Mar 31, 2008

Keller OnBoard and headlights

So, here I am, sitting at home and quietly finishing up super-detailing my 7 locomotive shells that I've been working on the past few weeks. Yesterday I finished up the last of the tiny decals I like to add - fuel & water labels, warnings, other little tidbits. I'm been using using diesel decals sets from Rail Graphics, purchased quite a few years ago - the last time I did this work, about 1991 believe it or not.

I also re-numbered one of the Athearn PA units (why must all PA-1 units out there be numbered 6009 !?), and finished up add numbers to the number boards of my Athearn PA and a pair of F7As. Not fun, but I did enjoy learning how to do it.

I suppose I should photograph these models when they're complete so you can see what I've been doing.

Anyway, back to Keller Onboard... I switch my focus to electronics, starting with the Lifelike Proto2000 PA / PB pair, which I thought would be the easiest. I had purchased a number of the 8-pin 2x4 NMRA plugs a while back, and soldered up just the track & motor connections on one of them, and plugged it into the NMRA plug on the PA to test it. The motor ran nicely w/ Keller, but no lights.

Of course - I hadn't jumpered the headlight connections. But, after some more experimenting, I found that the two headlight circuits are both polarity-sensitive (pins 6-7 for the oscillating headlight, and 3-7 for the lower headlight & numberboards). The oscillating headlight varies two circuits, to light two filaments inside one bulb to get the oscillating effect - about +/- 5v going to the bulb, but working on with pin 7 positive and 6 negative (I think - I might have it backwards). The lower headlight is just 1.5v, and again works w/ only one polarity.

With DCC, the decoder would provide the correct voltage and power to control these separately, and with normal DC both work but only when the locomotive's moving forward. The problem for me is that the rails will have a set DC polarity, regardless of which way the locomotive is moving. Thus, if the locomotive is on the rails facing west the headlights would work, but facing east they'd be off, regardless of whether it was moving or not.

I could replace both with my golden-white LEDs from NCE, and probably will I guess. I like the oscillating effect, but it uses the locomotives electronics and needs that certain polarity. I could light that bulb directly from track power (skip the electronics), and use a resistor to get down to 5v or so. No oscillating, but easier wiring. Or, I could replace both bulbs with the LEDs wired back-to-back, so that one LED would work for each direction the loco's facing.

I did finally figure out how to address the reverse-voltage problem... those LEDs don't like reverse voltage, and the Keller Onboard system puts 14.5v on the tracks. Turns out that with the LEDs back-to-back, one will light and the other won't, and the dark LED is protected from the reverse voltage by the current running thru the LED that does light. Assuming a current of 30 mA, I'll use a 3660 ohm resistor on one side of the two wires running to the LEDs. This is what I plan to use w/ the other locos.

What to do w/ the LifeLike unit? Not sure yet... I'm leaning now toward just lighting the two bulbs directly with appropriate resistors, if I can guess what their current draw is.