pseudo track occupancy detector

Posted on Feb 5, 2014

Track occupancy detectors are needed for hidden yards and other sections of track that are hard to see, but they are also necessary for block operation. The circuit described here uses an LED to indicate track occupancy for digitally controlled MG¤rklin HO model train systems (including Delta Control). In contrast to a real track occupancy detecto

pseudo track occupancy detector
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r, which detects all vehicles, it only responds to vehicles that draw traction current. This means it can be used without making complicated modifications to the rolling stock and track, since it is only necessary to gap the third rail. This circuit is thus especially suitable for retrofitting to existing installations, and it is equally well suited to M, K and C tracks. The basic idea of the circuit is simple. If a locomotive enters the monitored track section, a current flows through the motor. This current is sensed and generates an indication. With a MG¤rklin Digital system, power is provided to the locomotive via a controller or a booster in the form of a square-wave voltage. The voltage levels on the rails are approximately 15 V and +15 V. Digital control information is transferred by a continuous sequence of alternating plus and minus levels. This means that the detector circuit must be able to respond to AC signals. In Figure 1, the monitored track section on the left is connected to the ground terminal 0` via the rails. The third rail, with conducts the traction current to the locomotive, is isolated from the rest of the system (special third-rail insulators are available for this purpose), and it is connected to the B` terminal of the controller or booster via the detector circuit. If a locomotive travels over the monitored track section, the positive component of the drive current flows through diodes D1 and D2,...

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