Abstract: This invention relates to apparatus for continuously monitoring a light sensitive electronic component, forming part of a device for detecting light pulses of a specified character, the apparatus comprising a light source directing light through fiber optics at the component, the source being pulsed by a pulse generator which also sets an alarm monitor connected to operate a switch having its input taken to the component, the switch output being connected to a voltage sensor for resetting the alarm monitor if a signal received from the switch is above a predetermined minimum, the alarm monitor being adapted to activate an alarm if not reset.