New Circuits

Regrettably, because of my deteriorating eyesight, I just can't assemble electronic circuitry any more. For this reason, my ability to develop circuitry is severely handicapped. But here's where you may be able to help. I still enjoy the challenge of designing new circuits and improving old ones. I am publishing some proposed circuits on this site. If you build any of these, please let me know how they work.

Four-Aspect Searchlight Signal


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.

Modelling and Circuitry

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 universal track circuit 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.

Three-Aspect Signal Driver Using Discrete LEDs


I devised this circuit partly because the old 74-series TTL chips are becoming hard to obtain and partly because there is a new generation of LEDs which offer better colour, but require higher working voltages than are possible with TTL. The old-style green LEDs emitted yellowish green light which was hardly distinguishable from that from yellow LEDs. The Agilight blue/green LED (Maplin catalogue number N34BY) is an excellent match for the green of prototype colour-light signals (and traffic lights!), but requires a forward voltage of around 5V. I understand that Maplin may no longer stock the Agilight LED, but suitable alternatives may be available from other suppliers such as phenoptix.

This circuit is shown as operating from a 9V supply, but could operate from higher voltages if the 470R resistors are changed to a higher value. Note that the inputs of this circuit are intended for connection to the output of track circuit units; to activate an input current must flow into it from the 0V line. The signal driver must share the 0V line of the track circuit (or other train detector), but need not operate from the same power supply or supply voltage. Input 1 is from the train detector on the section which the signal is guarding and Input 2 the section beyond that.

How It Works

Current on Input 1 turns T1 on, lighting the red LED, so the signal shows danger. Input 1 also supplies bias to inhibitor transistors T2 and T4 ensuring that transistors T3 and T5 are non-conductive, so that no other LED can be illuminated, irrespective of the status of Input 2.

If there is no current on Input 1, current on Input 2 turns T3 on, lighting the yellow LED. It also supplies bias to T4, inhibiting T5 and ensuring that the blue/green LED remains off.

Only if there is no current on Input 1 or Input 2 will T5 receive bias via R8, lighting the blue/green LED, so that the signal shows green.

In this circuit the signal's three LEDs have a common-cathode arrangement and the signal requires a four-wire connection to the driver. The elegant three-wire arrangement with reverse-parallel pairs works well with TTL-based drivers, but is impractical in drivers using discrete transistors.

Important! Disclaimer

I have not built the above circuits and so cannot guarantee that they work. If you build any of these, please let me know the results.

Improved Directional Lighting


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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© Roger Amos 2013