Question About the RK Scale Pennsy Signal Bridge 30-11030

I have a couple of these bridges as well as the single head track side version.  I have never seen an N&W version with colors.

Too bad MTH did not offer interchangeable 3 color heads for these. I have a couple of ROWI single head cantilever type signals on the layout, with 3 colors, and they look great. I may pull one of the MTH heads and see if it's possible to open them up and modify them. We're really only talking changing 4 of the 7 leds, so it may be doable.

Rod

@Rod Stewart posted:

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. I may pull one of the MTH heads and see if it's possible to open them up and modify them. We're really only talking changing 4 of the 7 leds, so it may be doable.

You mean use the 3 vertical LEDs, change the top and bottom from yellow to red/green?  Wouldn't that have a odd-looking round signal head? Albeit it fastens to the bridge?  There have been previous OGR threads showing the guts; IIRC there's a round circuit board that may be more trouble than it's worth to cut traces etc. to wire up 3 independent LEDs.

I seems to me the key is the hook, pin, clamp, whatever that fastens the signal head to the bridge.  If it's some standard fastener then glue that to a we-honest 3-aspect signal head.

B&O  used a color position light signal much like the PRR one.    They did not have a center light.     The vertical two were green and the horizontal were red.    I think they had at least one diagonal, but I do not know the color.

Well I had a look today, and this turned out to be easier than anticipated. Like you said Stan, there is a small circular pcb inside the head, seen below. And I should have been clearer, I was thinking to maintain the circular head shape; just change the top and bottom vertical leds to green, and the horizontal left and right leds to red. Turns out it was easy peasy to remove 4 yellow leds, and solder in the new ones. No pcb mods needed, and no unwiring. The leds are spaced about 1mm away from the pcb, so I just used the front face as a template, set the leds one by one into their respective slots and soldered them in. This made the spacing and alignment near perfect.

it wound up pretty much as expected, see the clip below.

The ROWI 4040 signal that seems like it might be based on perhaps B&O and/or N&W does not have a center yellow, so this is not exactly like them. Still, it's not bad. I might paint the center yellow led black at some point, we shall see.

Rod

If you had removed the center LED is would like the N&W CPL signals.

@Danr posted:

If you had removed the center LED is would like the N&W CPL signals.

That's what I was thinking Dan, but then I would have had a hole in the middle. Could be plugged off pretty easy tho'. Like said, I might wind up just painting the center led black at some point.  Aside, what does "CPL" stand for?

Rod

CPL = color position light.

Ahh.  That's a pretty unique connector that attached the PRR to the MTH bridge structure!

If I had one my homwork, I would have realized it would be quite the hack job to mate a we-honest 3-aspect R-Y-G signal head to the MTH connector!  The orientations are not right but following shows what I was thinking....

@Rod Stewart posted:

That's what I was thinking Dan, but then I would have had a hole in the middle. Could be plugged off pretty easy tho'. Like said, I might wind up just painting the center led black at some point.  Aside, what does "CPL" stand for?

Rod

Many of the position light targets in use had holes at 5 and 10 o’clock which were covered with a plate.  In found a YouTube video were you can clearly see a cover over the center hole to covert a 7 light signal to a 6 light signal.

You could cover the center hole from the inside with black paper but you would still have the visor, which would not be prototypical.  Alternatively, snip one of the LED leads on the center diode.  Your average visitor will not know the difference; on the other hand, a rivet counter will quickly point out that your signal is wrong.

NS modified CPL

@stan2004 posted:

Ahh.  That's a pretty unique connector that attached the PRR to the MTH bridge structure!

If I had one my homwork, I would have realized it would be quite the hack job to mate a we-honest 3-aspect R-Y-G signal head to the MTH connector!  The orientations are not right but following shows what I was thinking....

Hey Stan, no harm, no foul. I knew from your description what you were driving at. But I was not clear originally that I wanted to retain the stock circular target head shape. The head plug and socket is different from anything I have seen. Its the same as the one used on the RK cantilever block signal, 5 flat pins arranged in a circle around a round center pin.

