An interface for a lamp operating device, in particular a ballast (EVG) for gas discharge lamps, has two input side terminals of a digital control input SE and a evaluation logic (AL) which is constituted for processing both digital signals applied to the terminals and also mains voltage signals. The amplitude of a mains voltage applied to the terminals is continuously settable by means of a voltage divider (ST) or a potentiometer. The amplitude of the applied mains voltage is converted into pulse width information (PWM) which in turn the evaluation logic converts into control commands, for example dimming setting values, for a ballast (EVG).