Linking signals to points

[Britannia Builder]

I have my points controlled by DCC Concepts ADS8FX decoders (the Hornby decoders were out of stock at the time) and I'd like to add some Train Tech SK2 and SK3  colour light home and distant signals so that they change to match the point position, a home and distant on each track leading into the points, ie four signals in total. I could buy separate accessory decoders for the signals and put them on the same DCC address as the points, but I notice that the ADS8FX has solder pads on the underside for connecting LEDs. These are described as being for lights on a control panel, but is there any reason why I shouldn't simply wire all four signals to these solder pads for the point in question and dispense with the need for additional decoders? Has anyone tried this?

 

Regards, John

[2e0dtoeric]

I would advise you to contact Train Tech, to see if the circuitry can cope with the load you are considering.

The answer would PROBABLY be yes, but it costs nowt to be sure! 😉

[96RAF]

David Palmer of TT is a forum member but a direct email to him (ref TT web site) would probably make for a quicker response.

Armed with that signal load info and protocol you could email Richard at DCC Concepts to garner his opinion of the project.

Rob

[Chrissaf]

John,

No need to bother the manufacturer contacts, all the information needed is contained within the product manuals.

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I have looked at the online manuals for the TrainTech SK2, SK3 and the DCC Concepts ADS8FX.

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The SK2 & SK3 are quite straight forward basic LED dual aspect signals with an external resistor. These signals are made as common cathode. The common side must go to a 'negative' voltage supply.

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Looking at the ADS8FX manual. The LED output is configured on the board as common anode. The common side of the decoder LED output is 'positive'.

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Thus the SK2 & SK3 signal products are not compatible for direct connection with the ADS8FX decoder board, because the decoder output voltages are the wrong way round.

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/media/tinymce_upload/3ffa3c286d4685bdc7297038ccdc5ca4.jpg

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As by far the majority of Accessory Decoders use 'common anode' it would not surprise me if Train-Tech purposely made their signals 'common cathode' to force you to use their own signalling decoders to maximise their revenue.

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PS - I am familiar with the construction of the Train-Tech kits. The LEDs are already pre-soldered to the long thin circuit board, thus they are not at all easy to modify to convert them to common 'anode' (the LEDs would need to be unsoldered and individually reversed on the PCB, these are tiny LEDs and thus this would be a very risky modification, particularly as these signal kits are not cheap).

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PPS - You could build a voltage inversion circuit (example below of one I have built previously), but to be honest, by the time you have faffed around building these (one per signal) you might as well just use the recommended Train-Tech signal decoder.

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[96RAF]

Depending upon how many signals you have and their construction method (dismantle-able or not) I would be tempted to get the soldering iron out and back engineer them to positive anode, seeing as there must be two separate leds in the signal and not a single three-legged bicolour led.

 

Disconnect the led legs that are common and reconnect the other pair as common, then put the external resistor on the common lead and connect to the acc decoder to suit. Worth a try.

[Britannia Builder]

Well spotted, Chris - that problem hadn't occurred to me. I've ordered the signals, so when they arrive I'll check to see if the wiring can be reversed (unlikely, since the LEDs are mounted on a long thin PCB). If not, I suppose I'll have to buy extra decoders unless you can come up with a voltage reversing circuit that I can make!

 

Regards, John

[Chrissaf]

Rob and John,

See my later edits that were added after your replies with regard to modifying the kit and a conversion circuit. If you need more in depth information regarding my inversion circuit, ask and I will post additional detail.

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PS - Those later edits were done BEFORE I saw the supplementary reply comments.

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PPS - Rob, the LEDs are 0403 SMD LEDs directly soldered to a very thin tall PCB board. The 0403 SMDs are so small you literally (well at least with my eyes) need a microscope to see what you are doing with a soldering iron. Which of course would need to be a very fine tipped iron. Alignment of the LEDs would be critical if they are going to end up being behind the lenses of the signal. In the factory, the LEDs would have been put precisely in place by a robot and not potentially shaky fingers.

[St1ngr4y]

...... OR you could go down the Railmaster route ....... 😉

 

Ray

[Chrissaf]

Issue is Ray, that these Train-Tech signals are just the basic signal head with the LEDs in them. No controller board is included, so you would still need to interface them with a decoder even if using RailMaster to control them.

