housing 74 includes an opening 76 to transmit the output from transmitter 50 to the road surface a second opening 78 is provided in the housing 74 to direct the returned signal from the roadway onto the optical detector 16 of optical system circuit 10 it is to be understood that while the above described circuits are found to be particularly adaptable to use in vehicles as a warning system the circuits may be used for other purposes than as described in relation to vehicle warning system fast event detector circuit 28 may for example be used in industrial applications wherein speeds of rotatory or linear moving objects are required to be monitored in such applications the stimulus for the fast event detector circuit may be obtained by illuminating the object by the transmitter a described above or by any of many other methods it is to be further understood that when the above described circuits are used in a warning system of a vehicle or the like the power sources 14 and 56 may be the operating battery of the vehicle or other batteries carried in the vehicle when the circuits are used in other environments such as industrial plant etc other sources of power may be utilized the outputs.