Signal monitoring circuit

Signals such as track signals in a railway signalling system are monitored by antiphase sequential application to an enabling input of a control circuit which starts the operation of a timer, providing an output signal only if the control circuit is enabled for a time interval during which all the signals being monitored are present sequentially. Absence of any one of the signals in this interval results in a fail-safe condition, resetting the timer, which then fails to provide an output. In the application to a railway signal monitoring system absence of any one signal would be indicative of occupation of an associated track section, resulting in a 'danger' condition being signalled.