There are five wires inside the head. One for reds, one for greens, one for yellows, one for the center yellow (which is always lit), and one common ground. I didn't bother to note which color wire does what. It's nice that the heads unplug, it sure makes them a bunch easier to take to the workbench to work on!

Thanks Danr for that addirtional information.

Rod

I'd just like to find a couple of the replacement heads, and since I'm a PRR man, the stock ones are just fine with me.

I gather they are in short supply John?

Rod

Here for comparison is a pic of the RK Cantilever Signal Head, the common red over yellow over green setup. And a close up of the same 6 pin connector as the Pennsy bridge head earlier. Note that these connectors are not keyed, and you can plug them in in any one of five positions. MTH must be counting on us being smart enough to know which way its supposed to look! Haha.

And here just for info is a video of the ROWI cantilever signal in operation, just past the bridge to the right. This is what got me thinking to convert the RK bridge signal heads. I kinda like this look.

Rod

@Rod Stewart posted:

I gather they are in short supply John?

Rod

They were supposed to be re-issued, but apparently that never happened.

I converted my railroad from MTH PRR signals to Custom Signals products two years ago.  I sold most of the old MTH signals but I do still have 6 of the separate signal heads from the PRR signal bridge.  E-mail me if any of you guys are interested in them.

Email on it's way.

Bob; I'll definitely take a set please. Email sent.

Rod

A fun thought exercise might be to sketch out what it would take to use red-green bi-color 3-pin LEDs such that the center LED can be either red, yellow, or green.  I realize this would not be prototypical.

By powering both the red-green LEDs simultaneously you can generate amber/yellow.  So the choices would be 3 green vertical LEDs, 3 yellow diagonal LEDs, 3 red horizontal LEDs.  This shows the range of colors...from green-only to red-only to various ratios of red and green to generate the yellow/amber.

Interesting idea Stan. The colors generated would need to be the same hue as the current greens, reds, and yellows to look right. So switching the center led from red to green is easy enough using the 3 leads. Yellow would be the tricky part since you have to power both anode leads to make yellow. I guess you would need small blocker diodes so as to prevent cross feed from red to green and vice versa when you only want one or the other. Hmmmm........

And there is very limited space within the signal head for added parts. My head is starting to hurt.......

Rod

I figure you'd probably want to change out ALL the LEDs...and use the bi-color red/green even for the red-only and green-only LEDs.  That way you know you'll have color matching.  I figure at about 5 cents a piece (I can't tell, are those 3mm or 5mm?) the LED cost won't be the show-stopper:

Like you'll say, there's some homework to work out some kind of diode-blocking.  And if this is the "standard" MTH timed-yellow circuit, one must confirm whether a common anode or common cathode bi-color is the best fit for what's available via that bizarre connector!

Well, if nothing else, I wouldn't be surprised if you'd be the only person in the world who has such a signal!

In thinking a bit more about your suggestion Stan, I came up with a bit of a conundrum. Having 3 green vertical and 3 red horizontal should be easy enough. But I think having the center 3 lead led show yellow on diagonal will be trouble. Problem is to make yellow with it you need both red and green power, but you have neither present when the yellows are on. What do you think?

The leds are 3mm by the way.

Rod

Separate power Rod, just have some power all the time.

So just for fun I sketched up (through the miracle of cut and paste once again) what may work to have the center led show green, red, and yellow when appropriate. The  upper sketch below shows how the leds are currently wired. For this exorcise we are assuming common cathode wiring. The basic idea would be the same if they turn out to be wired common anode; just polarities reversed. And the premise is that we are starting with the all yellow Pennsy style head already modified to 3 color CPL. There are 5 wires total including the common cathode ground, not shown. The center yellow led is lit full time; it never changes. The leds are shown oriented exactly as they are on the existing tiny circuit board (the signal head is only 1" OD).

