Toner concentration control apparatus

A xerographic copying apparatus having a developer operable to present a two component developer mix, i.e. carrier and toner, to the photoconductor's latent electrostatic image, and apparatus for automatically adding virgin toner to the developer as the toner concentration is depleted as a result of toning the photoconductor's image over a period of making a number of copies. Once the need to add toner is indicated, a known, unit quantity of toner is added to the developer. Immediately thereafter, the ability to sense toner concentration is inhibited until the next two photoconductor images have been toned. In the event that low concentration is sensed immediately thereafter, another unit quantity of toner is added, and sensing is again inhibited for the two-copy interval. In the additional event that low concentration is sensed seven consecutive times, each time being separated by a two-copy inhibit interval, a failure latch is set and further operation of the copying apparatus is inhibited. That latter event may be caused by toner exhaustion, failure to feed toner properly, or failure of the concentration sensing means.