Apparatus for comparing voltages with predetermined upper and lower threshold levels is described. A single differential circuit is connected to a junction point in a first voltage divider, which determines one of the above threshold levels. The differential circuit produces a first output signal level when that threshold value is exceeded. A transistor is connected to a junction point of a second voltage divider, which determines the other threshold level, as to have its conducting state controlled thereby. The transistor is rendered conductive when the other threshold level is exceeded switching the output signal.