over the sewing material and the thread lever is also in its uppermost position when the stopping tappet 54 comes in contact with the stopping lever 55 the latter is displaced to the right as viewed in fig6 with respect to the pin 56 against the action of a tension spring 57 until it actuates a switch 26 the above described mechanism works as follows for sewing the pedal 33 is swung forward to thereby pivot in the clockwise direction of fig2 so that the slide 41 moves downward and the brushes come in contact with the layers of the resistance 3 the value of the resistance has maximum 4 kohm so that a relatively small voltage appears at the inputs of the comparators 8 and 9 the two comparators then produce a positive output signal the capacitor 19 is charged through the diode 23 the schmidt trigger 27 is switched on and a signal is inputted to gate 11 through the inverter 28 so that the pulses of the pulse modulator 35 are delivered to the transistor 12 and the motor 13 is driven the length of the pulses increases when the input signal decreases and the number of turns of the motor increases if the foot control is let loose it comes back to its neutral position illustrated in fig2 and the driving circuit is interrupted the voltage at the inputs of the comparators 8 and 9 increases to the full voltage i e to a value above the reference voltages and the outputs x and y of the two comparators go to zero thus discharging the capacitor 19 through the resistance 24 the time delay which results from the discharge of the capacitor.