Drawbacks with the Current Design

While the use of LEDs, and the use of the LM334Z are highly suited to what we are doing, they are not the perfect solution. If we were living in a perfect world the light would start to shine at 0.1V, as it is it starts lighting at around 3V. It reaches its brightest at just over 4V, which is enough for many locomotives to be moving along the track.

Another issue I have found is the occasional flickering of the reverse light. This is an issue with any directional lighting system.

Voltage Loss, Causes and Solutions

The reason that the LED does not immediately light is because there is voltage loss in the circuit. LEDs use a PN junction, which just means it uses one kind of semiconductor butted up against another. Inherent in this junction is a voltage drop. This means that the junction will not allow the electricity to go through until it is over a certain voltage. It will then typically "short" all voltage past this point. For a typical diode used for signal processing or rectifying the voltage drop is around 0.5V. This would be grand, but for our LEDs (which are just a special kind of diode, a Light-Emitting-Diode) the voltage drop is much higher. The voltage drop across an LED relates to the frequency the LED has to generate. The lower the frequency the lower the voltage drop. This means that a Red LED has a voltage drop of around 1.7 volts, and a Blue LED has a voltage drop around 3.5V. I believe the white LEDs which we find so valuable use an ultraviolet frequency, then converting this to visible light with a mix of florescent dyes. This is much like the fluorescent tubes found in lighting. This is unfortunate for us as it means the frequency is even higher, giving us a voltage drop of around 4V. Actually all those rated voltage drops are slightly lower, but it takes a little above the actual voltage drop to illuminate the LED properly.

The solution? Well, under most scenarios this is not a real issue. I mean, we have immensely improved the lighting performance already. If we want to improve it further, we really have two options. We could either artificially raise the voltage experienced by the LED so that it will light sooner, or we could artificially lower the voltage the motor experiences, so that it moves off slightly later. It is much easier to take voltage away, rather than give extra, so we will look at the second option first.

The disadvantage to dropping the voltage the motor experiences is that the locomotive will no longer reach the same top speed. Usually this should not be a problem, as few people crank their locomotive up to full blast anyway. In order to lower the voltage to the motor we can simply use a semiconductor to our advantage. If we put two diodes (facing opposite ways) inline with the motor then the motor will experience a voltage drop of around 0.5-0.6V. We can just use common 1N4004 power diodes, available from any electronics store.

The other option is to raise the voltage. While this will preserve the locomotive's top speed, it is more difficult to implement. This would nominally involve some kind of voltage doubler. This would be space permitting, as the unit would take up around 1cm³ at a minimum. I offer the following simple circuit, although I have not finalised the designed, nor have I done testing on it yet.

Voltage Doubler Schematic

Reverse Light Flickering, Causes and Solutions

The reverse light flickering is caused by poor track contact. It is really caused by the motor when the power is instantaneously disconnected. This is due to back EMF (electromotive force). To understand this you must first understand that current through a wire generates a magnetic field, and a changing magnetic field generates a current through a wire. The coils in the motor are really a series of very long pieces of wire, and the current generates the magnetic field the motor uses to turn. If a coil is receiving a current, and the current is disconnected, then the magnetic field collapses. This collapsing magnetic field constitutes a changing magnetic field, and generates a reverse voltage in the coil (this is called back EMF). In a motor this would normally be offset by the changing magnetic field that occurs as the coil moves past the fixed magnet(s) in the motor, but not always (depending on the position of the coil when the power fails). Therefore we occasionally, when the power fails and the coil is in a certain position, get a reverse voltage applied to the circuit. This reverse voltage is enough to ever so briefly light the reverse light.

This occurrence is common to any directional lighting system (although it will be more apparent in ours due to its high efficiency and speed), and cures are usually easy. Firstly cleaning the track and wheels of the locomotive should fix the problem 99%. If it does not, then a small capacitor across the motor in the locomotive will provide a permanent fix. The capacitor does not have to be big, say a 0.047uF to 0.1 uF polyester capacitor (often called a "Greencap"). These are small and should be able to be tucked against the motor somewhere.