Adding LED's to decoder Light functions

[Padders]

Having successfully installed Digikeijs decoder lighting on the centre coach of a class 101 (Thanks Chris) I now want to utilise the decoders fitted to each end coach (engine and slave) to light these carriages. Is it possible to use the existing 6 pin decoder light functions to add more LED's for the coach sections?

 

The decoders fit into a board that runs the length of the coach and provides directional lighting. I was thinking of tapping off the board but not sure about current limits or how to wire so additional leds are active in both directtions

 

The decoders are Bachmann 36-558A 6 pin plug fitted to a Bachmann Farish Class 101 DMU

[Chrissaf]

It can be done this way. This is easier to implement than trying to 'back engineer' the existing PCB board to find appropriate connection points.

 

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You need to make the connections ideally where the decoder connector solders to the PCB, so that your custom added componentry is connected directly [electrically] to the four decoder connector pins.

 

The 1N4148 diodes are industry standard diodes. The 1N4148 is physically small and typically rated at 300mA, more than enough for this project. However, ensure that the TOTAL current load on the White and Yellow wires [custom LED additons PLUS the existing directional lights] does not exceed the function current rating of the decoder, typically about 100mA. It might be prudent to increase the value of the LED 1,000 ohm resistors to 1,800 ohms to reduce the current drawn by them by approximately half.

 

The diodes on the Yellow and White allow a 'negative' supply to be available for the custom coach lights regardless of which direction of travel is enabled. The diodes on the Red and Black produce the 'positive' supply.

 

Based upon the indicated component values, the current drawn for each LED added will be in the region of 10mA. If the coach lighting LEDs are too bright, then increase the resister values in 1,000 ohm steps until the desired brightness is achieved.

 

The coach lighting will be 'ON' whenever F0 'Directional Lighting' is switched on. Thus there is no separate 'ON/OFF' control for the coach lighting.

[Padders]

Thanks Chris for demonstarting your extensive knowledge once again. 1N4148's and 1.8k resistors on order.

Just to clarify, the new LED's would take it's  positive supply via the 1N4148's direct from pick-ups and negative return via the 1N4148's from FWD/RVS lighting from the decoder?

Regards

Martin

[Chrissaf]

The 'pickups' are electrically the same physical location as Pins 3 & 4. So yes, you could wire it as you stated. Just makes sure that the resistor is included in series in the circuit somewhere as indicated in my schematic.

 

Just to note: My sketch showing an 'Actual 1N4148' diode might be a little misleading. My sketch shows a 'Black Body' with a 'White Bar'. In reality, most 1N4148 diodes have a glassy red body with a black bar, but the functional orientation of the bar [white or black] is the same.

 

[Padders]

Chris

Would you recommend 1 resistor or 1 per LED as per diagram. I will be using 4 LED's each end left over from the Digikeijs install.

[Chrissaf]

The resistor values were arrived at on the basis of my schematic as drawn.

 

One resistor per LED is 'best engineering practice'. I have written reams of text in other threads about LED / Resistor design considerations, so not worth 'reinventing the wheel' and writing it all up again.

 

The left over part from the DR800 may not be suitable. I can't say without seeing a schematic of how the remaining LEDs on the board are connected, they may even have SMD resistors already included on the board, they might be connected in series and not parallel. These are all 'unknowns, that will affect how they can / could / might be incorporated into my basic design above.

 

I strongly suggest that unless you can trace out the tracks and component connections of the left over board bits and publish a sketch of the results for me to evaluate, that you follow my sketch with new components. SMD LEDs can easily be soldered across three tracks of Vero copper strip board. See example layout below.

 

/media/tinymce_upload/cc37f1226ab0f8a3bbcaa51141bb209c.jpg

Note that the resistor can also be sourced in an SMD [surface Mounted Device] version. If all SMD components are used, then they will all mount (solder) on the copper side of the strip. If radial resistors are used, then they will mount through the hole on the non copper side. If the resistor is too long to be mounted as drawn, then just mount them at a suitable angle to bridge the appropriate copper tracks, without having to stand upright on end.

