Abstract: The present invention relates to a sensor comprising an optical fibre (2), a light source (1) for passing light through the fibre (2), a plurality of sets of actuating means (3), each set having one or more actuation means (3) connecting the fibre (2) to a respective device to be monitored, each actuation means (3) within a set having a stored vibrational energy which is released to modulate the light in the optical fibre (2) when a parameter of the device to which it is connected changes by a predetermined amount and means (4,5,6 and 7) for converting the optical signal into an electrical signal wherein each set of actuation means (3) has a different natural vibrational frequency thus enabling the sensor to detect from which device the vibration originated by monitoring the change in transmitted light.