GB1020702A - Improvements in or relating to circuit arrangements for detecting the presence or absence of an electrical input signal - Google Patents

Abstract

1,020,702. Track circuits. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Jan. 13, 1964 [Jan. 16, 1963], No. 1418/64. Heading B7N. [Also in Division H3] A railway track circuit uses a frequencysensitive relay arrangement (see Division H3) in which the amplitude of alternating current signals is limited by a trigger circuit before being passed via a band pass filter to a further amplitude-limiting trigger circuit. The track circuit may be arranged by means (not shown) so that electrical signals of first and second frequencies are transmitted if the track is respectively occupied and non-occupied. Fig. 3 shows a receiver for such a system comprising a band pass filter 10 capable of passing the two frequencies (3260 and 3320 c/s.) and feeding a specified bi-stable transistor trigger circuit 2 which limits the amplitude of signals above a predetermined level. The square wave output from the trigger circuit 2 is fed via a band pass filter 4 (passing only one of said frequencies) to a similar amplitude-limiting trigger circuit 6, the output of which is rectified at 7 and operates a relay 8 thus indicating the presence of a train. A further channel 4<SP>1</SP> ... 81 (responsive to the second frequency) is provided to indicate the absence of a train. The threshold level of the trigger circuits 6, 6<SP>1</SP> is arranged in conjunction with the response curves of the filters 4, 4<SP>1</SP> so that the trigger circuits only operate on a band of frequencies closely adjacent to the wanted frequency. Spurious operation of relays 8, 8<SP>1</SP> by high level signals of incorrect frequency is thus prevented. Alternatively, as in Fig. 1 (not shown), the system may provide a signal of one frequency if the track is not occupied and no signal if the track is occupied.