Abstract: A control circuit having a difference unit to which first and second input signals are in operation applied to produce a signal representative of the differences between the two input signals, a logic circuit connected to receive a signal from the difference unit to produce in response to signals applied thereto exceeding a predetermined value in respect to a datum an output of a constant value equal to twice the said predetermined value and on the other side of the datum, an adding unit connected to receive the signal from the logic circuit and the signal from the difference unit to produce an output signal representative of the algebraic sum of the two signals applied to the adding unit, said switch means adapted to switch to a first condition in response to signals from the adding unit having values to one side of the datum and to switch to a second condition in response to signals from the adding unit having values to the other side of the datum.