that is tool 4 has regained the position at which its lifting was initiated up down counter 38 will then be blocked and gate 40 39 unblocked so that consuming device 42 will again receive pulses zd the state described above will accordingly commence again fig6 illustrates the different signals that occur during vibrations while diode 13b and phototransistor 14b are at the boundary between an opaque and a transparent zone with diode 13a and phototransistor 14a on the other hand being on different sides of an opaque zone in this case obviously a will equal 0 while b will supply a signal indicating a transition from an opaque to a transparent zone in a device of the known type signal b will mean that disk 10 continues to rotate and tool 4 to drill it will be evident on the other hand that in the device in accordance with the invention the outputs qma1 and qma2 of monostables ma1 and ma 2 will remain at zero so that signals xa 1 and ya 1 will also remain at zero signals zd 1 and zc 1 will then each generate a pulse for each period of b pulses that are out of phase it will then be immediately evident from fig3 that up down counter 38 will oscillate.