Now if we sub a bi-color led for the center one, the lower view shows how it just might work. Four added diodes are needed. (I realize that the bi-color diode is not represented correctly, but it's sufficient for purposes of this discussion) Diodes D1 and D3 provide red and green when the standard led pairs are so lit. Then when the yellows are lit the jumper through D2 and D4 provide power to both both red/green; thus yellow. Meanwhile D1 and D3 block cross-triggering of either the red or green leds when yellow is lit. For this case only 4 wires are needed including the common cathode wire. The previous center yellow led wire is not needed. None of this affects any MTH wiring other than in the signal head.

This could be breadboarded easy enough to verify and fine tune it, and to answer a number of pressing questions: 1) Would the bi-color center led have a close enough color hue to look right with the mono-color leds? 2) Would the added diode losses make the bi-color led too dim in any/all colors? 3) Is there room within the signal head to accommodate 4 added diodes; say tiny 1N4148's for sake of argument? 4) Would the result be worth all the trouble? (I'll answer this by saying "Probably not!") Anyway, interesting to contemplate.

Rod

Sounds like a custom PCB, surface mount parts will make it easier to fit.

@Rod Stewart posted:

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4) Would the result be worth all the trouble? (I'll answer this by saying "Probably not!") Anyway, interesting to contemplate.

Agreed.  "Probably not!" is the answer to 4).

Your circuit indeed reflects what I had in mind about diode-blocking.  Actually, I'd expect at least 1 resistor needs to be added to the center-LED to balance red and green current when both are on.  Red LED is about 2V, green LED is about 3V.  So with the ~0.7V diode drop to each, Red is now 2.7V and green is 3.7V.  Red will steal all the current unless a resistor is added in the red leg.

Ideally one would not want to mess with the driving circuit in the base.  But practically speaking, and since this would be a one-off, it may be advantageous to modify the base circuit (where there is presumably more room to play) to minimize the # of components crammed into the tiny signal head.

Like you say, interesting to contemplate.

It might not be a one-off if it worked really well, maybe make a new circuit for the base PCB.

I've never seen a prototype color position light with a center light.  I know that B&O and N&W had no center lights.  The quick and dirty approach would be to paint the center LED black.  Since you need to remove the face plate to install green and red LEDs anyway, you could remove the center light shade with a Dremel and fill the hole with a bit of styrene.  Repaint the face black and you will have a clean CPL.

@Bob posted:

I've never seen a prototype color position light with a center light.

I've never seen a prototype train track with a center rail.

If we are to pursue this if only "virtually" I'm curious to confirm that the 1" circuit board really parallels the outer LEDs (rather than placing 2 in series).  Also, I assume the board in the base is the MTH timed-yellow type?  The timed-yellow schematic has been posted many times on OGR; but if I recall correctly it uses a 555 timer chip where the 555's output pin drives yellow and green - one color when high, the other color when low.  In other words, it's NOT common cathode for all the LEDs.  Not sure I'm making sense but would like to see the base circuit board.  I realize this is the PRR "timed-yellow" that has to have an extra output to always power the center LED so maybe it's a different circuit than the R-Y-G timed-yellow.

Stan; if you are correct about the 555 timer circuit, that would mean that the yellow and green leds are wired common anode, right? And I assume that the circuit is basically the same with the timed yellow delay before greens are lit. The center yellow would be a separate output that is continuously powered.

And since the head needs to be disassembled anyway so as to swap to green and red leds, and a bi-color center led, I would think it best to make all needed changes in the heads if possible?

Another thought I just had is maybe forget the yellow center led lighting, and have it light only when red and green are lit. So the yellow caution would be 2 leds only, the all clear and stop & hold would be 3 green and red leds respectively.  I would be OK with that, and it eliminates all the diode business. Only 2 tiny jumpers would be needed to power the center led when needed.

I am not sure whether the pairs of leds on the current board are series or parallel lit; I did not bother to check that out when I had the converted head apart. That would be good to know. Meanwhile I tried blacking out the continuously lit yellow center led on the converted head, and that looks better to my eye.

