Recently, while looking for signalling projects, I dug up an old copy of Model Railroader (April 2001) and found a electronics project for a automatic 1-lamp searchlight.
It uses a bipolar LED (red in one direction, green in the other, and yellow when sending a low frequency block signal through its leads.
Testing this circuit however I found it to be not working. there were a few mistakes and omissions in the circuit as published in the Model Railroader issue.
Attached is a copy of the article in question, as well as my schematic for a working circuit.
