail]

Thanks for the really useful info on leds Chris, the website link for leds will prove especially useful  😎