Control unit for recording the charging operation in a 12 volt on-board network (engine running). If additional consumers are to be switched on only when the engine is running (the alternator supplies current) or if the on-board battery is to be charged via an isolating relay by interconnecting the on-board and starter battery, then the D+ contact of the alternator is usually used to control a relay. Often, however, this contact is not accessible in the vehicle, is no longer available in newer alternators, is not available in a container superstructure, or voltage-dependent control is generally required. This is where the D+ Simulator comes in. It generates a voltage-controlled (e.g. from the starter battery) signal for a subsequent powerful (cut-off) relay. Other possible applications include the control of additional consumers that should only be activated when the battery is sufficiently charged, such as switching on an air conditioning system while the vehicle is in motion or switching a refrigerator, cooling or heating box to 12 V on-board operation. Intelligent voltage evaluation of the switching thresholds avoids false activations during voltage dips or peaks. The dynamic threshold detection also avoids oscillation around the switching points when switching very large loads. The switching status is indicated by a light emitting diode. The electronics are protected against humidity, the connection is made in the rear terminal compartment by a plug-in screw terminal. The switching output is loadable with 12 V / 0.5 A, short-circuit and overload-proof and of type PNP, plus output, load ground side.