I am in the process of modifying several engines to reduce the brightness of the current red marker leds, and I decided I needed a convenient way to quickly test what value resistor is needed. Since I don't have a decade resistor box, this little board is the result. It's a whole lot easier than trial and error with a whole pile of different value resistors. I thought I would post it here for the benefit of others who might want to do the same. Note this can also be used to test LED headlights, backup lights, ditch lights, etc, etc. The value of resistor needed can vary dramatically depending on whether the leds are powered by low voltage from a command board for instance, or full 18 volt track voltage.
The circuit pdf is attached in the next post below; and it's dead easy to build. I used SBC-2 and SBH-2 connectors (available from AllElectronics) for input and output, which should be compatible with about 90% of engine wiring inter-connects. The black ground wire connects straight through from input to output. The red has the surplus 1K trim pot inserted between input and output; one to one end of the pot, and the other to the center wiper. The red to red polarity must be maintained from input to output for the LED's to work. The 3 pin Dupont header you see has the outside pins connected to either side of the pot. The center pin is connected to black. Note that as assembled above the board is fully reversible. Connect it up whichever way your engine connectors require.
To test you simply insert the board into the existing wire harness, power up the engine, and adjust the trim pot to your LED brightness liking. (Since you are adding resistance to the circuit, you don't need to worry about overpowering the led and burning it out) Then power down and use the outer pins of the Dupont header to measure the resistance across the pot, and insert a resistor of near that value in series in the led wiring. Dead easy. Before powering down you can measure between either outer pin to the center pin of the Dupont header to read the effective voltage in or out of the board. You must remember that since most command board sources will be PWM, they won't be voltages that you might expect to see. This has been covered elsewhere on this forum.
I have found that red tender markers driven by a railsounds board needs about 620R added to look right. Since there was already a 200R resistor inline, I replaced it with an 820R one. For PS-2 engines, red tender leds look about right with a 470R-620R resistor inserted in the wiring. I also found that led headlights (warm white, or bright white), and B/U lights driven by PS-2 look really good with a 220R resistor inline, as grj posted in another thread a short while ago. Note that once the correct value is found, any similar engine and led sets will behave the same in terms of resistor value to use. Next on my list is to reduce the values for various existing green engine marker lights, to try to brighten them up a bit.
I have no plans to produce diptrace board files for this project, since each user would likely only want one, and they are really easy to build. Having said that, if anyone would like help with the schematic or other details, contact me offline and I'll be happy to help out.
Rod