[Britannia Builder]

Chris, thanks for the inverter diagram, but it's probably beyond my electronics skills, and I won't risk trying to modify the signal PCB. The Train Tech SC1 controller has 2 ports to control two 2-light signals independently, but it's not clear if I can economise by connecting 2 or even 4 signals onto one of its ports (ie home and distant on each branch of the point). I suspect not. Perhaps another option would be to use a latching relay activated by the point solenoid output on the ADS8FX so that I can drive all four signals without any worries about power capacity?

 

Regards, John

[Chrissaf]

John, Gaugemaster make a suitable ready made Latching Relay Board GM500D. If you use this product make sure you procure the GM500D version and not the GM500. The D version has the correct operating polarity when being used with a common positive solenoid decoder.

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http://www.gaugemaster.com/item_details.asp?code=GM500D&r=1

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It has TWO change-over contacts on the board, thus if not using switched frog polarity points then the second set of contacts could be used for a second signal.

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Wiring diagram here

http://www.gaugemaster.com/instructions/gm500D.pdf

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[Britannia Builder]

Thanks Chris, I've now ordered a GM500D. I checked earlier today with DCC Concepts and they say that the ADS8FX LED outputs are low power and would probably not drive multiple signals, so my original idea wouldn't have worked quite apart from the polarity issue.

 

Regards, John

[Chrissaf]

On the ADS8FX manual data-sheet, it states that when connecting LEDs to the solder tabs that an external resistor is not required. This can only mean one thing. The circuit design on the ADS8FX board includes components to limit the current output. This may be a simple resistor, or it may be a 'constant current' electronic driver circuit. Either way, the output current may be configured to be limited to a value not exceeding 20mA. DCC Concepts have not stated in their documentation what the maximum current is. It might be less than 20mA.

[Britannia Builder]

Just looking again at the ADS8FX, it has 3 terminals for frog control and it appears to connect the centre frog terminal to one or other of the two outer terminals depending on the point position. I'm not using live frogs, so could I feed a DC supply through these to power the signal LEDs? In practice the GM500D may be worth the small extra cost since it can be placed near the point solenoid and avoids running wires back to the ADS8FX.

 

Regards, John

[Chrissaf]

That is certainly feasible, just make sure that the two outer terminals are not in contact internally within the decoder to the track DCC supply. I don't think they are, but worth checking with a meter all the same.

[Britannia Builder]

Thanks Chris, DCC Concepts have confirmed that it's OK to use the frog terminals for LED switching so I might give it a try. I've now wired up one of the signals via the GM500D latching relay and it works nicely.

 

I'm now thinking of installing a red/yellow/green home/distant signal at the end of the platform, where it is immediately followed by a point feeding in from a siding and then a little further down the track by the passing loop point that I've been talking about above. This signal would be controlled by both points so that it's red if the first point is against it, yellow if the second is against it and green if both are clear. I think I can use a relay on the first point to switch power either to the red LED or to a feed to the second set of terminals on the GM500D on the second point, which would feed back power to either the green or yellow LED as appropriate. Couldn't do this with Railmaster!

 

Regards, John

[Britannia Builder]

I've now wired up some of my signals to the ADS8FX frog terminals, and they work nicely. Interestingly this frog relay is not latching - when DCC power is off it reverts to one particular side, so switching on the DC signal power to test signal polarity without the DCC power can be misleading. The home/distant signal controlled by two sets of points as described in my post above also works nicely.

 

I found that the red LEDs in the Train Tech signals are very much brighter than the green and yellow, using the supplied 1000 ohm resistor on the common negative. I've had to add a 4700 ohm resistor in the red feed to bring the red brightness down to just a bit more than the others.

 

Regards, John

[St1ngr4y]

Interestingly this frog relay is not latching - when DCC power is off it reverts to one particular side, so switching on the DC signal power to test signal polarity without the DCC power can be misleading. 

This "feature" of the ADSn-FX decoders is a problem when used for frog polarity switching. If a point is left switched the wrong way, you can get a short circuit next time you power everything up. I think the manufacturers are aware of this, however.

Ray

[Britannia Builder]

Hi Ray

 

That's interesting. On my layout I have an emergency power off switch which cuts power to the track, but not to the accessory decoders. This allows me to correct point settings before restoring the power, in the event that a train was heading for an adverse point. This setup would also allow you to power up the ADS8FXs before powering the track, to ensure the correct frog relay settings.

 

Regards, John