I saw these on the bay and was wondering if R1 could be replaced with a variable pot to adjust the output voltage. They come as a kit and are less than a buck a piece.
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R1 is there just to limit the LED current. There is no regulator. DC out with be 1.41 times the RMS AC voltage at no load, then decrease with current draw. Probably worth a buck for the circuit board and connectors but will need added components to drive specific devices.
Pete
I'm looking to drive LED lights around the layout and just want to tap into the existing track wireing without having to run new wireing. Mainly just for street lamps and yard lights. also for some caboose and in car lighting
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I don't think there's a one-size-fits-all AC/DC converter on eBay or anywhere for O-gauge. The applications vary too widely in terms of voltages, currents, size/space available, etc. especially between trackside/layout and rolling-stock.
That said, if you don't mind some DIY soldering/wiring how about the following combo. Note that the same guy is selling both items and accepts offers. So if buying a handful of DC-DC converters as well as a 10-pack of bridge rectifiers, I'd think you could get it below $1 per AC/DC converter.
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A much better alternative than either of the above IMO.
AC/DC to DC Buck Converter Step Down Module LM2596 Power Supply Output DC1.5-27V, eBay: 321657019357
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Yes, all-in-one but $5.30 each. 
Perhaps I was reading too much into OP's comments about less than $1 and in kit form.
No shortage of options. Again, I don't think there's a universal solution for O-gauge. Earlier John H module is nice because it has a convenient hand adjustable knob vs. a tiny screwdriver-required adjustment. The screw-terminal inputs/output are also handy. The built-in digital voltmeter is nice. But $9.23.
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Stan, if you buy more than one, shipping is only $1.30 for the additional pieces. I also see the same supply for less at times, I bought some of them about a year ago for $2.80 shipped free.
The diodes are rated for 1 amp. You could drive a lot of LEDs with that one board. Best suited for buildings and street lights where you can use a fixed voltage and fixed resistor. Depending on the application the DC-DC convertors might be overkill.
Pete
261180606444 John’s right about price. Here’s 10, shipped free. $26.22
I picked these up on eBay, 10 pieces @2.95 each, with free shipping.
Parts would probably cost more. Delivery was about 3 weeks.
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I believe I will just buy the AC/DC to DC Buck Converters, the price is right and I can tap into any AC line or just run a seperate supply line from one of my transformers. Just one could run quite a few leds.
Thanks for all the info
The buck converters are a better choice than the LM317 based packages. Much less heat and much more current capacity, they're way more efficient than the LM317 designs.
