The sheets in each tray of a multi tray sheet feeder are of the same thickness, but the sheets in one tray may be of a different thickness than the sheets in another tray. The sheets are fed from each sheet feeder tray to an intermediate stacker and then to a printer. A first sensor is provided just prior to entry of a sheet into the intermediate stacker and a second sensor is provided to sense a sheet as it is fed from the intermediate stacker. If the paper weight of sheets of paper on a tray fall within a first range of paper weight values, each sensor is designed to have a first given voltage response condition when sensing those sheets and if the paper weight of the sheets falls within a second range of paper weight values each sensor is designed to have a second given voltage response condition sensing the latter sheets. A current value supplied to the emitter of a sensor can be controlled to provide the desired voltage response or a resistance in a phototransistor collector circuit can be varied to provide the desired voltage response condition. If the first range of paper weight values is lighter than the second range of paper weight values, each sensor, when in the first given voltage response condition, will have a voltage response, when sensing a sheet of a given paper weight, which is higher than the voltage response when the same sensor senses a sheet of the same paper weight, when each sensor is in the second given voltage response condition. The first sensor is placed in a proper voltage response condition to sense the thickness of a sheet that has been fed from a particular tray and that thickness value is compared with a single sheet thickness value, sensed by the first sensor when in the same voltage response condition, that has been placed in memory for the sheets on that particular tray to detect a multi sheet feed from the tray. The sheet then enters the intermediate stacker. When the same sheet is fed from the intermediate stacker, it is sensed by the second sensor. The second sensor is placed in the same voltage response condition as the first sensor when it sensed the same sheet and the thickness value sensed by the second sensor is compared with the thickness value sensed of the same sheet by the first sensor to detect a multi sheet feed from the intermediate stacker.