To many people I'm best known for the books I've written on the application of electronics to model railways. At last, by popular request, here's a page on the subject.
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 have published some proposed circuits below. If you build any of these, please let me know how it works.
For the benefit of those new to this subject, a track circuit on a full-size railway is a device which detects the presence of a train in a section of track using the wheel/axle assemblies of the rail vehicles to complete an electric circuit between the two rails. Track circuits supply inputs to mimic diagrams and automatic signalling systems.
They are used for the same purposes on model railways, but there is a complication in that the rails may be carrying the traction current for the train. So a little ingenuity is needed. We isolate the section of track (in either or both rails) so that it needs its own supply from the controller(s). The feed from the controller passes through the track circuit electronics, which monitors the current flowing into the section. To allow for occasions when the controller is at zero, switched off or disconnected, a separate "trickle" of current from the track circuit's own power supply is also provided; this is deliberately kept too low to cause unwanted movement of trains, while being sufficient to activate the current monitoring circuitry.
Of course, track circuits only detect rail vehicles that draw current, such as locomotives and vehicles fitted with lights powered from the track. For most purposes, this is sufficient. Other vehicles can be made conductive if it is necessary that they activate track circuits.
The heart of the track circuit is its current monitoring circuitry. This must be capable of passing and detecting current of either polarity from a few milliAmps to as much as 1A (e.g. a heavily-loaded OO/HO loco) without causing adverse voltage drop. In this design the current passes through the reverse-parallel base/emitter junctions of a pair of power transistors. The voltage loss is therefore a fairly constant 0.7V which is usually tolerable. The collectors of the two transistors are bonded. The presence of a train in the section will always stimulate collector current from one or other of these transistors. This is amplified, smoothed (essential if dirt on the track causes temporary interruptions of the traction current) and delivered as a TTL/CMOS-compatible Boolean output signal (low if a train is present).
The circuit that has been improved is that shown in Figure 14.4 on page 68 of my Complete Book of Model Railway Electronics. This circuit, known sometimes as Tektor, is unusual in that it can be wired into either the live rail side of the track, i.e. between the controller's live terminal and the live rail, or the return rail side of the track, i.e. between the return rail and the controller's return terminal. With the latter configuration it is necessary to divide the return rail into sections which is not always convenient. With live-rail operation the return rail can be continuous, but live-rail operation is not possible with a controller that delivers continuous DC. As most controllers, however, deliver a pulsed output, they work satisfactorily with Tektor's live-rail operating mode.
The original Tektor had one disadvantage, a pre-set potentiometer which was a sensitivity control. Too high a setting causes spurious train detection; too low a setting and trains may not be detected. There is a broad spread in which the device gives consistently accurate results, but if the supply voltage is changed, the control may need adjustment.
Below is the circuit of an improved Tektor with no such pre-set control:
The control has been replaced by the combination of R2 and D3 which should provide T3 with suitable base bias whenever a train is detected, irrespective of the supply voltage.
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 the track circuit unit shown above 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.
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.
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 circuits such as the one shown above - 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.
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 LEDs 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.
I have not built the above circuits and so cannot guarantee that they work. As stated earlier, I hope that someone will build them, test them and let me know the results. In particular, I should like to know if the improved track circuit performs reliably over the supply range from +5V to +12V and if that supply range can be extended upwards, e.g. to +18V or +22V.
So far as I am concerned, anyone may freely build and use the circuits described above. Anyone may freely manufacture the circuit or a kit of parts for it and sell this for a profit. I am not seeking royalties. My only wish is to be credited as the original designer.
I am happy to discuss model railway electronics topics, whether or not they relate to the projects in my books. Just email me (click on the Contact Me button on the left).
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