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. If I publish a proposed circuit here, maybe someone out there could build a prototype and let me know how it works. So, here goes with number one.
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.
I have not built the above circuit and so cannot guarantee that it works. As stated earlier, I hope that someone will build it, test it and let me know the results. In particular, I should like to know if the 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 circuit 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 its 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 envelope icon to the left).
If you entered my site directly using a link from a search engine, you may have missed out on the site navigation system. If you cannot see six buttons along the left edge of the window, please click the Continue button below.
