Detector for model train presence in signalling district - comprises back to back diodes sensing traction current to drive photocoupler and comparator circuit

摘要

A graphite deposit in the insulating rings on the wheels of a railway vehicle ensures that the vehicle acts as a resistor shunting the rails. A back to back diode pair (D1,D2) acting as a detector is located in series with the tractive power supply. A voltage drop across the diodes turns on a transistor (T) to pass its collector current through an LED in a photocoupler when a vehicle is present on the rails. A capacitor (C1) allows the high frequency telecommand signals to flow through the detector circuit. The LED output is transferred to a phototransistor with the optocoupler. Its output is integrated by an RC circuit (R3C3) and applied to a comparator. This compares the integrator voltage with a reference value set on a potentiometer (P) to provide an output (S) when a vehicle is present to drive the signals and automatic controls via wired or programmed logic circuits.