Many a model railway is enhanced by having in place operational signals. These can of course be either semaphore or colour light and this article focuses on the use of Light Emitting Diodes (LEDs) for colour light signals. LEDs are easy to obtain and devices emitting red, yellow and green light are readily available. As a general statement, LEDs have the convenient property of passing current in one direction only, blocking the supply if connected in the opposite direction or polarity. This ability to conduct current in one direction only can be exploited in the design of the signal circuits.

The control switch itself may be a two-position toggle switch, or a relay with sets of contacts which can be similarly wired, either of which can be obtained from electrical component suppliers such as RS Components Ltd. Or it could be a signal control switch or lever designed specifically for model railway use. An example would be a Cobalt-S signal lever from DCC Concepts, these levers having ample internal contacts which can be connected so as to reverse the polarity of the circuit.

At the other end of the circuit, the signal or set of points is arranged to change when the polarity is reversed. In using LEDs for the signals, this is simplicity itself. The circuits operate on 12V direct current (DC) and the connection to the two wires is arranged so that the LEDs are fed by opposite poles.

At this stage it is important to note that a suitable resistor must be connected in series with each LED. This is to limit the current, without which the LED would be damaged and may even explode. A resistor of a suitable resistance value may be provided by the supplier of the LED. If not, it is possible to calculate the value of the resistance and this is described next.

So how to connect up? Initially, have a look at the wiring for a two-aspect signal. Diagram 2 shows the connections. The resistors are shown by rectangular boxes. The 12V DC power supply is at the lefthand side of the diagram and the control switch is shown in the red or danger position. RED GREEN Current flows through the first resistor and the red LED, which lights up. No current flows through the green circuit as the green LED is connected the other way round, so blocking the current flow. Once the switch is operated, the polarity changes and current can then flow through the green circuit and not the red.

With two colour light signals in succession, the first signal to be encountered by an approaching train can be provided with a third aspect, indicating the state of the signal ahead.

Three aspect signals display a red, yellow or green light. Red of course, means ‘stop’. Green is ‘go’ or ‘all clear’. A yellow light tells the driver it is safe to proceed but that the next signal is at red. This is illustrated in diagrams 3a, b and c.

Please refer to the connection diagrams 4a, 4b and 4c. Both the signals are controlled by a similar two-wire arrangement and, as can be seen, the connections are basically the same as those shown for a two-aspect signal. As before, each LED has an associated resistor. The threeaspect signal on the left of the diagram (signal 1) is arranged to show red with the controlling switch ‘normal’; and yellow when cleared to proceed.

An additional connection is provided to indicate the state of the signal ahead. There is a relay at the three-aspect signal with its coil connected through a diode in the feed to the green LED. The relay contacts are normally closed and are in the circuit of the yellow LED.

To signal a train, first signal 1 is cleared. On operating the control switch, the polarity is reversed and signal 1 changes from red to yellow.

At this point signal 2 is still showing red. When signal 2 is cleared to the green (proceed) indication, the positive feed connects to the green LED on signal 2 and, through the diode, to the green circuit on signal 1. There is also a feed to the relay coil. The green aspect on signal 1 is displayed and the relay picks up, opening the relay contacts which extinguish the yellow LED. Now both signals are displaying green aspects, giving the train driver a clear run ahead.