 

The best way to solder SMD devices to copper, when no specialist SMD soldering equipment is available and only a normal soldering iron is being used, is to tin the two placement areas of the copper strip with a small blob of solder. Position the SMD device with long nose pliers or tweezers, holding down the SMD against the soldered area. Then use the iron to dab one end of the SMD as you apply slight pressure on the SMD component to let it bed into the molten solder [a very fine iron tip is needed and pressed against the solder blob, rather than the SMD device itself]. Wait for it to cool down, then perform the same dabbing action with the iron on the other end of the SMD. I've done loads of SMD LEDs this way without any issues. Working quickly with a hot iron is 'of the essence' here. I also 'test' each soldered LED as I go.

 

I have written a 'How to Solder' tutorial which can be read here:

https://www.hornby.com/uk-en/forum/faq-how-to-solder-for-model-railways/

 

[96RAF]

0603 size SMDs will bridge the gap between standard veroboard tracks.

[Chrissaf]

One additional bit of information regarding SMD LEDs.

 

Like any LED, the SMD LED has a positive side and negative side. Connect them the wrong way round and they won't light up. This online guide gives photographic examples of various SMD LED types and how to identify the positive (anode) and negative (cathode) sides.

 

So using my previous schematics as a reference. The SMD LED Anode will connect towards the resistor (positive voltage side in my circuit schematics).

[Chrissaf]

Padders,

 

It looks like you tried to 'copy and paste' the image into your removed post or some other unsupported image posting method. 'Copy and paste' does not work on this forum for new images. Not only does it not work, but the corrupted residual page that got published, corrupted the rest of this thread page and prevented new posts being added. Which is why it had to be removed before I could post this reply.

 

Read TIP 8 in my TIPs page that explains how to correctly post an image. Including what file types are supported etc.

 

https://www.hornby.com/uk-en/forum/tips-on-using-the-forum/

 

If you did use the method described in TIP 8 then there must have been something else about your image or posting method that the forum software didn't like. Possibly an unsupported file type for example. Or a URL link to the image, instead of the image itself, and using the URL link text incorrectly in the upload tool.

 

[Padders]

Hi Chris

 

Thanks for the info. I did try cut and paste which caused the problem. I now have working lights wired exactly as your circuit diagram but there is one slight issue. There appears to be a residual current which, when the head/tail lights are off, the cabin lights remain dimly lit. It's not a major issue but wondered where it's getting this residual current?

[Chrissaf]

There appears to be a residual current which, ............. It's not a major issue but wondered where it's getting this residual current?

 

When the 'white and yellow' wires are switched off i.e F0 OFF. The 'white and yellow' wires do not go 'open circuit'. Because you are creating what is effectively a 'fudge' circuit to use the decoder in a 'non standard' manner, you really need the white wires to go either completely 'open circuit' or completely full positive to ensure that the diodes that have been 'custom' added are not even slightly 'forward' biased.

 

When disabled [F0 OFF], the 'white and yellow' wires go to a condition called 'floating' where they are neither full positive nor full negative. This 'floating' residual voltage is probably allowing the diodes to be 'partially' forward biased allowing a very slight current to flow, and since LEDs, unlike bulbs, can still glow on very low currents, you have got your observed results.

 

I suggested the 1N4148 diodes because of their very small physical size to better suit a N gauge carriage. 1N4148 are technically very fast signal switching diodes and not really meant for power rectification applications. This means that their forward and reverse bias voltages are very much more sensitive compared to a Silicon Rectification diode such as a 1N4001 - 1N4007. In an ideal world I would have suggested the 1N400x, but this diode is about four times physically bigger than the 1N4148 and four of them would have taken up far more space in your N gauge carriage.

 

In my reply above, I am assuming you are using the LED circuit and resistor values exactly as per my earlier reply. If you are using the recycled lighting PCB for which your image failed to publish. Then it maybe that board circuit is contributing to the observed effect. As I said before, I would need to see the traced out board circuit and component types and values to fully analyse the circuit. If there are SMD resistors in series with the LEDs on your recycled board, they may have very low ohmic values and not the 1,800 ohms that are recommended.

[Padders]

There are no additional components on the recycled board, just 2 leds which I have added resistors to the anodes, linked cathodes and connected to the 1n4148's, so I guess it's the sensitivity of the diodes. As I say not a major issue, but thanks anyway.

[Chrissaf]

😎

[ColinB]

You need to add a 22 uF capacitor across the LED and resistor chains, otherwise it will flicker like hell. You only need one for all four chains. It is a lot easier if you use caravan leds that come in strips. From China they work out cheaper than buying the individual components.