Multiple copy and transparency detector with illumination control

In a printing machine having a photosensitive element and a plurality of operating components cooperating with one another and the photosensitive element to produce images on copy sheets, a supply of copy sheets providing sheets along a copy sheet path for receiving images, and a detector along the copy sheet path for detecting superposed copy sheets within the path, the detector comprising

• an illumination means and a phototransistor for detecting light rays from the illumination means, the illumination means and phototransistor disposed along the copy sheet path,
• a driving circuit including a driver transistor to power the illumination means,
• a first operational amplifier connected to the phototransistor for providing an output signal from the phototransistor,
• a feedback loop connected between the first operational amplifier and the driver transistor to regulate the detector, and
• an output circuit connected to the feedback loop, the output circuit including a second operational amplifier and an RC circuit, the output circuit providing a first pulse for the detection of a copy sheet along the copy path and a second immediate pulse for the detection of a second copy sheet along the paper path indicating multiple or superposed copy sheets.

In a printing machine having transparencies advanced through copy processing stations along a predetermined transparency path, a device for detecting transparencies at a transparency detection station comprising

• means for feeding transparency material along the transparency path past the copying processing stations, illumination means positioned along the transparency path at the transparency detection station, a photosensitive means for receiving illumination from said illumination means, the amount of illumination received by the photosensitive means being dependent upon whether a transparency is present or not present at the transparency detection station, and
• circuitry for detecting transparencies in said transparency path, said circuit comprising
• feedback means electrically connected to the photosensitive means for maintaining a constant illumination from the illumination means, and
• signal producing means connected to the feedback means to provide a signal in response to the stepped change in the amount of illumination received by the photosensitive means due to the presence of a copy transparency in the transparency path.