Modifying PF lights for alternating flash

[davidzq]

Some years back, someone had made a tutorial on modifying the circuitry inside the Power Functions LED (8870) which allowed them to flash alternately by alternating the input polarity.
I've been unable to locate this tutorial, or remember exactly where it was I found it. I believe it was a photo on Flickr, but hours of searching have yielded nothing. It was posted somewhere train related, as the application was specifically for use as ditch lights.
I am well aware of the circuit breakdown on Philo's page, and I am competent in circuitry so I can figure this out on my own if need be.
But I'm hoping to double check my modifications with the guide before I destroy a light (I believe I'll need to cut some of the circuit lines on the PCB).

Anyone know where it is?

[JopieK]

I haven't seen it before, but it would be doable, creating basically an oscillating circuit like the 12V level crossing box. Either use an NE555 or go for something like this I would say:

https://electronics.stackexchange.com/questions/45505/transistor-alternating-between-multiple-leds

You could also check this: 

 

[davidzq]

While that solution is the best way in terms of performance, it unfortunately cannot be done in the small space of the 2x2x2/3 splitter, and would require an external enclosure.

The method I had seen involved removing the bridge rectifier, jumping some pads, and cutting some of the PCB circuits. The end result is outwardly identical to an unmodified PF light.

[Brickblock1]

i Believe this may be what your looking for https://brickmodelrailroader.com/wp-content/uploads/railbricks/railbricks_9-print.pdf

[Toastie]

On 10/26/2021 at 10:25 PM, davidzq said:

which allowed them to flash alternately by alternating the input polarity.

I believe the term flashing is a bit unclear here. @JopieK was looking for a periodically flashing light (maybe even alternating), I believe, whereas one can also read for your post: One goes on only for one polarity, the other for the other. Is that what you meant or rather the periodically flashing light?

Best
Thorsten 

[davidzq]

7 hours ago, Brickblock1 said:

i Believe this may be what your looking for https://brickmodelrailroader.com/wp-content/uploads/railbricks/railbricks_9-print.pdf

YES!

That was it!

 

I really need to go through RailBricks again. It's getting dated now, but it's still a wealth of information.

4 hours ago, Toastie said:

I believe the term flashing is a bit unclear here. @JopieK was looking for a periodically flashing light (maybe even alternating), I believe, whereas one can also read for your post: One goes on only for one polarity, the other for the other. Is that what you meant or rather the periodically flashing light?

Best
Thorsten 

Yes, one light for one polarity. The other light for the other polarity.

Thus allowing a motorized switch (or a programmable output) to flash the LEDs in an alternating pattern. Left, right, left, right...

[JopieK]

14 hours ago, Toastie said:

I believe the term flashing is a bit unclear here. @JopieK was looking for a periodically flashing light (maybe even alternating), I believe, whereas one can also read for your post: One goes on only for one polarity, the other for the other. Is that what you meant or rather the periodically flashing light?

Best
Thorsten 

Hehe, good point :) 

@davidzq, you might already know this, but an LED is a diode (light emitting diode = LED) which is an electrical part that allows current to flow only into one direction, that is why changing polarity allows to activate one LED or the other.

I always use that trick for the headlights. I made a custom PCB for the signals of the Crocodile, setting it into one direction enables the yellow lights, setting it in reverse enables the red lights.

[davidzq]

2 hours ago, JopieK said:

Hehe, good point :) 

@davidzq, you might already know this, but an LED is a diode (light emitting diode = LED) which is an electrical part that allows current to flow only into one direction, that is why changing polarity allows to activate one LED or the other.

I always use that trick for the headlights. I made a custom PCB for the signals of the Crocodile, setting it into one direction enables the yellow lights, setting it in reverse enables the red lights.

Well aware. And of how the bridge rectifier works.

But Philo's diagrams show a connection between the LEDs after the bridge, so I just wanted to ensure there was no trace between the pads which could result in a dead short if I were to remove the bridge and jumper it's pads. And knowing I had seen the diagram before, I knew someone else had already done the legwork for me, so sought it out. 🙂