rapid than the phototransistors in certain cases it may be appropriate to replace the two potentiometers by a single potentiometer which is coupled in between the phototransistors and whose slider is connected to earth the embodiment of the present invention illustrated in fig1 13 corresponds in principle to the above described embodiment and differs substantially from the above described embodiment in that the recesses 17 39 and 18 39 for the phototransistors tr1 and tr2 are located substantially straight above one another and in that the optical axes 11 39 12 39 and 13 39 make other angles with the longitudinal axis 19 39 of the arm 8 39 than in the above described embodiment in this new embodiment the optical axis 11 39 for the recess 16 39 makes an angle of 45 u00b0 55 u00b0 preferably 48 u00b0 with the longitudinal axis 19 of the arm 8 39 which is located in the surface of the arm 8 39 facing towards the reed as is apparent in fig1 the recess 16 39 is intended for an led as in the above described embodiment seen in the projection shown in fig1 the optical axes 11 39 12 39 and 13 39 make an angle of 10.