Rod

Above is the timed-yellow circuit used in a variety of MTH 3-aspect red-yellow-green LED signals.  This was posted many years ago by member fec_fan.

What I was getting at is the 555 output (on pin 3) drives the yellow and green LEDs.  In principle these would be the diagonal and vertical LEDs in a PRR signal.  As drawn you could not have a common anode or common cathode signal head.  As I think about it, I believe the base circuit board in the PRR signal bridge MUST BE a different circuit from this.  It may still be based on a 555 timer for the timed-yellow function...but to reduce the wire count from base to head, I'd think they had to design a slightly different circuit including at minimum a resistor to continuously power the center LED.

Your idea to power the "center" LED only when it needs to be red or green is a clever compromise that keeps things simple.  That's a good thing!

But if going for the full-monty, I suppose not having to modify the base circuit board is the way to go.  That said, if you can take multiple close-ups of the top and bottom of the base circuit board, I will volunteer to draw out the circuit in the way fec_fan did above for the R-Y-G timed-yellow circuit.

Stan that could very well be the circuit used in the Pennsy bridge. From what I see you need 3 wires for the yellow, green circuit, plus one wire for red (I presume red returns to the same ground rail as the yellow cathode/black wire). Plus for this device a fifth wire to power the center yellow led full time, which could come off the 12V Vcc hot rail thru a suitable resistor.

Today I did some playing with the pins on one of the heads using 5 vdc and a resistor. I got both greens and yellows to light, which I thought was odd. But it would be explained by the fact they are in series in the diagram. I also got the reds to light separately, and the center yellow to light separately. But I could not conclude which pin might be the ground rail. So armed with this diagram I might have another go at it.

Any idea what the other devices fecfan mentions are? Like 11011, 11013, 11035, 11024, 11025? I guess I can search the MTH site and find out. Also he does not list the Pennsy bridge 11030 specifically, so perhaps he was not sure about it. He lists the 11009 cantilever signal bridge as a probable, which I have to believe is correct, since it's just like 11023, just a different frame.

I have a spare NIB Pennsy bridge, so maybe I can dig it out and pull one of the circuit boards out and get some pics for you. It uses two boards of course, one in each base. They are not interconnected in any way as far as I know. That would also enable tracing board outputs to the signal head socket, which would be good to know. Wow, talk about mission creep!

Rod

Is it time for a microprocessor based board in the signal bridge yet?

@stan2004 posted:

I've never seen a prototype train track with a center rail.   ...snip...

Check out the Liverpool overhead railway, portions did use a center third rail:

https://en.wikipedia.org/wiki/...ool_Overhead_Railway

OK here are 3 pics of one of the two Pennsy Bridge circuit boards, at varying distances. The biggest one is a little fuzzy; borderline for my camera phone I guess. Note the board appears to be two identical circuits; one for each of the four head positions on the bridge. Each has red-green-black inputs; green from the NC contacts, red from the NO contacts, black ground. Each has 5 wires going to the respective head socket. These colors are consistent with the 5 wires inside the signal head that I modded earlier.

There is nothing on the bottom of the board at all. The screw hole in the middle is used to mount it to the bridge base. You can see what appears to be a 3 pin Vreg, and an 8 pin 555 package. There are of course rectifier diodes and filter caps to make the 12vdc operating voltage.

Does that help Stan?

Rod

I'm not a fan of the timed yellow position.  I know this was done so that a single block could control all three aspects of the signal, but it would have been nice if a jumper could have been included to allow the yellow to be controlled by its own block.

Dale

That board cries out to be redone with a uP.

@gunrunnerjohn posted:

That board cries out to be redone with a uP.

A more versatile board would be way better John, but such a design is not in my wheelhouse that's for sure! Are you volunteering?

Rod

My plate is way past full Rod, but it would be a fun little engineering exercise.  Maybe it's time you learned some programming.