Abstract: Apparatus for controlling the density of a reproduced image of an original to be copied in an electrophotographic copying machine by maintaining a fixed, predetermined image-developing bias voltage during development of a reference patch image and controlling the bias voltage in response to settings of image density selectors during the development of the original image.

Abstract: An apparatus for controlling the toner concentration in electrophotographic copying machines produces a toner concentration signal in response to the density of a reference patch image formed on a moving electrophotosensitive medium. The patch image density is detected at a time corresponding to a predetermined extent of movement of the electrophotosensitive medium following receipt of a signal indicating scanning of the original to be copied.

Abstract: A level shift circuit for indicating the open or closed condition of an access door of a copier senses the powered condition of operating portions of the copier to which the supply of power is interrupted when the access door is open and provides a corresponding logic signal to a sequence control circuit of the copier. In the disclosed embodiment, a series-connected resistor and zener diode are connected to the copier operating portions to provide an output signal through an inverting buffer to the sequence control circuit.

Abstract: A bias voltage switching circuit for an electrophotographic copying apparatus includes a series-connected string of resistors,, a constant bias voltage source in parallel with the string of resistors and a plurality of switching transistors. Each of the transistors is connected between a different junction of the series connected resistors and a common reference potential to which the constant voltage source and one end of the string of resistors is connected. Selective operation of the transistors varies the voltage output of the bias voltage switching circuit for use in controlling the concentration of the image produced by the copying apparatus.