GB1233811A - - Google Patents

Abstract

1,233,811. Signalling on trains. WESTINGHOUSE AIR BRAKE CO. 4 Feb., 1969 [6 Feb., 1968], No. 5866/69. Heading B7N. [Also in Divisions Gl and H3] A fail safe circuit for detecting the presence of a signal (e.g. an A.C. signal indicating that a train is moving and thus that the doors must not be opened) comprises a capacitor which is charged in the presence of a signal and thus interrupts a further charging supply, the arrangement being such that in the absence of the signal the capacitor is charged from the further supply, the supply is interrupted and discharge takes place, these operations being repeated thus to produce an output signal. Rotation of a disc 22 when the train moves induces an A.C. signal in winding 18 and this is amplified and rectified by T8, T9 and 36 to charge a four-terminal capacitor 40. When the train stops the output of the rectifier ceases and the capacitor discharges. This switches over a Schmittt trigger T4, T5 allowing T6 to conduct and thus T9 to oscillate. The oscillator is rectified via T2, T3 and 36 to charge the capacitor 40 to cause the Schmitt trigger T4, T5 to interrupt the supply to the oscillator, whereby the capacitor is allowed to discharge again. Cyclic operation in this manner produces an oscillation which is amplified by T7 and T8 and rectified at 70, 73 to produce a signal representing no input signal and no movement. It is explained that failure of any component produces a safe output. The toothed disc 22 could be replaced by an oscillation source (Fig. 2, not shown).