You might think that directionally controlled lighting (normally white headlights and red tail lights which switch over automatically when the loco changes direction) is easy. Surely all you have to do is to wire reverse-parallel LED pairs (with a series resistor of course) in parallel with the motor, so that they share its power pick-ups. The polarity of the power supply ought to ensure that the appropriate LEDs are illuminated when the loco moves. It works fine on the test bed, but put it in a model loco running on a track and all hell breaks loose. All the LEDs come on together. Why?
Because a moving model locomotive is a very harsh operating environment for electronic circuitry. The power supply from the controller to the motor (and lighting in this circuit) has to pass through the ever-changing contact which the loco wheels (which are often dirty or oily) make with the track (usually also dirty or dusty) and that which the wheels make with the power pick-ups which may themselves be dirty. Moreover this applies on both sides of the circuit. The consequence: in the real world there are frequent momentary interruptions to the supply from the controller received by the motor and the LEDs. While the circuit is complete the appropriate LEDs will be illuminated, but during the interruptions they are extinguished. But that's only half the story: during the interruptions the motor keeps spinning and it acts as a generator, providing an alternative power supply for the LEDs. Unfortunately, the voltage that it generates is, from the LEDs' point of view, of opposite polarity to that from the controller. So during the interruptions the wrong set of LEDs lights up! As the loco runs the right and wrong sets of LEDs light alternately in rapid succession and, to the human eye, it appears that all are on together.
To remedy the situation is electrically very simple, but mechanically fiddly. You must give the LED circuits their own power pick-ups, separate from those supplying the motor, as in the circuit shown below. The theory is as follows. When the motor loses its power supply and keeps spinning, there is now nowhere for the electricity that it generates to go. So the only power supply available to the LEDs is the current from the controller through the track. This may experience interruptions, but they are usually very brief and the appropriate LEDs should appear to be be illuminated.
Let's suppose that you're fitting head and tail lights to a diesel- or electric-outline locomotive (two yellow headlights and two red tail lights at each end). The circuit is shown above. Let's also assume that the loco has two bogies (trucks), one of which is powered. It may be that there are power pick-ups (for the motor) only on the power bogie (truck). If so, fit power pick-ups to the other bogie (truck) and use these for the LEDs. If your loco has motor power pick-ups on both bogies, you have no option but to fit additional power pick-ups (for the LEDs) to one of them. Resist the temptation to share power pick-ups; that's what causes the problem we're trying to eliminate.
Incidentally, use yellow LEDs for headlamps, not white ones. White LEDs have a higher operating voltage than red ones and when connected in reverse parallel with red ones, may cause the red LEDs to be subjected to a catastrophically high inverse voltage. The problem does not arise with yellow LEDs. The circuit diagram specifies 1K8 series resistors, but you may need to adjust this depending on the LEDs used and the brightness required.
I'm grateful to Ger Hayden who has built and tested this circuit and is delighted with the results. So we know that this works!
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