Many model train enthusiastic UV find the mechanical reversing system for trains in the HO series from Marklin and other manufacturers primitive and unreliable. The system is based on a.c. motors and a mechanical reversing assembly operated by a small electromagnet. The motor speed is determined by the track voltage, which can lie between 4 V and about 16 V. When the knob on the speed controller has turned fully anti-clockwise, the a.c. the voltage on the track is briefly increased to 24 V. Ideally, this causes the electromagnet in the loco to be actuated and overcome the counterforce of a small spring. In practice, this way of changing the direction of a model train is fraught with difficulties as the tension of the spring is a very critical factor. In not a few cases, the voltage pulse fails to actuate the reversing mechanism. and instead, cause the loco to hurl itself at a turnout Where it is derailed. When the spring is too, loose. it may happen that a loco, running at full speed, reverses suddenly with ‘disastrous’ results.
Some ten years ago Marklin recognized the disadvantages of the voltage-operated reversing system, and carne up with an electronic alternative in the form of a zener diode and two transistors. Unfortunately, this upgrade proved expensive and difficult to fit in existing locos, which many modelers would be loathed to give up. In all-electronic reversing systems developed a few years ago, the direction of the loco is ‘stored’ in a small button cell. This is necessary to prevent the information being lost as there is no supply voltage when the locostands still. The present circuit uses a 100 pF tantalum capacitor to keep the control circuit powered for up to 8 hours. The capacitor, in the author’s opinion. is more elegant and environmentally safer than the battery. The circuit described below is based partly on SMDS (surface-mount devices) and is designed to be as economical as possible as regards power consumption. When the circuit is not, actuated, transistors T1 and T2 are off.