Network Rail, the company responsible for the UK rail network's infrastructure, is introducing new signals using LED technology as it upgrades the network. Four-aspect signals are being replaced by searchlight signals in which the lower unit normally displays red, green or yellow using separate arrays of LEDs and the upper unit is used only for double yellow. The photograph below shows RY 903 at the north end of Rugby station, which is fitted with a "feather" indicator which appears to use white LEDs.
Multicolour LEDs make modelling searchlight signals easy. The following signal driver assumes that the multicolour LED is a common cathode type - suitable 3mm diameter devices are available from Maplin (catalogue numbers GW62S and CH09K). Because 74-series TTL chips are becoming hard to find, the circuit shown below uses discrete components. It is shown as using a 9V supply, but lower or higher voltages could be used by adjusting the resistor values.
Input 1 is from the train detector on the section which the signal is guarding; Input 2 the section beyond that and Input 3 the section beyond that. Negative logic is assumed, that is a "0" or "low" on an input indicates the presence of a train in the relevant section; the output from a TEKTOR track circuit unit is ideally suitable. A "0" on Input 1 brings T1 into conduction, illuminating the red junction in the multicolour LED and also turns on inhibitor transistors T2 and T4 preventing any other LED junctions from being illuminated.
If Input 1 is high, T2 is unbiased and T3 receives bias from the 0V line, illuminating the green junction in the multicolour LED. A "0" on either Input 2 or Input 3 brings T1 into conduction, illuminating the red junction in the multicolour LED which now shows yellow, a mixture of red and green. You may need to adjust the values of R4 and R7 to obtain a lifelike yellow. Unless there is a "0" on Inputs 1 or 2, a "0" on Input 3 enables T5, illuminating the separate yellow 2 LED giving the double-yellow aspect.
If all three Inputs are high, the only transistor to receive any bias is T3, which illuminates the green junction in the muilticolour LED.
The following diagram shows a three-aspect version of the same circuit. Input 3 and the circuitry around the yellow 2 have been removed.
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