Signal failure circuit for automatic transmissions

A signal failure circuit for incorporation in a control circuit of vehicle automatic transmission system of the type having a generator which produces a fluctuating signal, comprises an interval timer including a comparator and a capacitor, the latter being charged by means of a charging circuit and discharged by means of a transistor which is rendered conductive by pulses of a pulse train derived from said fluctuating signal. When said pulses cease the comparator provides an output to a monostable circuit which thereby switches to its unstable state to activate safety circuits associated with the transmission system. The monostable circuit will remain in its unstable state for a predetermined time after the return of the pulses of the pulse train. The charging circuit of the capacitor is conveniently controlled by a further transistor the conduction of which is dependent upon the output voltage of an a.c.-d.c. converter which also receives said train of pulses.